V. Ostwald was interested instrumental methods of analysis. In particular, he studied the absorption spectra of various solutions in the visible region and, using 300 different systems, showed that the color of an electrolyte solution under conditions of complete dissociation is determined by the additive absorption of light by its ions; the idea of V. Ostwald, expressed by him in a letter to S., was of greater practical importance. Arrhenius back in 1892. The discussion was about the possibility of direct potentiometric determination of very low concentrations of metal ions from the electromotive force of a suitable galvanic cell. However, methods for direct potentiometric analysis of real objects appeared much later - already in the 20th century.
Unfortunately, in domestic textbooks on analytical chemistry, the name of W. Ostwald is usually mentioned only in connection with (the law of dilution and the theory of indicators; his other theoretical achievements are presented without reference to the author. But the point is not in particulars. It must be recognized that it was W. Ostwald and his school raised chemical methods of analysis from empirical to a higher, theoretically based level. This made it possible to select optimal analysis conditions and suitable reagents in advance, as well as predict systematic errors. The famous Russian chemist P. I. Walden wrote that V. Ostwald’s books are a true guide to discoveries in analytical chemistry.
The underestimation of the significance of W. Ostwald's chemical-analytical (and other) works in our country can be explained by two relatively random circumstances. Firstly, W. Ostwald's philosophical ideas caused fierce debate, in which he was sharply criticized by many famous scientists and public figures. In particular, V.I. Lenin considered Ostwald “a very great chemist and a very confused philosopher.” The negative attitude towards the philosophical works of V. Ostwald, wittingly or unwittingly, was transferred to his work as a whole. Secondly, as one of the creators of the “physical” theory of solutions, V. Ostwald sharply polemicized with Russian chemists, who were mainly supporters of the “chemical” theory of solutions (D. I. Mendeleev, D. P. Konovalov, N. N. Beketov, etc. .). Both sides made unnecessarily harsh statements. Later it became clear that essentially neither side in this dispute was completely right, the “chemical” and “physical” theories of solutions came closer in studies of ion hydration (I.A. Kablukov), and then merged, but echoes of the old controversy were still felt throughout the 20th century.
After W. Ostwald retired (1906) and he stopped active research in the field of chemistry, his students and followers developed the theory of titrimetric analysis, and they often consulted with their teacher. In particular, the dissociation constants of many indicators were determined (E. Zalm, 1907). A major achievement of the Ostwald school was the modeling of the titration process in the form of neutralization curves (J. Hildebrandt, 1913). On this basis, it was possible to evaluate the possibility of titrating strong and weak electrolytes, calculate dilution limits and errors associated with inaccurate selection of indicators. The monograph by the Dane Niels Bjerrum “The Theory of Alkalimetric and Acidimetric Titration” (1914) was of great importance. In this book, for the first time, a clear recommendation appeared on which acids and which bases can be titrated with sufficiently high accuracy, and Bjerrum theoretically proved that if the dissociation constant is less than 10 -10, titration is impossible even in relatively concentrated solutions.
The invention of buffer solutions had the greatest practical significance for the development of analytical chemistry in those years. From a theoretical point of view, the introduction of the concept of “hydrogen index” (pH) was very important. Both innovations arose as a result of the use of Ostwald's ideas about ionic equilibria, but the first to use them were not analysts, but biochemists. In 1900, O. Fernbach and L. Euben studied the activity of some enzymes at different acidity of the solution and came to the conclusion that the relative constancy of enzyme activity when adding acids or alkalis is explained by the presence in the same solution of a mixture of mono- and dihydrogen phosphates (“a mixture like buffer disk of the car, weakens the effects of acids and bases"). A little later, the Hungarian biochemist P. Seely began to specifically introduce buffer solutions with a known and approximately constant concentration of hydrogen ions into the blood serum samples under study. Apparently, B. Fels (a student of Nernst) was the first to use acetate and ammonia buffer solutions in the practice of chemical analysis in 1904.
Studying the activity of enzymes at different acidity of solutions, the Danish biochemist Søren Sørensen in 1909 established that the change in enzyme activity is determined not by the nature of the added acid or even its concentration, but by the concentration of hydrogen ions created when the acid is added. Calculations of enzymatic activity were significantly simplified if the decimal logarithm of the concentration of these ions, taken with the opposite sign, was used as an argument, i.e. pH indicator. True, S. Sørensen used rounded (integer) pH values. Somewhat later, fairly accurate methods for measuring this indicator were developed. They were based on the use of a set of colored acid-base indicators or on potentiometric measurements. A significant contribution to the development of both methods of measuring pH was made by the then young (1921) Dutch analytical chemist Isaac Moritz Kolthoff (1894-1997). Later (1927) he moved to the USA, where for many decades he was a generally recognized leader of American analytical chemists.
In 1926, I.M. Kolthoff published an excellent monograph, “Volume Analysis,” which summarized the theoretical foundations of titrimetric analysis as a whole. It was as important for the fate of this method as Mohr's monograph was in its time. Subsequently, on the basis of this book, I. M. Kolthof compiled a two-volume manual on titrimetric analysis, and then a textbook on analytical chemistry for students of American universities.
In the preface to the monograph, I.M. Kolthoff wrote: “The fact that I dare to bring together in the proposed book the scientific foundations of volumetric analysis is justified by the possibility, with the help of theoretical knowledge, not only to improve known methods, but also to find new ones. To do this, you need to consider the appropriate reaction, as well as the action of the indicator, in detail from the point of view of the law of mass action. When the system is in equilibrium, mathematical analysis relatively simply determines the possibility of titration, finding optimal conditions, as well as titration errors... Thus, new methods do not have to be looked for purely empirically, but for the most part they can already be derived theoretically.” In addition to the generalization and experimental verification of the results previously obtained by J. Hillebrandt, E. Salm, N. Bjerrum and other physical chemists of the school of W. Ostwald, to which I. M. Kolthoff himself ideologically belonged, the author put forward many new provisions, confirming them with calculations and experiments. A detailed mathematical analysis of the titration curves was carried out (jump height, calculation of the potential at the equivalence point, criteria for titration of mixtures, etc.). Titration errors caused by different factors are compared. At the same time, I.M. Kolthof believed that reliable theoretical predictions can be made only for the reactions of neutralization, precipitation and complex formation. The low rate at which equilibrium is achieved and the stepwise nature of many oxidation-reduction processes should significantly reduce the value of theoretical predictions related to redoxmetry.
R. Petere began to create the theoretical foundations of redox titration in 1898. He also tested in numerous experiments the applicability and correctness of the famous Nernst formula (1889), used to construct redoxmetric titration curves. F. Crotogino (real potentials, influence of pH) and other authors successfully worked in the same area. Kolthoff's own work in the field of redoxmetry was also related to the Nernst equation, but the author examined in detail the kinetic aspects of redox processes, including catalytic effects and induced reactions, and studied the factors influencing the potential at the equivalence point. In his other works, I.M. Kolthoff actually created the theory of potentiometric and amperometric titration. And the very terms “potentiometric titration and “amperometric titration” were introduced into science by him. Comparing I.M. Kolthoff’s book on the theory of titrimetric analysis with the corresponding sections of today’s textbooks on analytical chemistry, one can only be surprised at how modern the content and style of a book written 80 years ago seem.
n The set of traditional methods for determining the composition of a substance by its sequential chemical decomposition is called “wet chemistry” (“wet analysis”). These methods have relatively low accuracy, require relatively low qualifications of analysts, and are now almost completely replaced by modern instrumental methods for determining the composition of a substance.
n However, “wet chemistry” has its advantage over instrumental methods - it allows, through standardized procedures (systematic analysis), to directly determine the composition and different oxidative states of elements.
Along with classical chemical methods, physical and physicochemical (instrumental) methods are widely used. They are based on measuring the optical, electrical, adsorption, catalytic and other characteristics of the analyzed substances, depending on their quantity (concentration).
Typically, these methods are divided into the following groups: electrochemical (conductometry, polarography, potentiometry, etc.); spectral or optical (emission and absorption spectral analysis, photometry, colorimetry, nephelometry, luminescent analysis, etc.); X-ray (absorption and emission X-ray spectral analysis, X-ray phase analysis, etc.); chromatographic (liquid, gas, gas-liquid chromatography, etc.); radiometric (activation analysis, etc.); mass spectrometric.
The listed methods, while inferior to chemical ones in accuracy, are significantly superior to them in sensitivity, selectivity, and speed of execution. The accuracy of chemical methods is usually in the range of 0.005-0.1%; errors in determination by instrumental methods are 5-10%, and sometimes significantly more.
