All methods of quantitative analysis, depending on the nature of the experimental technique used for the final determination of the components of the analyzed substance or mixture of substances, are divided into three groups: chemical, physical and physicochemical (instrumental) methods of analysis.
Chemical methods of analysis include:
1. Weight analysis - measurement of the mass of the substance being determined or its components, isolated in a chemically pure state or in the form of corresponding compounds.
2. Volumetric analysis - measurement of the volume of liquid, solid and gaseous products or their aqueous and non-aqueous solutions.
Various volumetric methods are known:
1) volumetric titrimetric - measurement of the volume of a reagent of precisely known concentration spent on a reaction;
2) gas volumetric - analysis of gas mixtures, based on selective absorption of the determined component from the analyzed gas mixture by suitable absorbers;
3) volumetric sedimentation, based on the stratification of dispersed systems under the influence of gravity, accompanied by the separation of the dispersed phase in the form of sediment and subsequent measurement of the volume of the sediment in a calibrated centrifuge tube. For example, in micro- and ultramicroanalysis, the sulfur content is found by oxidizing it to sulfate and subsequent precipitation in the form of a precipitate of barium sulfate, determined by this method.
In a broader sense, sedimentation analysis is a method for determining in disperse systems the size and relative content of particles of various sizes based on the rate of sedimentation (settling or floating).
The sedimentation rate of spherical particles under known conditions is described by the Stokes equation:
where v is the sedimentation rate;
Particle radius;
Particle material density;
Density of dispersed medium;
Viscosity of the medium;
Acceleration of gravity.
Very often in laboratory practice, gravimetric methods of sedimentation analysis are used, based on hydrostatic weighing of sediment during its accumulation using sedimentation glass balances by N. A. Figurovsky.
In some cases, the division of analysis methods into chemical and physicochemical is arbitrary, since it is sometimes difficult or practically impossible to resolve the issue of whether a particular analysis method belongs to any of these groups.
The listed methods are only methods for the final determination of the analyte or its components and do not reflect all the features of chemical analysis.
An essential part of chemical analysis, on which the analytical chemist sometimes has to spend more time and labor than on the final determination of the analyte, are methods of decomposition of the analyte, as well as methods of separation, isolation and concentration of the elements (or ions) being determined.
Analytical chemistry methods can be classified based on different principles. Depending on the measured property of the substance, the following methods are distinguished: chemical; physico-chemical; physical (Table 14). The basis of chemical methods are analytical chemical reactions. Physicochemical methods are based on the measurement of any physical parameters of a chemical system, which depend on the nature of the components of the system and change during the chemical reaction. Such parameters include, for example, potential values in potentiometry, optical densities in spectrophotometry, etc. Physical methods do not involve the use of chemical reactions. The composition of a substance is determined by changing any physical properties of the object (density, viscosity, radiation intensity, etc.). There are no clear boundaries between chemical and physicochemical and physicochemical and physical methods. Physical and physicochemical methods are often called instrumental. Recently, so-called “hybrid” methods have been used, combining two or more methods. For example, gas chromatography-mass spectrometry.
Quantitative analysis methods
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Analysis methods |
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Chemical |
Physico-chemical |
Physical |
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gravimetry titrimetry |
electrochemical spectroscopic (optical) luminescent kinetic thermometric chromatographic |
spectroscopic (not optical) nuclear physics radiochemical |
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Analytical signal (a value functionally related to the content of the component being determined) |
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change in indicator color, release of gas, sediment, etc. |
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Analysis of a substance involves obtaining experimentally data on its chemical composition. Regardless of the methods used, the following requirements apply to the analysis:
- 1. Analytical accuracy is a collective characteristic of a method, including its accuracy and reproducibility.
- 2. Correctness of the analysis results - obtaining results close to the actual ones.
- 3. Reproducibility - obtaining the same or similar results with repeated determinations.
- 4. Express - speed of analysis.
- 5. Sensitivity - the minimum amount of a substance that can be determined by this method.
- 6. Versatility - the ability to define many components. It is especially important to determine them simultaneously in one sample.
- 7. Automation of analysis. When conducting mass homogeneous analyses, you should choose a method that allows automation, which reduces labor intensity and errors, increases speed, and reduces the cost of analysis.
- 21. Characteristics of the analysis method
Quantitative analysis, a set of chemical, physicochemical and physical methods for determining the quantitative ratio of the components that make up the substance being analyzed. Along with the qualitative analysis of K. a. is one of the main branches of analytical chemistry. Based on the amount of substance taken for analysis, macro-, semi-micro, micro- and ultra-micro methods of analysis are distinguished. In macromethods, the sample weight is usually >100 mg, the solution volume is >10 ml; in ultramicromethods - 1-10-1 mg and 10-3-10-6 ml, respectively (see also Microchemical analysis, Ultramicrochemical analysis). Depending on the object of study, a distinction is made between inorganic and organic analytes, which in turn are divided into elemental, functional, and molecular analysis. Elemental analysis allows you to determine the content of elements (ions), functional analysis - the content of functional (reactive) atoms and groups in the analyzed object. Molecular K. a. involves the analysis of individual chemical compounds characterized by a certain molecular weight. The so-called phase analysis is a set of methods for separating and analyzing individual structural (phase) components of heterogeneous systems. In addition to specificity and sensitivity (see Qualitative analysis), an important characteristic of K. a. methods. - accuracy, that is, the value of the relative error of determination; accuracy and sensitivity in CA. expressed as a percentage.
To the classical chemical methods of CA. include: gravimetric analysis, based on precise measurement of the mass of the substance being determined, and volumetric analysis. The latter includes titrimetric volumetric analysis - methods for measuring the volume of a reagent solution consumed in a reaction with the analyte, and gas volumetric analysis - methods for measuring the volume of analyzed gaseous products (see Titrimetric analysis, Gas analysis).
Along with classical chemical methods, physical and physicochemical (instrumental) methods of CA are widely used, based on the measurement of 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. Accuracy of chemical methods of CA. is usually in the range of 0.005-0.1%; errors in determination by instrumental methods are 5-10%, and sometimes significantly more. Sensitivity of some methods K. a. is given below (%):
Volume................................................. ......10-1
Gravimetric........................................ 10-2
Emission spectral........................10-4
Absorption X-ray spectral...... 10-4
Mass spectrometric...................................10-4
Coulometric............................................. 10-5
Laboratory work No. 9
Chemical identification and analysis of the substance
Analytical chemistry is a scientific discipline that develops and applies methods, general approaches and instruments to obtain information about the composition and nature of matter in space and time. Chemical composition is understood as elemental (the most important and common type of analysis), molecular, phase, and isotopic composition. When determining the chemical composition of organic compounds, functional analysis is often used to determine the presence of specific functional groups in the molecule of the analyzed compound.
There are methods of qualitative and quantitative analysis. The purpose of qualitative analysis is the detection of elements, ions, molecules, functional groups, free radicals, phases contained in the sample under study based on a comparison of their experimentally obtained characteristics with available reference data, in other words, chemical identification. When analyzing organic compounds, individual elements (for example, carbon, oxygen, nitrogen) or functional groups are found directly. When analyzing inorganic compounds, it is determined which ions, molecules, groups of atoms, and chemical elements make up the substance being analyzed. The task of quantitative analysis is to determine the quantitative content and ratio of components in the analyzed substance or mixture.
Chemical identification (detection) is the establishment of the type and state of phases, molecules, atoms, ions and other constituent parts of a substance based on a comparison of experimental and corresponding reference data for known substances. Identification is the goal of qualitative analysis. During identification, a set of properties of substances is usually determined, for example: color, phase state, density, viscosity, melting, boiling and phase transition points, solubility, electrode potential, ionization energy.
Qualitative analysis is characterized by the detection limit (opening minimum) of dry matter, i.e. the minimum amount of a reliably identifiable substance, and the maximum concentration of the substance C min ,. These two quantities are related to each other by the relationship:
Methods of qualitative analysis
Dry methods of analysis. Volatile metal compounds color the burner flame in one color or another. Therefore, if you introduce the substance under study on a platinum wire into a colorless flame of a burner, then the flame becomes colored in the presence of certain elements in the molecule of the substance
Wet methods of analysis. Qualitative analysis methods are based on ionic reactions, which make it possible to identify elements in the form of certain ions. During the reaction, sparingly soluble compounds, colored complex compounds are formed, oxidation or reduction occurs with a change in the color of the solution. Any cation can be identified by a specific reaction if other cations that interfere with this identification are removed.
For identification through the formation of sparingly soluble compounds, both group and individual precipitants are used.
Anions are usually classified according to their salt solubility or redox properties.