When using physical and physicochemical methods, microquantities of substances are usually required. The analysis can in some cases be performed without destroying the sample; Sometimes continuous and automatic recording of results is also possible. These methods are used to analyze high-purity substances, evaluate product yields, and study the properties and structure of substances.
n POTENTIOMETRY (from Latin potentia-strength, power and Greek metreo-measure), an electrochemical method of research and analysis of substances, based on the dependence of the equilibrium electrode potential E on the thermodynamic activity (concentration) of the components of the electrochemical reaction.
where E 0 standard potential, R-gas constant, T-temperature, F-Faraday constant, n-number of electrons participating in the reaction, a, b, . . . , t, r. . . -stoichiometric coefficients for reaction components A, B, . . . , M, P (which can be ions and molecules in the liquid, solid or gas phase). n The activities of solid and gaseous components and solvents are taken as unity. n n n
n In potentiometric measurements, a galvanic cell is formed with an indicator electrode, the potential of which depends on the activity of at least one of the components of the electrochemical reaction, and a reference electrode, and the electromotive force (EMF) of this element is measured.
n Among these methods, a distinction is made between direct potentiometry and potentiometric titration. Direct potentiometry is used to directly determine the activity of ions (for example, Ag+ in a solution of Ag. NO 3) from the E value of the corresponding indicator electrode (for example, silver); in this case, the electrode process must be reversible.
n Historically, the first methods of direct potentiometry were methods for determining the pH value p. N. The appearance of membrane ion-selective electrodes led to the emergence of ionometry (the Khmetria river), where the river. X = - log ax, ax-activity of component X of the electrochemical reaction.
n Sometimes p. H-metry is considered as a special case of ionometry. Calibration of potentiometer instrument scales according to p values. X is difficult due to the lack of appropriate standards. Therefore, when using ion-selective electrodes, the activity (concentration) of ions is determined, as a rule, using a calibration curve or the addition method. The use of such electrodes in non-aqueous solutions is limited due to the instability of their body and membrane to the action of organic solvents.
n Calibration curve method. To do this, build a calibration graph in advance in EMF - lg coordinates. SI using standard solutions of the analyzed ion, having the same ionic strength of the solution. n The graph is linear. Then the EMF of the circuit with the analyzed solution is measured by ionic strength and the concentration of the solution is determined from the graph. An example of the definition is shown in Fig.
Additive method. n This is a group of methods based on the addition of a solution of the analyzed ion with a known concentration to the analyzed solution. The additive can be one-time - single additive method; two-time - double addition method; reusable - a method of repeated additions.
n Direct potentiometry also includes redoxmetry - the measurement of standard and real oxidation. - will restore. potentials and equilibrium constants oxidize. - will restore. reactions. Oxidize. - will restore. the potential depends on the activities of the oxidized and reduced forms of the substance. Redoxmetry is also used to determine the concentration of ions in solutions. By direct potentiometry using metallic. electrodes, the mechanism and kinetics of precipitation and complexation reactions are studied.
n Direct potentiometry has important advantages. During the measurement process, the composition of the analyzed solution does not change. In this case, as a rule, preliminary separation of the analyte is not required. The method can be easily automated, which allows it to be used for continuous monitoring of technological processes.
n More common are potentiometric titration methods, which are used to determine a wide range of substances in aqueous and non-aqueous media. In these methods, changes in the potential of the indicator electrode are recorded during the titration of the test solution with a standard reagent solution, depending on the volume of the latter. Potentiometric titration is carried out using various reactions: acid-base and redox interactions, precipitation and complexation.
n The equivalence point for potentiometric titrations is determined graphically on the titration curve. Usually one of the following types of titration curves is used: integral, differential or Gran curve.
n The integral titration curve (Fig. a) is plotted in coordinates E - VT. The equivalence point is in the middle of the titration jump. n The differential titration curve (Fig. b) is plotted in coordinates: n ∆E / ∆V-VT. The equivalence point is at the top of the titration curve. The differential titration curve gives a more accurate determination of the equivalence point than the integral one. n The titration curve in the Gran method (Fig. c) is plotted in coordinates: ∆V / ∆E -VT. The equivalence point is at the intersection of two straight lines. This curve is convenient to use to determine the equivalence point when titrating dilute solutions.
n In acid-base titration methods, any electrode reversible to H+ ions (hydrogen, quinhydrone, antimony, glass) can be used as an indicator electrode; The most common glass electrode. Redox titration is carried out with electrodes made of noble metals (most often platinum).
1. A strong acid (HCl) is titrated with a strong base (Na. OH) to e.g. p. H = -lg V i.e. = p. H=p. OH = 7 After t.e. p. Н=14+ lg 2. A strong base (Na. OH) is titrated with a strong acid (HCl) to e.g. p. H=14+ log V i.e. = p. H=p. OH = 7 After t.e. p. H = -lg
3. A weak acid (CH 3 COOH) is titrated with a strong base (Na. OH) Before titration p. H = 0.5 rub. K – 0.5 lg. Sour to t.e. p. N = r. K - lg. Sour + lg. Soli V t. e. = p. H = 7 + 0.5 rub. K + 0.5 lg. Soli After t.e. p. H=14+ lg. Pine 4. A weak base (NH 4 OH) is titrated with a strong acid (HCl) Before titration p. H = 14 - 0.5 rub. K + 0.5 lg. Sosn Before t.e. p. N= 14 - r. K + lg. Pine – lg. Soli V t. e. = p. H = 7 - 0.5 rub. K - 0.5 lg. Sosn After t.e. p. H = - lg. Sour
n In precipitation and complexometric titration methods, the indicator (ion-selective or metal) electrode must be reversible with respect to one of the ions participating in the reaction. Near the equivalence point, a sharp change (jump) in the electrode potential E is observed, caused by the replacement of one electrochemical reaction by another with a corresponding change in E0.
n Potentiometric titration has a number of advantages compared to titrimetric methods that use chemical indicators: objectivity and accuracy in establishing the titration end point, low limit of determined concentrations, the ability to titrate cloudy and colored solutions, the possibility of differentiated (separate) determination of components of mixtures from one portion solution if the corresponding E 0 are sufficiently different.
n Potentiometric titration can be carried out automatically to a given potential value; titration curves are recorded in both integral and differentiated form. From these curves one can determine the “apparent” equilibrium constants of decomposition processes.
CLASSIFICATION OF ELECTRODES n For potentiometric measurements, electrochemical circuits containing two electrodes are used: an indicator and a reference electrode. If both electrodes are immersed in the solution being analyzed, then such a circuit is called a non-transfer circuit. If the reference electrode is connected to the test solution through a liquid contact (salt bridge), the circuit is called a transfer circuit. n
In potentiometric analysis, transfer chains are predominantly used. Schematically, such a circuit is depicted as follows: Indicator electrode Analyzed Salt solution bridge Reference electrode
n An indicator electrode is an electrode whose potential determines the activity of the analyzed ion in accordance with the Nernst equation. A reference electrode is an electrode whose potential is constant and does not depend on the concentration of ions in the solution. The salt bridge serves to prevent mixing of the analyzed solution and the reference electrode solution. n Saturated solutions of salts KCl, KNO 3 and others with similar mobilities of the cation and anion are used as a salt bridge.
n The following are used as indicators in potentiometric analysis: n 1. Electrodes on the surface of which reactions involving the exchange of electrons occur. They are called electron exchange, or redox. Electrodes made of chemically inert metals - platinum, gold, etc. are used as such electrodes. In analytical practice, the industrially produced platinum point electrode EPV-1 -100 and the membrane redox electrode EO - 1, made of special glass, are used.
n 2. Electrodes on the surface of which ion exchange reactions occur. They are called ion exchange or ion selective electrodes. The main element of ion-selective electrodes is an ion-sensitive membrane. Therefore, they are also sometimes called membrane ones. n Ion-selective electrodes are made: n - with solid membranes; n - with glass membranes; n - with liquid membranes.
n Electrodes with solid membranes. In such electrodes, the membrane is made of a slightly soluble crystalline substance with an ionic type of electrical conductivity. Structurally, the electrode is a tube with a diameter of about 1 cm made of an inert polymer (usually polyvinyl chloride), to the end of which a thin (~0.5 mm) membrane is glued. An internal reference solution is poured into the tube, into which the reference electrode is immersed. Currently, the industry produces electrodes with solid membranes that are selective to F-ions (membrane based on La. F 3 single crystal), to CI-, Br- and I-ions (membranes based on a mixture of silver sulfide and the corresponding silver halide) .
n Electrodes with glass membranes. They are made from special electrode glass, which contains oxides of aluminum, sodium, potassium, boron, etc. The membrane of such electrodes is a thin-walled ball (~0.1 mm) with a diameter of 5 - 8 mm. n Currently, the industry produces glass electrodes that are selective only for the cations H+, Na+, K+, Ag+, NH 4+. In these electrodes, not only the membrane, but also the body itself is made of glass.
n Electrodes with liquid membranes. In such electrodes, liquid membranes, which are ion exchange substances dissolved in organic solvents, are separated from the analyzed solution by hydrophobic finely porous films, porous disks or hydrophobized ceramic diaphragms. Their main disadvantage is the gradual leaching of the ion exchanger by the analyzed solution, which shortens the life of the electrode.
n These difficulties were avoided after the development of electrodes with film membranes. In such electrodes, a plasticizer and an electrode-active substance dissolved in it are introduced into a thin membrane made of a hydrophobic polymer (polyvinyl chloride), which enters into an ion-exchange reaction with the analyzed ion in solution. Currently, industry produces film ion-selective electrodes for Na+, K+, NH 4+, Ca 2+, Mg 2+ cations; electrodes for determining the total hardness of water; on halide anions, CNS-, NO 3 -. There are electrodes for other ions.
n Silver chloride electrodes are currently used as reference electrodes. A silver chloride electrode is a silver wire coated with an Ag layer. Cl and immersed in a saturated solution of KS 1. The modern design of reference electrodes also includes a salt bridge.
n Potentiometric methods of analysis are widely used to automate the control of technological processes in the chemical, petrochemical, food and other industries, in medicine, biology, geology, as well as in the control of environmental pollution.
n COULOMETRY n Coulometry is an electrochemical method of analysis that is based on measuring the amount of electricity (coulombs) spent on the electrooxidation or reduction of the analyte.
The amount of substance contained in the analyzed sample is calculated according to the equation: m = M Q / F n n where m is the amount of substance in the analyzed solution, g; n M – molar mass of the analyzed component (substance or ion); Q is the amount of electricity spent on the electrochemical oxidation or reduction of the analyzed component, C; F - Faraday number equal to 96,500 C/mol; n is the number of electrons involved in the electrochemical process. The amount of electricity is calculated by the formula: Q = I t n where I is the current strength, A; t is the duration of electrolysis, s.
n In coulometry, there are two types of analysis: n 1) direct coulometry; n 2) coulometric titration. n For both types of coulometry, the following condition must be met: only the analyte with 100% current efficiency must undergo electrochemical reduction or oxidation.
n The direct coulometry method is very sensitive. They can determine up to 10 -9 g of a substance in a sample. The error of determination does not exceed 0.02%. n Coulometric titration has significant advantages over conventional titration. Its use eliminates the need to prepare and standardize the titrant; it becomes possible to use unstable titrants: silver (I), tin (II), copper (II), titanium (III), etc. n Any type of titration can be performed coulometrically: acid-base, precipitation, complexometric, redox. The coulometric titration method is superior in accuracy and sensitivity to other titration methods. It is suitable for titrating very dilute solutions with a concentration of up to 10 -6 mol/dm 3, and the error of determination does not exceed 0.1 -0.05%.
Conductometry (from the English conductivity - electrical conductivity and metry) is a set of electrochemical methods of analysis based on measuring the electrical conductivity of solutions. Conductometry is used to determine the concentration of solutions of salts, acids, bases, and to control the composition of some industrial solutions. Conductometric analysis is based on changes in the concentration of a substance or the chemical composition of the medium in the interelectrode space; it is not related to the electrode potential, which is usually close to the equilibrium value. Conductometry includes direct methods of analysis (used, for example, in salinity meters) and indirect (for example, in gas analysis) using direct or alternating current (low and high frequency), as well as chronoconductometry, low-frequency and high-frequency titration.
n PHOTOMETRIC ANALYSIS (PA), a set of molecular absorption spectral analysis methods based on the selective absorption of electromagnetic radiation in the visible, IR and UV regions by molecules of the component being determined or its compound with a suitable reagent. The concentration of the component being determined is determined according to the Bouguer-Lambert-Beer law.
The law is expressed by the following formula: n where I 0 is the intensity of the incoming beam, l is the thickness of the layer of substance through which the light passes, kλ is the absorption index.
n Colorimetry (from Latin color - color and Greek metreo - measure) is an analysis method based on determining the concentration of a substance by the intensity of the color of solutions (more precisely, the absorption of light by solutions). The color intensity is determined either visually or using instruments such as colorimeters.
Photometry differs from spectrophotometry in that light absorption is measured in the visible region of the spectrum, less often in the near UV and IR regions (i.e., in the wavelength range from ~ 315 to ~ 980 nm), and also in that to select the desired area spectrum (width 10 -100 nm) do not use monochromators, but narrow-band light filters.
The instruments for photocolorimetry are photoelectrocolorimeters (PEC), characterized by the simplicity of their optical and electrical circuits. Most FECs have a set of 10 -15 light filters and are two-beam devices in which a beam of light from a radiation source (incandescent lamp, rarely a mercury lamp) passes through a light filter and a light flux divider (usually a prism), which divides the beam into two, directed through cuvettes with the test solution and with the reference solution.
After the cuvettes, parallel light beams pass through calibrated attenuators (diaphragms), designed to equalize the intensities of light fluxes, and fall on two radiation receivers (photocells), connected via a differential circuit to a null indicator (galvanometer, indicator lamp). The disadvantage of the instruments is the absence of a monochromator, which leads to a loss of selectivity of measurements; advantages: simplicity of design and high sensitivity due to high aperture ratio.
The measured range of optical density is approximately 0.05 -3.0, which makes it possible to determine many elements and their compounds in a wide range of contents - from ~ 10 -6 to 50% by mass. To further increase the sensitivity and selectivity of determinations, the selection of reagents that form intensely colored complex compounds with the substances being determined, the choice of solution composition and measurement conditions are essential. The errors of determination are 5%.
ABSORPTION SPECTROSCOPY studies the absorption spectra of electromagnetic radiation by atoms and molecules of matter in various states of aggregation. The intensity of the light flux as it passes through the medium under study decreases due to the conversion of radiation energy into various forms of internal energy of the substance and (or) into the energy of secondary radiation.
The absorption capacity of a substance depends mainly on the electronic structure of atoms and molecules, as well as on the wavelength and polarization of the incident light, layer thickness, concentration of the substance, temperature, and the presence of electric and magnetic fields.
The application of absorption spectroscopy is based on the Bouguer-Lambert-Beer law - a physical law that determines the attenuation of a parallel monochromatic beam of light as it propagates in an absorbing medium.
To measure absorbance, spectrophotometers are used - optical instruments consisting of a light source, a sample chamber, a monochromator (prism or diffraction grating) and a detector. The signal from the detector is recorded in the form of a continuous curve (absorption spectrum) or in the form of tables if the spectrophotometer has a built-in computer.
To determine the concentration of the test substance, the following is used: Calibration curve method. The intensity of the analytical signal is measured for several standard samples or standard solutions and a calibration graph is constructed in the coordinates I = f(c) or I = f(lgc), where c is the concentration of the component in the standard solution or standard sample. Under the same conditions, the signal intensity of the analyzed sample is measured and the concentration is found using the calibration graph. .
Additive method. The intensity of the analytical signal of the sample Ix is measured, and then the intensity of the signal of the sample with a known addition of the standard solution Ix + stt. The concentration of a substance in a sample is calculated using the relation cx = cst. Ix/(Ix+st - Ix).
Theoretical and experimental methods of phosphorus are used in lighting and signaling technology, in astronomy and astrophysics, in calculating the transfer of radiation in the plasma of gas-discharge light sources and stars, in the chemical analysis of substances, in pyrometry, in calculating heat transfer by radiation, and in many other fields of science. and production.
n Determination of the content of ingredients in the atmospheric air of populated areas and the air of the working area: nitrogen oxide (II), nitrogen oxide (IV), ammonia, sulfur dioxide, arsenic, sulfuric acid content, sulfates, hydrogen sulfide, phenol, formaldehyde. n In drinking water: ammonia and ammonium ions, arsenic, nitrates and nitrites, selenium, sulfates, total iron. n In the soil: aluminum (mobile), nitrates, ammonium, calcium, magnesium, mobile forms of sulfur, phosphorus, sulfates, gross content and mobile forms of iron, cobalt, copper, manganese, nickel, chromium. n Analysis of petroleum products, mineral oils and other organic substances.
PHYSICAL METHODS OF ANALYSIS are based on measuring the effect caused by the interaction of radiation with matter - a flow of quanta or particles. Radiation plays approximately the same role as a reagent in chemical methods of analysis. The physical effect being measured is a signal. As a result of several or many measurements of the signal magnitude and their statistical processing, an analytical signal is obtained. It is related to the concentration or mass of the components being determined.
n Based on the nature of the radiation used, F. m.a. can be divided into three groups: n 1) methods using primary radiation absorbed by the sample; n 2) using primary radiation scattered by the sample; n 3) using secondary radiation emitted by the sample.
n 1) spectroscopic methods of analysis - atomic emission, atomic absorption, atomic fluorescence spectrometry, ultraviolet spectroscopy, X-ray spectroscopy, X-ray fluorescence method and X-ray spectral microanalysis, mass spectrometry, electron paramagnetic resonance and nuclear magnetic resonance, electron spectrometry;
n 2) nuclear physical and radiochemical methods - radioactivation analysis, n nuclear gamma resonance or Mössbauer spectroscopy, isotope dilution method, n 3) other methods, for example, X-ray diffractometry.
In recent years, instrumental methods of analysis have been increasingly used, which have many advantages: speed, high sensitivity, the ability to simultaneously determine several components, a combination of several methods, automation and the use of computers to process analysis results. As a rule, instrumental methods of analysis use sensors (probes), and, above all, chemical sensors, which provide information about the composition of the environment in which they are located. Sensors are connected to a system for storing and automatically processing information.
Conventionally, instrumental methods of analysis can be divided into three groups: spectral and optical, electrochemical and chromatographic methods of analysis.
Spectral and optical methods of analysis are based on the interaction of the analyte and electromagnetic radiation (EMR). Methods are classified according to several criteria - whether EMR belongs to a certain part of the spectrum (UV spectroscopy, photoelectrocolorimetry, IR spectroscopy), the level of interaction of substances with EMR (atom, molecule, atomic nucleus), physical phenomenon (emission, absorption, etc.). ). The classification of spectral and optical methods according to their main characteristics is given in Table. 12.
Atomic emission spectroscopy is a group of analysis methods based on measuring the wavelength and intensity of the light flux emitted by excited atoms in the gaseous state.
Table 12.