Quantitative analysis methods
Determination methods are often divided into chemical, physical-chemical, sometimes a group is identified physical methods of analysis. Chemical methods are based on chemical reactions. For analysis, only those reactions are used that are accompanied by external effects, for example, a change in the color of the solution, the release of gas, the precipitation or dissolution of a precipitate, etc. These externalities are, in this case, analytical signals. The chemical changes that occur are called analytical reactions, and the substances that cause these reactions are chemical reagents. In the case of physicochemical methods, the chemical changes that occur, entailing changes in parameters such as the color intensity of the solution in spectrophotometry, the magnitude of the diffusion current in voltmetry, etc., are recorded using physical instruments. When analyzing by physical methods, chemical reactions are not used, but the physical properties of a substance are studied using instruments. Physical methods include chromatography, X-ray diffraction, luminescent, radioactivation methods of analysis, etc.
The titrimetric method is based on the fact that all substances react with each other in strictly equivalent quantities. The analytical signal in titrimetry is volume. An equivalent is some real or fictitious particle that can add, release, or be otherwise equivalent to one hydrogen ion in acid-base reactions or one electron in redox reactions.
A conditional particle can be an atom, a molecule, an ion, or part of a molecule. For example, in the reaction
Na 2 CO 3 + HCl = NaHCO 3 + NaCl
the conventional particle is the Na 2 CO 3 molecule, and in the reaction
Na 2 CO 3 + 2HCl = Na 2 CO 3 + 2NaCl
the conventional particle is ½ Na 2 CO 3 .
In reaction
KMnO 4 + 5 e + 8H + → Mn 2+ + 4 H 2 O + K +
conventional unit – 1/5 KMnO 4.
A number indicating what fraction of a molecule is equivalent to one hydrogen ion or electron in a given reaction is called equivalence factor (f) . For example, f Na 2 CO 3 = 1 for the first reaction, f Na 2 CO 3 = 1/2 for the second reaction and f KMnO 4 = 1/5 for the third reaction.
In practice, it is inconvenient to use molecules, ions, and equivalents, since they are very small (~ 10 -24 g). Used mole, which contains 6.02·1023 conventional particles. The mass of one mole is called molar mass, and the mass of one mole equivalent is called molar mass of the equivalent of E. The molar mass of the equivalent of substance X is the mass of one mole of the equivalent of this substance, equal to the product of the equivalence factor by the molar mass of substance X:
E = molecular weight ∙f (9)
Molar mass has the dimension g/mol. For example, they say. mass of Na 2 CO 3 = 106 (g/mol), molecular weight of ½ Na 2 CO 3 = 53 (g/mol) or, in other words, E Na 2 CO 3 (f=1) = 106, E Na 2 CO 3 (f=1/2) =53.
Solutions are used in titrimetry. The concentration of a solution is expressed by the amount of substance per unit volume. A liter (1 dm3) is taken as a unit of volume in titrimetry. A solution containing 1 mole of conventional particles per liter is called molar. For example, C HCl = 1 M (one-molar solution of HCl), C HCl = 0.1 M (decimolar solution of HCl), C ½ Na 2 CO 3 = 0.1 M (decimolar solution of ½ Na 2 CO 3). A solution containing 1 mole equivalents per liter is called normal; in this case, it is necessary to indicate the equivalence factor. For example, 0.1 n Na 2 CO 3 (f = 1) or 0.1 n Na 2 CO 3 (f = 1/2), decimolar solution of Na 2 CO 3. If f = 1, then the molar and normal concentrations are the same .
If two substances reacted in equivalent quantities, then the amount of substance 1 (n 1) is equal to the amount of substance 2 (n 2). Since n 1 = M 1 V 1 and n 2 = M 2 V 2, then
M 1 V 1 = M 2 V 2.
Knowing the concentration of one of the substances and the volumes of solutions, it is possible to find the unknown concentration and, therefore, the mass of another substance:
M 2 = (10) or N 2 = (11) and
m = M 2 molecular weight (12) or m = N 2 E (13).
In addition to the molar and normal concentrations, a titer of the solution is also used. The titer shows the number of grams of solute in 1 ml of solution. Titre for the analyte shows the mass of the analyte with which 1 ml of this solution reacts; for example, T HCl /Ca CO 3 = 0.006 g/cm 3, this means that 1 ml of HCl solution reacts with 0.006 g of CaCO 3.
titrated, or standard, solution – a solution whose concentration is known with high accuracy. Titration – adding a titrated solution to the test solution to determine an exactly equivalent amount. The titrating solution is often called working solution or titrant. The moment of titration when the amount of added titrant is chemically equivalent to the amount of the titrated substance is called equivalence point(t,e.) . Methods of detection i.e. varied: visual (with the help of an indicator and without an indicator), physical and chemical.