Classification of spectral and optical methods
| Physical phenomenon | Interaction Level | |
| Atom | Molecule | |
| Spectral methods | ||
| Light absorption (adsorption) | Atomic adsorption spectroscopy (AAS) | Molecular adsorption spectroscopy (MAS): photoelectrocolorimetry, spectrophotometry |
| Emission of light (emission) | Atomic emission spectroscopy (AES): flame photometry | Molecular emission spectroscopy (MES): luminescence analysis |
| Secondary emission | Atomic fluorescence spectroscopy (AFS) | Molecular fluorescence spectroscopy (MFS) |
| Light scattering | - | Scattering spectroscopy: nephelometry, turbidemetry |
| Optical methods | ||
| Light refraction | - | Refractometry |
| Rotation of plane polarized light | - | Polarimetry |
In emission analysis, the analyte, which is in the gas phase, is excited, imparting energy to the system in the form of EMR. The energy required for the transition of an atom from a normal to an excited state is called excitation energy (excitation potential ) . The atom remains in an excited state for 10 -9 - 10 -8 s, then, returning to a lower energy level, emits a quantum of light of a strictly defined frequency and wavelength.
Flame photometry– an analysis method based on photometric measurements of the radiation of atoms excited in a flame. Due to the high temperature in the flame, the spectra of elements with low excitation energy - alkali and alkaline earth metals - are excited.
Qualitative analysis is carried out based on the color of the flame pearls and the characteristic spectral lines of the elements. Volatile metal compounds color the burner flame in one color or another. Therefore, if you add the substance under study on a platinum or nichrome wire into a colorless burner flame, then the flame becomes colored in the presence of substances of certain elements, for example, in the colors: bright yellow (sodium), violet (potassium), brick red (calcium) ), carmine red (strontium), yellow-green (copper or boron), pale blue (lead or arsenic).
Quantitative analysis is based on the empirical dependence of the intensity of the spectral line of the element being determined on its concentration in the sample using a calibration graph.
Photoelectrocolorimetry based on the absorption of light by the analyte in the visible region of the spectrum (400 – 760 nm); This is a type of molecular adsorption spectroscopy. During the analysis, the light flow passing through the light-absorbing solution is partially scattered and refracted, but most of it is absorbed, and therefore the intensity of the light flow at the output is less than at the input. This method is used for qualitative and quantitative analysis of true solutions.
Turbidimetric method is based on the absorption and scattering of monochromatic light by suspended particles of the analyte. The method is used for the analysis of suspensions, emulsions, for the determination of substances (chlorides, sulfates, phosphates) capable of forming sparingly soluble compounds in solutions, natural and process waters.
TO optical analysis methods include refractometry and polarimetry.
Refractometric method based on the refraction of light when a beam passes through the interface between transparent homogeneous media. When a beam of light falls on the interface between two media, partial reflection from the interface and partial propagation of light in the other medium occurs. The method is used for identification and frequency of substances, quantitative analysis.
Polarimetry– an optical non-spectral method of analysis based on the rotation of a plane-polarized monochromatic beam of light by optically active substances. The method is intended for qualitative and quantitative analysis of only optically active substances (sucrose, glucose, etc.) capable of rotating the plane of polarization of light.
Electrochemical methods of analysis are based on measuring potentials, current and other characteristics during the interaction of the analyte with an electric current. These methods are divided into three groups: methods based on electrode reactions occurring in the absence of current ( potentiometry ); methods based on electrode reactions occurring under the influence of current ( voltammetry, coulometry, electrogravimetry ); methods based on measurements without an electrode reaction ( conductometry – low frequency titration and oscillometry – high-frequency titration).
According to application methods, electrochemical methods are classified into straight , based on the direct dependence of the analytical signal on the concentration of the substance, and indirect (establishing the equivalence point during titration).
To register an analytical signal, two electrodes are required - an indicator electrode and a reference electrode. An electrode whose potential depends on the activity of the detected ions is called indicator. It must respond quickly and reversibly to changes in the concentration of detected ions in solution. An electrode whose potential does not depend on the activity of the detected ions and remains constant is called reference electrode . For example, when determining the pH of solutions, a glass electrode is used as an indicator electrode, and a silver chloride electrode is used as a reference electrode (see Topic 9).
Potentiometric method is based on measuring the electromotive forces of reversible galvanic elements and is used to determine the concentration (activity) of ions in a solution. The Nernst equation is used for calculations.
Voltammetry– a group of methods based on the processes of electrochemical oxidation or reduction of the analyte, occurring on a microelectrode and causing the occurrence of a diffuse current. The methods are based on the study of current-voltage curves (voltammograms), reflecting the dependence of the current on the applied voltage. Voltammograms make it possible to simultaneously obtain information about the qualitative and quantitative composition of the analyzed solution, as well as about the nature of the electrode process.
In voltammetry methods, two- and three-electrode cells are used. Indicator electrodes are working polarizable electrodes on which processes of electro-oxidation or electro-reduction of a substance occur; reference electrodes – electrodes of the second type (saturated silver chloride or calomel).
If a dripping mercury electrode with a constantly renewed surface is used as a working polarizable electrode, and a layer of mercury at the bottom of the cell serves as a reference electrode, then the method is called polarography .
In modern voltammetry, any indicator electrodes are used (rotating or stationary platinum or graphite, stationary mercury), except for the dripping mercury electrode.
Conductometric method is based on measuring the electrical conductivity of solutions depending on the concentration of charged particles present. Objects of analysis are electrolyte solutions. The electrical conductivity of dilute solutions is proportional to the concentration of electrolytes. Therefore, by determining the electrical conductivity and comparing the obtained value with the value on the calibration graph, you can find the concentration of the electrolyte in the solution. The conductometry method, for example, determines the total content of impurities in high-purity water.
Chromatographic methods separation, identification and quantitation are based on different rates of movement of individual components in a mobile phase flow along a stationary phase layer, with the analytes present in both phases. The efficiency of separation is achieved through repeated sorption-desorption cycles. In this case, the components are distributed differently between the mobile and stationary phases in accordance with their properties, resulting in separation. Conventionally, chromatographic methods can be divided into gas chromatography, ion exchange and paper.
Gas chromatography– a method for separating volatile thermostable compounds, based on the distribution of substances between phases, one of which is a gas, the other is a solid sorbent or viscous liquid. The separation of the components of the mixture occurs due to the different adsorption capacity or solubility of the analyzed substances when their gaseous mixture moves in a column with a flow of the mobile phase along the stationary phase.
Objects of analysis in gas chromatography are gases, liquids and solids with a molecular weight of less than 400 and a boiling point of less than 300 0 C. During chromatographic separation, the analyzed compounds should not be subject to destruction.
Ion exchange chromatography– a method of separation and analysis of substances, based on the equivalent exchange of ions of the analyzed mixture and an ion exchanger (ion exchanger). There is an exchange of ions between the phases of the heterogeneous system. The stationary phase is ion exchangers; As a rule, water is mobile, since it has good dissolving and ionizing properties. The ratio of the concentrations of exchanged ions in the solution and the sorbent (ion exchanger) phase is determined by the ion exchange equilibrium.
Paper chromatography refers to plane chromatography, it is based on the distribution of analytes between two immiscible liquids. In partition chromatography, the separation of substances occurs due to differences in the distribution coefficients of components between two immiscible liquids. The substance is present in both phases as a solution. The stationary phase is retained in the pores of the chromatographic paper without interacting with it; the paper acts as a carrier of the stationary phase.
Thus, the use of the laws of electrochemistry, sorption, emission, absorption or reflection of radiation and the interaction of particles with magnetic fields has made it possible to create a large number of instrumental methods of analysis, characterized by high sensitivity, speed and reliability of determination, and the ability to analyze multicomponent systems.
Questions for self-study:
1. What is chemical identification of a substance?
2. What types of analysis do you know?
3. What is the purity of substances?
4. How is the identification of cations of inorganic substances carried out?
5. How are anions of inorganic substances identified?
6. How are quantitative analysis methods classified?
7. What are the basics of the gravimetric method of analysis?
8. What are the characteristics of titrimetric methods of analysis?
9. What are the characteristics of chemical methods of analysis?
10. How are instrumental methods of analysis classified?
11. What are the basics of electrochemical methods of analysis?
12. What are the basics of chromatographic methods of analysis?
13. What are the basics of optical analysis methods?
Literature:
1. Akhmetov N.S. General and inorganic chemistry. M.: Higher school. – 2003, 743 p.
2. Akhmetov N.S. Laboratory and seminar classes in general and inorganic chemistry. M.: Higher school. – 2003, 367 p.
3. Vasiliev V.P. Analytical chemistry. - M.: Higher. school – 1989, Part 1, 320 p., Part 2., 326 p.
4. Korovin N.V. General chemistry. - M.: Higher. school – 1990, 560 p.
5. Glinka N.L. General chemistry. – M.: Higher. school – 1983, 650 p.
6. Glinka N.L. Collection of problems and exercises in general chemistry. – M.: Higher. school – 1983, 230 p.
7. General chemistry. Biophysical chemistry. Chemistry of biogenic elements./ Ed. Yu.A. Ershova - M.: Higher. school – 2002, 560 p.
8. Frolov V.V. Chemistry. – M.: Higher. school – 1986, 450 p.
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Voronezh 2011
Lecture No. 1 (2 hours) Introduction Questions: 1. Subject of chemistry. The importance of chemistry in the study of nature and the development of technology. 2. Base
Basic quantitative laws of chemistry
The basic quantitative laws of chemistry include: the law of constancy of composition, the law of multiple ratios and the law of equivalents. These laws were discovered at the end of the 13th - beginning of the 19th centuries, and
Modern model of the structure of the atom
The modern theory of atomic structure is based on the work of J. Thomson (who discovered the electron in 1897, and in 1904 proposed a model of the structure of the atom, according to which the atom is a charged sphere with
Orbital quantum number 0 1 2 3 4
Each value of l corresponds to an orbital of a special shape, for example, the s-orbital has a spherical shape, the p-orbital has a dumbbell shape. In the same shell, the energy of sublevels increases in the series E
Structure of multi-electron atoms
Like any system, atoms strive for a minimum of energy. This is achieved at a certain state of the electrons, i.e. at a certain distribution of electrons over orbitals. Record
Periodic properties of elements
Since the electronic structure of elements changes periodically, then, accordingly, the properties of elements determined by their electronic structure, such as ionization energy,
Periodic table of elements by D.I.Mendeleev
In 1869, D.I. Mendeleev announced the discovery of the periodic law, the modern formulation of which is as follows: the properties of elements, as well as the forms and properties of their compounds
General characteristics of chemical bonds
The doctrine of the structure of matter explains the reasons for the diversity of the structure of substances in various states of aggregation. Modern physical and physicochemical methods make it possible to experimentally determine
Types of chemical bond
The main types of chemical bonds include covalent (polar and nonpolar), ionic and metallic bonds. A covalent bond is a chemical bond formed
Types of intermolecular interactions
Bonds in the formation of which the rearrangement of electronic shells does not occur are called interactions between molecules. The main types of interaction between molecules include:
Spatial structure of molecules
The spatial structure of molecules depends on the spatial direction of the overlap of electron clouds by the number of atoms in the molecule and the number of electron pairs of bonds due to
General characteristics of the state of aggregation of matter
Almost all known substances, depending on conditions, are in a gaseous, liquid, solid or plasma state. This is called the state of aggregation of matter. Ag
Gaseous state of a substance. Laws of ideal gases. Real gases
Gases are common in nature and are widely used in technology. They are used as fuel, coolants, raw materials for the chemical industry, working fluid for performing mechanical work.
Characteristics of the liquid state of a substance
Liquids in their properties occupy an intermediate position between gaseous and solid bodies. Near the boiling point they are similar to gases: fluid, have no definite shape, amorphous
Characteristics of some substances
Substance Type of crystal Energy of the crystal lattice, kJ/mol Temperature
General concepts of thermodynamics
Thermodynamics is a science that studies the transformation of various forms of energy into each other and establishes the laws of these transformations. As an independent discipline
Thermochemistry. Thermal effects of chemical reactions
Any chemical processes, as well as a number of physical transformations of substances (evaporation, condensation, melting, polymorphic transformations, etc.) are always accompanied by a change in the reserve of internal
Hess's law and consequences from it
Based on numerous experimental studies, the Russian academician G.I. Hess discovered the fundamental law of thermochemistry (1840) - the law of constancy of the sums of heats of the real world.
The operating principle of a heat engine. System efficiency
A heat engine is a device that converts heat into work. The first heat engine was invented at the end of the 18th century (steam engine). Now there are two
Free and bound energy. Entropy of the system
It is known that any form of energy can be completely converted into heat, but heat is converted into other types of energy only partially, conditionally the reserve of internal energy of the system
The influence of temperature on the direction of chemical reactions
DH DS DG Reaction direction DH< 0
DS >0 DG< 0
The concept of chemical kinetics
Chemical kinetics is the study of the rate of chemical reactions and its dependence on various factors - the nature and concentration of reactants, pressure,
Factors influencing the rate of chemical reactions. Law of mass action
The rate of chemical reactions is influenced by the following factors: the nature and concentration of the reacting substances; temperature, nature of the solvent, presence of a catalyst, etc.
Theory of activation of molecules. Arrhenius equation
The rate of any chemical reaction depends on the number of collisions of the reacting molecules, since the number of collisions is proportional to the concentrations of the reacting substances. However, not everything is table
Features of catalytic reactions. Theories of catalysis
The rate of a chemical reaction can be controlled using a catalyst. Substances that participate in reactions and change (most often increase) its speed, remaining at the end of the reaction
Reversible and irreversible reactions. Signs of chemical balance
All reactions can be divided into two groups: reversible and irreversible. Irreversible reactions are accompanied by precipitation, the formation of a poorly dissociating substance, or the release of gas. Reversible rea
Chemical equilibrium constant
Let us consider a reversible chemical reaction of a general type, in which all substances are in the same state of aggregation, for example, liquid: aA + bB D cC + dD, where
Gibbs phase rule. Water diagram
Qualitative characteristics of heterogeneous equilibrium systems in which no chemical interaction occurs, but only a transition of the constituent parts of the system from one state of aggregation is observed
The phase rule for water has the form
С = 1+ 2 – Ф = 3 – Ф if Ф = 1, then С = 2 (the system is bivariant) Ф = 2, then С = 1 (the system is single-variant) Ф = 3, then С = 0 (the system is non-variant) Ф = 4, then C = -1 (
The concept of chemical affinity of substances. Equations of isotherms, isobars and isochores of chemical reactions
The term “chemical affinity” refers to the ability of substances to enter into chemical interactions with each other. For different substances it depends on the nature of the reacting substances
Solvate (hydrate) theory of dissolution
Solutions are homogeneous systems consisting of two or more substances, the composition of which can vary within fairly wide limits, permissible solution.
General properties of solutions
At the end of the 19th century, Raoult, van't Hoff, and Arrhenius established very important laws that connect the concentration of a solution with the saturated vapor pressure of the solvent above the solution, the rate
Types of liquid solutions. Solubility
The ability to form liquid solutions is expressed to varying degrees in different individual substances. Some substances can dissolve unlimitedly (water and alcohol), others - only to a limited extent.
Properties of weak electrolytes
When dissolved in water or other solvents consisting of polar molecules, electrolytes undergo dissociation, i.e. to a greater or lesser extent break down into positive and negative
Properties of strong electrolytes
Electrolytes that dissociate almost completely in aqueous solutions are called strong electrolytes. Strong electrolytes include most salts that are already in the
If these conditions are met, colloidal particles acquire an electrical charge and a hydration shell, which prevents their precipitation
Dispersion methods for producing colloidal systems include: mechanical - crushing, grinding, grinding, etc.; electric – production of metal sols under the action
Stability of colloidal solutions. Coagulation. Peptization
The stability of a colloidal solution is understood as the constancy of the basic properties of this solution: preservation of particle sizes (aggregative stability
Properties of colloidal disperse systems
All properties of colloidal disperse systems can be divided into three main groups: molecular kinetic, optical and electrokinetic. Let us consider the molecular kinetic
Features of metabolic processes
Chemical reactions are divided into exchange and redox (Ox-Red). If the reaction does not change the oxidation state, then such reactions are called exchange reactions. They are possible
Features of redox processes
During redox reactions, the oxidation state of a substance changes. Reactions can be divided into those that take place in the same reaction volume (for example, in
General concepts of electrochemistry. Conductors of the first and second kind
Electrochemistry is a branch of chemistry that studies the patterns of mutual transformations of electrical and chemical energy. Electrochemical processes can be divided
The concept of electrode potential
Let us consider the processes occurring in galvanic cells, i.e., the processes of converting chemical energy into electrical energy. A galvanic element is called an electrochemical
Galvanic Daniel-Jacobi cell
Consider a system in which two electrodes are in solutions of their own ions, for example, a Daniel-Jacobi galvanic cell. It consists of two half-elements: from a zinc plate, immersed
Electromotive force of a galvanic cell
The maximum potential difference between the electrodes that can be obtained when operating a galvanic cell is called the electromotive force (EMF) of the cell.
Polarization and overvoltage
During spontaneous processes, an equilibrium potential of the electrodes is established. When an electric current passes, the potential of the electrodes changes. Electrode potential change
Electrolysis. Faraday's laws
Electrolysis is the name given to processes occurring on electrodes under the influence of electric current supplied from an external current source through electrolytes. When elect
Metal corrosion
Corrosion is the destruction of metal as a result of its physical and chemical interaction with the environment. This is a spontaneous process that occurs with a decrease in the Gibbs energy system
Methods for producing polymers
Polymers are high-molecular compounds characterized by a molecular weight from several thousand to many millions. Polymer molecules are called
Polymer structure
Polymer macromolecules can be linear, branched and network. Linear polymers are polymers that are built from long chains of one-dimensional elements, i.e.
Properties of polymers
The properties of polymers can be divided into chemical and physical. Both properties are associated with the structural features of polymers, the method of their preparation, and the nature of those introduced into
Application of polymers
Fibers, films, rubbers, varnishes, adhesives, plastics and composite materials (composites) are produced from polymers. Fibers are obtained by squeezing solutions or
Some reagents for identifying cations
Reagent Formula Cation Reaction product Alizarin C14H6O
1. Classification of instrumental methods of analysis according to the measuring parameter and method of measurement. Examples of instrumental analytical methods for qualitative analysis of substances
In one of the methods of classifying instrumental (physicochemical) methods, the analysis is based on the nature of the measured physical parameter of the analyzed system and the method of its measurement; the value of this parameter is a function of the amount of substance. In accordance with this, all instrumental methods are divided into five large groups:
Electrochemical;
Optical;
Chromatographic;
Radiometric;
Mass spectrometric.