Reactions used in titrimetry must meet the following requirements:
- the reaction must proceed quantitatively, i.e. the equilibrium constant must be large enough;
- the reaction must proceed at high speed;
- the reaction should not be complicated by adverse reactions;
- there must be a way to fix i.e.
According to the method of fixing the equivalence point, titration methods with color indicators, potentiometric titration methods, conductometric, photometric, etc. are distinguished. When classifying according to the type of main reaction occurring during titration, the following methods of titrimetric analysis are usually distinguished:
- Acid-base interaction methods involve the process of proton transfer:
H + + OH - = H 2 O
CH 3 COOH + OH - = CH 3 COO - + H 2 O
- Complexation methods use reactions of the formation of coordination compounds:
Hg 2+ + 2Cl - = HgCl 2 (mercurimetry)
Mg 2+ + H 2 Y 2- = MgY 2- + 2H + (complexonomerism)
- Precipitation methods are based on the formation reactions of poorly soluble compounds:
Ag + + Cl - = AgCl (argentometry)
Hg + 2Cl - = Hg 2 Cl 2 (mercurometry)
- Oxidation-reduction methods combine a large group of redox reactions:
MnO + 5 Fe 2+ + 8H + = Mn 2+ + 5Fe 3+ + 4 H 2 O (permanganatometry)
2S 2 O + I 2 = S 4 O + 2I - (iodometry)
To find the equivalence point, a differential curve is often constructed in the coordinates ΔрН/ΔV – V, i.e. determine the rate of pH change when changing the amount of added solution at different titration points. The equivalence point is indicated by the maximum of the resulting curve, and the reading along the abscissa axis corresponding to this maximum gives the volume of titrant spent on titration to the equivalence point. Determining the equivalence point from a differential curve is much more accurate than from a simple pH – V relationship.
Example. To titrate 20 cm 3 of 0.02 M HCl solution, 15.00 cm 3 of NaOH solution is consumed. Determine the molar concentration of this solution.
Solution. Since substances react with each other in strictly equivalent quantities, the amount of HCl at the equivalence point must be equal to the amount of NaOH, i.e.
n(HCl) = n(NaOH); n(HCl) = C(HCl) V(HCl) ; n(NaOH)= C(NaOH) V(NaOH);
C(NaOH)= 
;
C(NaOH) = 
= 0.02667 mol/dm3.
Goal of the work: study “dry” and “wet” methods of chemical identification, become familiar with the basic principles of the titrimetric method of analysis and methods for determining the concentration of acids and alkalis.
Equipment and materials:
1. gas burner,
2. platinum wire,
3. test tubes,
4. rack for test tubes,
5. tripod
6. burette,
7. titration flask
8. set of reagents: dry salts - KCl, LiCl, NaCl, CaCl 2, BaCl 2, SrCl 2, CuCl 2, 0.5 N solutions of Na 3 PO 4, AgNO 3, FeSO 4, K 3, K 4, KOH, FeCl 3, KSCN, KI, NaCl, NaBr, HNO 3.
Analytical chemistry deals with the study of experimental methods for determining the composition of substances. Determining the composition of substances includes identifying the nature of the components that make up the substance under study and establishing the quantitative relationships of these components.
First, the qualitative composition of the object under study is established, i.e. solve the question of what it consists of, and then proceed to determine the quantitative composition, i.e. find out in what quantitative ratios the detected components are found in the object of study.
Qualitative analysis substances can be carried out using chemical, physical, physicochemical methods.
Chemical methods of analysis are based on the use of characteristic chemical reactions to determine the composition of the analyte.
Chemical analysis of a substance is carried out in two ways: “dry way” or “wet way”. Dry analysis- these are chemical reactions that occur with substances during incandescence, fusion and coloring of the flame.
Wet analysis- These are chemical reactions that occur in electrolyte solutions. The analyzed substance is pre-dissolved in water or other solvents. Depending on the mass or volume of the substance taken for analysis, on the technique used, macro-, semi-micro- and micromethods are distinguished.
Macro method. To carry out the analysis, take 1-2 ml of a solution containing at least 0.1 g of the substance and add at least 1 ml of the reagent solution. The reactions are carried out in a test tube, the precipitate is separated by filtration. The filter cake is washed to remove impurities.
Semi-micromethod. For analysis, 10-20 times less substance is taken (up to 0.01 g). Since this method works with small quantities of a substance, microtubes, watch glasses or slides are used. Centrifugation is used to separate the precipitate from the solution.