Electrochemical methods analyzes are based on the use of the electrochemical properties of the analyzed substances. These include the following methods.
Electrogravimetric method is based on the accurate measurement of the mass of the substance being determined or its components, which are released on the electrodes when a direct electric current passes through the analyzed solution.
The conductometric method is based on measuring the electrical conductivity of solutions, which changes as a result of ongoing chemical reactions and depends on the properties of the electrolyte, its temperature and the concentration of the dissolved substance.
Potentiometric method - based on measuring the potential of an electrode immersed in a solution of the substance under study. The electrode potential depends on the concentration of the corresponding ions in the solution under constant measurement conditions, which are carried out using potentiometers.
Polarographic method is based on the use of the phenomenon of concentration polarization that occurs on an electrode with a small surface when passing an electric current through the analyzed electrolyte solution.
Coulometric method is based on measuring the amount of electricity spent on the electrolysis of a certain amount of a substance. The method is based on Faraday's law.
Optical methods analysis are based on the use of the optical properties of the compounds under study. These include the following methods.
Emission spectral analysis is based on the observation of line spectra emitted by vapors of substances when they are heated in the flame of a gas burner, spark or electric arc. The method makes it possible to determine the elemental composition of substances.
Absorption spectral analysis in the ultraviolet, visible and infrared regions of the spectrum. There are spectrophotometric and photocolorimetric methods. The spectrophotometric method of analysis is based on measuring the absorption of light (monochromatic radiation) of a certain wavelength, which corresponds to the maximum of the absorption curve of the substance. The photocolorimetric method of analysis is based on measuring light absorption or determining the absorption spectrum in devices - photocolorimeters in the visible part of the spectrum.
Refractometry is based on measuring the refractive index.
Polarimetry - based on measuring the rotation of the plane of polarization.
Nephelometry is based on the use of the phenomena of reflection or scattering of light by uncolored particles suspended in solution. The method makes it possible to determine very small quantities of a substance present in a solution in the form of a suspension.
Turbidimetry - based on the use of the phenomena of reflection or scattering of light by colored particles that are suspended in solution. The light absorbed by or transmitted through a solution is measured in the same way as in photocolorimetry of colored solutions.
Luminescent or fluorescent analysis - based on the fluorescence of substances that are irradiated with ultraviolet light. This measures the intensity of the emitted or visible light.
Flame photometry (flame photometry) is based on spraying a solution of the substances under study in a flame, isolating radiation characteristic of the element being analyzed and measuring its intensity. The method is used for the analysis of alkali, alkaline earth and some other elements.
Chromatographic methods analyzes are based on the use of selective adsorption phenomena. The method is used in the analysis of inorganic and organic substances for separation, concentration, isolation of individual components from a mixture, and purification from impurities.
Radiometric methods analyzes are based on measuring the radioactive radiation of a given element.
Mass spectrometry analysis methods are based on determining the masses of individual ionized atoms, molecules and radicals as a result of the combined action of electric and magnetic fields. Registration of separated particles is carried out by electrical (mass spectrometry) or photographic (mass spectrography) methods. The determination is carried out using instruments - mass spectrometers or mass spectrographs.
Examples of instrumental methods of analysis for the qualitative analysis of substances: X-ray fluorescence, chromatography, coulometry, emission photometry, flame photometry, etc.
2.
2. 1 The essence of potentiometric titration. Reaction requirements. Examples of oxidation-reduction, precipitation, complexation reactions and corresponding electrode systems. Graphical methods for determining titration end point
Potentiometric titration is based on determining the equivalent point by the change in potential on electrodes immersed in a titrated solution. In potentiometric titration, both non-polarizing (without current flowing through them) and polarizing (with current flowing through them) electrodes are used.
In the first case, during the titration process, the concentration in the solution of one of the ions is determined, for which there is a suitable electrode for recording.
The potential E x on this indicator electrode is set according to the Nernst equation. For example, for oxidation-reduction reactions, the Nernst equation is as follows:
where E x is the potential of the electrode under these specific conditions; A ok - concentration of the oxidized form of the metal; A reduced - concentration of the reduced form of the metal; E 0 - normal potential; R - universal gas constant (8.314 J/(deg*mol)); T - absolute temperature; n is the difference between the valences of the oxidized and reduced forms of metal ions.
To form an electrical circuit, a second so-called reference electrode, for example a calomel electrode, is placed in the titrated solution, the potential of which remains constant during the reaction. In addition to the mentioned oxidation-reduction reactions, potentiometric titration on non-polarizing electrodes is also used for neutralization reactions. Metals (Pt, Wo, Mo) are used as indicator electrodes for oxidation-reduction reactions. In neutralization reactions, a glass electrode is most often used, which has a characteristic over a wide range similar to a hydrogen electrode. For a hydrogen electrode, the dependence of the potential on the concentration of hydrogen ions is expressed by the following dependence:
Or at 25°C:
In potentiometric titration, titration is often used not to a certain potential, but to a certain pH value, for example, to a neutral pH=7. Somewhat apart from the generally accepted methods of potentiometric titration (without current flowing through the electrodes), discussed above, are methods of potentiometric titration at constant current with polarizable electrodes. Most often, two polarizing electrodes are used, but sometimes one polarizing electrode is used.
Unlike potentiometric titration with non-polarizing electrodes, in which virtually no current flows through the electrodes, in this case a small (about a few microamperes) direct current is passed through the electrodes (usually platinum), obtained from a stabilized current source. The current source can be a high-voltage power supply (about 45 V) with a relatively large resistance connected in series. The potential difference measured at the electrodes increases sharply as the reaction approaches the equivalent point due to the polarization of the electrodes. The magnitude of the potential jump can be much greater than during titration at zero current with non-polarizing electrodes.
Requirements for reactions during potentiometric titration are the completeness of the reaction; a sufficiently high reaction speed (so that you don’t have to wait for results, and there is the possibility of automation); obtaining in the reaction one clear product, and not a mixture of products that can be obtained at different concentrations.
Examples of reactions and corresponding electrode systems:
Oxidation-recoverye:
Electrode system:
In both cases, a system is used that consists of a platinum electrode and a silver chloride electrode.
ABOUTfathomse:
Ag + + Cl - =AgClv.
Electrode system:
TOcomplexatione:
Electrode system:
Graphical methods for determining the titration end point. The principle is to visually examine the complete titration curve. If we plot the dependence of the potential of the indicator electrode on the titrant volume, then the resulting curve has a maximum slope - i.e. maximum value DE/DV- which can be taken as an equivalence point. Rice. 2.1, showing just such a dependence, was built according to the data in Table. 2.1.
Table 2.1 Results of potentiometric titration of 3.737 mmol chloride with 0.2314 F solution of silver nitrate
Rice. 2.1 Titration curves for 3.737 mmol chloride with 0.2314 F silver nitrate solution: A- a conventional titration curve showing the region near the equivalence point; b- differential titration curve (all data from Table 2.1)
Gran's method. You can build a graph DE/DV- change in potential per titrant portion volume as a function of titrant volume. Such a graph obtained from the titration results given in table. 2.1, shown in Fig. 2.2.
Rice. 2.2 Gran curve, constructed from potentiometric titration data presented in table. 2.1
2.2 Task: V calculate the potential of a platinum electrode in a solution of iron (II) sulfate titrated with a solution of potassium permanganate by 50% and 100.1%; if the concentration of FeI ions ? , H? and MnO?? equal to 1 mol/dm3
The potential of a platinum electrode - an electrode of the third kind - is determined by the nature of the conjugated redox couple and the concentration of its oxidized and reduced forms. This solution contains a pair:
Fe 3+ + e - Fe 2+ ,
for which:
Since the original solution is titrated by 50%, then / = 50/50 and 1.
Therefore, E = 0.77 + 0.058 log1 = 0.77 V.
3. Amperometric titration
3.1 Amperometric titration, its essence, conditions. Types of titration curves depending on the nature of the titrated substance and titrant using examples of specific reactions th
Amperometric titration. For amperometric indication in titration, you can use a cell of the same basic design as for direct amperometry. In this case, the method is called amperometric titration with a single polarized electrode. During titration, the current caused by the analyte, titrant, or reaction product is controlled at a constant value of the working electrode potential, located in the potential region of the limiting diffusion current.
As an example, let us consider the precipitation titration of Pb 2+ ions with a solution of potassium chromate at different potentials of the working electrode.
The regions of limiting diffusion currents of redox pairs Pb 2+ /Pb and CrO 4 2- /Cr(OH) 3 are located in such a way that at a potential of 0 V the chromate ion is already reduced, but the Pb 2+ ion is not yet (this process occurs only at more negative potentials).
Depending on the potential of the working electrode, titration curves of various shapes can be obtained.
a) The potential is - 1V (Fig. 3.1):
Up to the equivalence point, the current flowing through the cell is the cathodic current for the reduction of Pb 2+ ions. When titrant is added, their concentration decreases and the current drops. After the equivalence point, the current is due to the reduction of Cr(VI) to Cr(III), as a result of which the cathodic current begins to increase as the titrant is added. At the equivalence point (φ = 1) a sharp break is observed in the titration curve (in practice it is less pronounced than in Fig. 3.1).
b) Potential is 0 V:
At this potential, Pb 2+ ions are not reduced. Therefore, only a small constant residual current is observed up to the equivalence point. After the equivalence point, free chromate ions appear in the system, capable of reduction. In this case, as the titrant is added, the cathodic current increases, as during titration at - 1V (Fig. 3.1).
Rice. 3.1 Curves of amperometric titration of Pb 2+ with chromate ions at working electrode potentials - 1 V and 0 V
Compared to direct amperometry, amperometric titration, like any titrimetric method, is characterized by higher accuracy. However, the amperometric titration method is more labor-intensive. The most widely used in practice are amperometric titration techniques with two polarized electrodes.
Biamperometric titration. This type of amperometric titration is based on the use of two polarizable electrodes, usually platinum, to which a small potential difference of 10-500 mV is applied. In this case, the passage of current is possible only when reversible electrochemical reactions occur at both electrodes. If at least one of the reactions is kinetically hindered, polarization of the electrode occurs and the current becomes insignificant.
Current-voltage dependences for a cell with two polarizable electrodes are shown in Fig. 3.2. In this case, only the potential difference between the two electrodes plays a role. The potential value of each electrode individually remains uncertain due to the absence of a reference electrode.
Figure 3.2 Current-voltage dependences for a cell with two identical polarizable electrodes in the case of a reversible reaction without overvoltage ( A) and irreversible reaction with overvoltage ( b).
Depending on the degree of reversibility of the electrode reactions, titration curves of various shapes can be obtained.
a) Titration of a component of a reversible redox couple with a component of an irreversible pair, for example, iodine thiosulfate (Fig. 3.3, A):
I 2 + 2S 2 O 3 2- 2I - + S 4 O 6 2- .
Up to the equivalence point, a current flows through the cell due to the process:
2I - I 2 + e - .
The current increases up to a titration degree of 0.5, at which both components of the I 2 /I - pair are in equal concentrations. Then the current begins to decrease until the equivalence point. After the equivalence point, due to the fact that the S 4 O 6 2- /S 2 O 3 2- pair is irreversible, polarization of the electrodes occurs and the current stops.
b) Titration of a component of an irreversible pair with a component of a reversible pair, for example, As(III) ions with bromine (Fig. 3.3, b):
Up to the equivalence point, the electrodes are polarized, since the As(V)/As(III) redox system is irreversible. No current flows through the cell. After the equivalence point, the current increases, since a reversible redox system Br 2 / Br - appears in the solution.
c) The substance being determined and the titrant form reversible redox pairs: titration of Fe(II) ions with Ce(IV) ions (Fig. 3.3, V):
Here, polarization of the electrodes is not observed at any stage of the titration. Up to the equivalence point, the course of the curve is the same as in Fig. 3.3, A, after the equivalence point - as in Fig. 3.3, b.
Rice. 3.3 Biamperometric titration curves of iodine with thiosulfate ( a), As(III) bromine ( b) and Fe(II) ions with Ce(IV) ions ( V)
3.2 Task: V an electrochemical cell with a platinum microelectrode and a reference electrode was placed with a 10.00 cm3 NaCl solution and titrated with a 0.0500 mol/dm3 AgNO solution 3 volume 2.30 cm. Calculate NaCl contentin solution (%)
The reaction occurs in the solution:
Ag + + Cl - =AgClv.
V(AgNO 3) = 0.0023 (dm 3);
n(AgNO 3) = n(NaCl);
n(AgNO 3)=c(AgNO 3)*V(AgNO 3)=0.0500*0.0023=0.000115,
or 1.15*10 4 (mol).
n(NaCl) = 1.15*10 -4 (mol);
m(NaCl) = M(NaCl)* n(NaCl) = 58.5*1.15*10 -4 = 6.73*10 -3 g.
Let’s take the density of the NaCl solution as 1 g/cm3, then the mass of the solution will be 10 g, hence:
n(NaCl) = 6.73*10 -3 /10*100% = 0.0673%.
Answer: 0,0673 %.
4. Chromatographic methods of analysis
4.1 Phases in chromatographic methods of analysis, their characteristics. Liquid Chromatography Basics
The liquid partition chromatography method was proposed by Martin and Synge, who showed that the height equivalent to a theoretical plate of a suitably packed column can reach 0.002 cm. Thus, a column 10 cm long can contain about 5000 plates; High separation efficiency can be expected even from relatively short columns.
Stationary phase. The most common solid carrier in partition chromatography is silicic acid or silica gel. This material absorbs water strongly; thus the stationary phase is water. For some separations, it is useful to include some kind of buffer or strong acid (or base) in the film of water. Polar solvents, such as aliphatic alcohols, glycols or nitromethane, have also been used as stationary phases on silica gel. Other carriers include diatomaceous earth, starch, cellulose, and crushed glass; Water and various organic liquids are used to wet these solid carriers.
Mobile phase. The mobile phase can be a pure solvent or a mixture of solvents that are not appreciably miscible with the stationary phase. Separation efficiency can sometimes be increased by continuously changing the composition of the mixed solvent as the eluent progresses (gradient elution). In some cases, separation is improved if the elution is carried out with a number of different solvents. The mobile phase is selected mainly empirically.
Modern instruments are often equipped with a pump to accelerate the flow of liquid through the column.
The main LC parameters that characterize the behavior of a substance in a column are the retention time of the mixture component and the retention volume. The time from the moment of introduction of the analyzed sample to the registration of the peak maximum is called retention time (elution) t R. The retention time consists of two components - the residence time of the substance in the mobile t 0 and motionless t s phases:
t R.= t 0 +t s. (4.1)
Meaning t 0 is actually equal to the time of passage of the adsorbed component through the column. Meaning t R does not depend on the amount of sample, but depends on the nature of the substance and the sorbent, as well as the packaging of the sorbent and can vary from column to column. Therefore, to characterize the true holding capacity, one should introduce corrected retention time t? R:
t? R= t R -t 0 . (4.2)
To characterize retention, the concept is often used retained volume V R - the volume of the mobile phase that must be passed through the column at a certain speed in order to elute the substance:
V R= t RF, (4.3)
Where F- volumetric flow rate of the mobile phase, cm 3 s -1.
The volume for washing out the non-sorbable component (dead volume) is expressed through t 0 :V 0 = t 0 F, and includes the volume of the column not occupied by the sorbent, the volume of communications from the sample injection device to the column and from the column to the detector.
Corrected retention volume V? R respectively equal to:
V? R= V R -V 0 . . (4.4)
Under constant chromatography conditions (flow rate, pressure, temperature, phase composition), the values t R And V R are strictly reproducible and can be used to identify substances.
Any process of distribution of a substance between two phases is characterized by distribution coefficient D. Magnitude D attitude c s/c 0 , Where With T And With 0 - concentrations of the substance in the mobile and stationary phases, respectively. The distribution coefficient is related to the chromatographic parameters.
The retention characteristic is also the capacitance coefficient k", defined as the ratio of the mass of a substance in the stationary phase to the mass of a substance in the mobile phase: k" = m n/m P. The capacity coefficient shows how many times longer a substance remains in the stationary phase than in the mobile phase. Size k" calculated from experimental data using the formula:
The most important parameter of chromatographic separation is the efficiency of the chromatographic column, the quantitative measure of which is the height N, equivalent to a theoretical plate, and the number of theoretical plates N.
A theoretical plate is a hypothetical zone whose height corresponds to the achievement of equilibrium between two phases. The more theoretical plates in a column, i.e. The more times equilibrium is established, the more efficient the column. The number of theoretical plates can be easily calculated directly from the chromatogram by comparing the peak width w and stay time t R component in column:
Having determined N and knowing the length of the column L, easy to calculate N:
The efficiency of a chromatography column is also characterized by the symmetry of the corresponding peak: the more symmetrical the peak, the more efficient the column is. Numerically, symmetry is expressed through the symmetry coefficient K S, which can be determined by the formula:
Where b 0.05 - peak width at one twentieth of the peak height; A- the distance between the perpendicular dropped from the maximum of the peak and the front edge of the peak at one twentieth of the height of the peak.
To assess the reproducibility of chromatographic analysis, the relative standard deviation ( RSD), characterizing the dispersion of results in the sample population:
Where n- number of parallel chromatograms; X- the content of the component in the sample, determined by calculating the area or height of the corresponding peak in the chromatogram; - average value of the component content, calculated based on data from parallel chromatograms; s 2 - dispersion of the obtained results.
The results of a chromatographic analysis are considered probable if the conditions for the suitability of the chromatographic system are met:
The number of theoretical plates calculated from the corresponding peak must be no less than the required value;
The separation factor of the corresponding peaks must be no less than the required value;
The relative standard deviation calculated for the height or area of the corresponding peak should be no more than the required value;
The symmetry coefficient of the corresponding peak must be within the required limits.
4.2 Forcountry house: R Calculate the content of the analyte in the sample using the internal standard method (in g and %) if the following data are obtained during chromatography: during calibration: qB=0.00735,SВ =6.38 cmІ,qST=0.00869 g,SST=8.47 cm², -when analyzing:SВ=9.38 cmІ,VВ=47 mmі,qST=0.00465 g,SST=4.51 cm²
SCT/SV = k*(qCT/qB);
k = (SCT/SV)/(qCT/qB) = (8.47/6.38)/(0.00869/0.00735) = 1.123;
qB = k*qST*(SV/SST) = 1.123*0.00465*(9.38/4.51) = 0.01086 g.
x, % = k*r*(SV/SCT)*100;
r = qCT/ qB = 0.00465/0.01086 = 0.4282;
x, % = 1.123*0.4282*(9.38/4.51) = 100%.
5. Photometric titration
5.1 Photometric titration. The essence and conditions of titration. Titration curves. Advantages of photometric titration in comparison with direct photometry
Photometric and spectrophotometric measurements can be used to record the titration end point. The end point of a direct photometric titration occurs as a result of changes in the concentration of the reactant and the reaction product, or both simultaneously; Obviously, at least one of these substances must absorb light at the selected wavelength. The indirect method is based on the dependence of the optical density of the indicator on the volume of the titrant.
Rice. 5.1 Typical photometric titration curves. The molar absorption coefficients of the analyte, reaction product and titrant are indicated by the symbols e s, e p, e t, respectively
Titration curves. A photometric titration curve is a graph of corrected absorbance versus titrant volume. If the conditions are chosen correctly, the curve consists of two straight sections with different slopes: one of them corresponds to the beginning of the titration, the other to the continuation beyond the equivalence point. There is often a noticeable inflection point near the equivalence point; The end point is considered to be the point of intersection of straight line segments after extrapolation.
In Fig. Figure 5.1 shows some typical titration curves. When titrating non-absorbing substances with a colored titrant to form colorless products, a horizontal line is obtained at the beginning of the titration; beyond the equivalence point, the optical density increases rapidly (Fig. 5.1, curve A). When colored products are formed from colorless reagents, on the contrary, a linear increase in optical density is first observed, and then a region appears in which absorption does not depend on the titrant volume (Fig. 5.1, curve b). Depending on the spectral characteristics of the reagents and reaction products, curves of other shapes are also possible (Fig. 5.1).
For the end point of a photometric titration to be sufficiently distinct, the absorbing system or systems must obey Beer's law; otherwise, the linearity of the titration curve segments required for extrapolation is disrupted. It is further necessary to introduce a correction for volume changes by multiplying the optical density by the factor (V+v)/V, Where V- initial volume of solution, a v- volume of added titrant.
Photometric titration often provides more accurate results than direct photometric analysis because data from multiple measurements are combined to determine the end point. In addition, in photometric titration the presence of other absorbing substances can be neglected since only the change in absorbance is measured.
5.2 Task:n a weighed portion of potassium dichromate weighing 0.0284 g was dissolved in a volumetric flask with a capacity of 100.00 cm3. Optical density of the resulting solution at l max=430 nm is equal to 0.728 with an absorbed layer thickness of 1 cm. Calculate the molar and percentage concentration, molar and specific absorption coefficients of this solution
where is the optical density of the solution; e - molar absorption coefficient of the substance, dm 3 *mol -1 *cm -1; With - concentration of the absorbing substance, mol/dm 3 ; l is the thickness of the absorbing layer, cm.
Where k- specific absorption coefficient of the substance, dm 3 * g -1 * cm -1.
n(K 2 Cr 2 O 7) = m(K 2 Cr 2 O 7)/ M(K 2 Cr 2 O 7) = 0.0284/294 = 9.67*10 -5 (mol);
c(K 2 Cr 2 O 7) = 9.67*10 -5 /0.1 = 9.67*10 -4 (mol/l);
Let’s take the density of the solution K 2 Cr 2 O 7 as 1 g/cm 3 , then the mass of the solution will be 100 g, hence:
n(NaCl) = 0.0284/100*100% = 0.0284%.
e = D/cl =0.728/9.67*10 -4 *1 = 753 (dm 3 *mol -1 *cm -1).
k = D/cl =0.728/0.284 *1 = 2.56(dm 3 *g -1 *cm -1).
6. Describe and explain the possibility of using instrumental methods of analysis (optical, electrochemical, chromatographic) for the qualitative and quantitative determination of zinc chloride
ZnCl 2 chloride; M=136.29; bts. trig., blur; с=2.91 25 ; tmel=318; tboil=732; С°р=71.33; S°=111.5; DN°=-415.05; ДG°=-369.4; DNpl=10.25; DNsp=119.2; y=53.83 20; 53.6 400; 52.2 700; р=1 428; 10 506 ; s=208 0 ; 272 10; 367 20; 408 25 ; 438 30; 453 40 ; 471 50 ; 495 60 ; 549 80; 614 100; h.r.eff.; r.et. 100 12.5, ac. 43.5 18; feast. 2.6 20; n.r.z. NH3.
Zinc chloride ZnCl 2, the most studied of the halides, is obtained by dissolving zinc blende, zinc oxide or zinc metal in hydrochloric acid. Anhydrous zinc chloride is a white granular powder consisting of hexagonal-rhombohedral crystals, melts easily and, upon rapid cooling, solidifies into a transparent porcelain-like mass. Molten zinc chloride conducts electricity quite well. When heated, zinc chloride evaporates and its vapor condenses in the form of white needles. It is very hygroscopic, but at the same time it is easy to obtain anhydrous. Zinc chloride crystallizes without water at temperatures above 28°C, and from concentrated solutions it can be isolated anhydrous even at 10°C. Zinc chloride dissolves in water, releasing a large amount of heat (15.6 kcal/mol). In dilute solutions, zinc chloride dissociates well into ions. The covalent nature of the bond in zinc chloride is manifested in its good solubility in methyl and ethyl alcohols, acetone, diethyl ether, glycerin, ethyl acetate and other oxygen-containing solvents, as well as dimethylformamide, pyridine, aniline and other nitrogen-containing compounds of a basic nature.
Zinc chloride tends to form complex salts corresponding to the general formulas from Me to Me 4 , but the most common and stable are salts in which four chlorine anions are coordinated around the zinc atom, and the composition of most salts corresponds to the formula Me 2 . As the study of Raman spectra has shown, in solutions of zinc chloride itself, depending on its concentration, ions 2+, ZnCl + (ad), 2- may be present, and ions - or 2- are not detected. Mixed complexes with anions of several acids are also known. Thus, potentiometric methods have proven the formation of zinc sulfate-chloride complexes in solutions. Mixed complexes were discovered: 3-, 4, 5-.
ZnCl 2 can be determined quantitatively and qualitatively by Zn 2+. It can be determined quantitatively and qualitatively using the photometric method from the absorption spectrum. For example, with reagents such as dithizone, murexide, arsazene, etc.
Spectral determination of zinc. Spectral analysis methods are very convenient for detecting zinc. The analysis is carried out on a group of three lines: 3345, 02 I; 3345.57 I 3345.93 I A, of which the first is the most intense, or a pair of lines: 3302.59 I and 3302.94 I A.
Depending on the task at hand, there are 3 groups of analytical chemistry methods:
- 1) detection methods allow you to determine which elements or substances (analytes) are present in the sample. They are used to conduct qualitative analysis;
- 2) determination methods make it possible to establish the quantitative content of analytes in a sample and are used to carry out quantitative analysis;
- 3) separation methods allow you to isolate the analyte and separate interfering components. They are used in qualitative and quantitative analysis. There are various methods of quantitative analysis: chemical, physicochemical, physical, etc.
Chemical methods are based on the use of chemical reactions (neutralization, oxidation-reduction, complexation and precipitation) into which the analyte enters. A qualitative analytical signal in this case is the visual external effect of the reaction - a change in the color of the solution, the formation or dissolution of a precipitate, the release of a gaseous product. In quantitative determinations, the volume of the released gaseous product, the mass of the formed precipitate, and the volume of a reagent solution with a precisely known concentration spent on interaction with the substance being determined are used as an analytical signal.
Physical methods do not use chemical reactions, but measure any physical properties (optical, electrical, magnetic, thermal, etc.) of the analyzed substance, which are a function of its composition.
Physicochemical methods use changes in the physical properties of the analyzed system as a result of chemical reactions. Physicochemical methods also include chromatographic methods of analysis, based on the processes of sorption-desorption of a substance on a solid or liquid sorbent under dynamic conditions, and electrochemical methods (potentiometry, voltammetry, conductometry).
Physical and physicochemical methods are often combined under the general name instrumental methods of analysis, since analytical instruments and devices that record physical properties or their changes are used to carry out the analysis. When conducting a quantitative analysis, the analytical signal is measured - a physical quantity associated with the quantitative composition of the sample. If quantitative analysis is carried out using chemical methods, then the basis of the determination is always a chemical reaction.
There are 3 groups of quantitative analysis methods:
- - Gas analysis
- - Titrimetric analysis
- - Gravimetric analysis
The most important among chemical methods of quantitative analysis are gravimetric and titrimetric methods, which are called classical methods of analysis. These methods are standard for assessing the accuracy of a determination. Their main area of application is the precision determination of large and medium quantities of substances.
Classical methods of analysis are widely used at chemical industry enterprises to monitor the progress of the technological process, the quality of raw materials and finished products, and industrial waste. On the basis of these methods, pharmaceutical analysis is carried out - determining the quality of drugs and medicines that are produced by chemical and pharmaceutical enterprises.