Micromethod. When performing an analysis using this method, take one or two drops of the solution, and dry matter - within 0.001 g. Typical reactions are carried out on a watch glass or porcelain plate.
When carrying out the analysis, the following operations are used: heating and evaporation, sedimentation, centrifugation, checking the completeness of sedimentation, separation of the solution (centrifuge) from the sediment, washing and dissolving the sediment.
Heating solutions can be carried out directly with the flame of a gas burner, on an asbestos grid or in a water bath. A small amount of the solution is heated to a temperature not exceeding 100°C in a water bath, in which the water should boil evenly.
For concentration solutions use a water bath. Evaporation the solution to a dry residue is carried out in porcelain cups or crucibles, heating them on an asbestos mesh. If the dry residue after evaporation needs to be calcined to remove volatile salts, then the crucible is placed on a porcelain triangle and heated with the flame of a gas burner.
Precipitation. The precipitation reaction is carried out in conical flasks or cylindrical test tubes. A precipitating reagent is pipetted into the test solution. The precipitant is taken in excess. The mixture is thoroughly mixed with a glass rod and rubbed against the inner walls of the test tube, this accelerates the process of sediment formation. Precipitation is often carried out from hot solutions.
Centrifugation. The precipitate is separated from the solution by centrifugation using a manual or electric centrifuge. The test tube with the solution and sediment is placed in a sleeve. The centrifuge must be loaded evenly. With rapid rotation, the centrifugal force throws sediment particles to the bottom and compacts it, and the solution (centrifuge) becomes transparent. The rotation time ranges from 30 s to several minutes.
Checking the completeness of deposition. The test tube is carefully removed from the centrifuge and 1-2 drops of the precipitating reagent are added along the wall to the clear solution. If the solution does not become cloudy, the precipitation is complete. If cloudiness of the solution is observed, then a precipitant is added to the test tube, the contents are mixed, heated and centrifuged again, then the completeness of precipitation is checked again.
Separation of the solution (centrifugate) from the sediment. After making sure that precipitation is complete, separate the solution from the precipitate. The solution is separated from the precipitate using a drop pipette. The pipette is closed with the index finger and carefully removed from the test tube. If the selected solution is needed for analysis, then it is transferred to a clean test tube. For complete separation, the operation is repeated several times. During centrifugation, the precipitate may settle tightly to the bottom of the test tube, then the solution is separated by decantation (carefully drained).
Washing the sediment. The sediment (if it is examined) must be washed well; To do this, a washing liquid is added, most often distilled water. The contents are thoroughly mixed with a glass rod and centrifuged, then the washing liquid is separated. Sometimes in work this operation is repeated 2-3 times.
Dissolution of sediment. To dissolve the precipitate, add a solvent to the test tube, stirring with a glass rod. Often the precipitate is dissolved by heating in a water bath.
For determining quantitative composition substances or products, reactions of neutralization, precipitation, oxidation - reduction, complex formation are used. The amount of a substance can be determined by its mass or the volume of solution spent on interaction with it, as well as by the refractive index of the solution, its electrical conductivity or color intensity, etc.
According to the amount of substance taken for research, analytical methods of quantitative analysis are classified as follows: macroanalysis - 1-10 g of solid substance, 10-100 ml of the analyzed solution; semi-microanalysis - 0.05-0.5 solids, 1-10 ml of analyzed solution; microanalysis - 0.001-1-10-4 g of solid substance, 0.1-1 * 10-4 ml of the analyzed solution. In merchandising practice, gravimetric (weight) and titrimetric (volume) methods are often used.
Gravimetric (weight) analysis- one of the methods of quantitative analysis, which allows you to determine the composition of the analyte by measuring mass. Mass measurement (weighing) is performed on an analytical balance with an accuracy of 0.0002 g. This method is often used in food laboratories to determine moisture, ash content, and the content of individual elements or compounds. The analysis can be performed in one of the following ways.
1. The component to be determined is quantitatively (as completely as possible) isolated from the test substance and weighed. This is how the ash content of products is determined. The initial product (sample) weighed on an analytical balance is burned, the resulting ash is brought to a constant mass (calcined until the mass stops changing) and weighed.
The ash content of the product x (%) is calculated using the formula
where B is the mass of calcined ash, g;
A is the initial weight of the product, g.
2. The component being determined is completely removed from the sample of the starting substance and the residue is weighed. This is how the moisture content of the products is determined, while a sample of the starting substance is dried in an oven to constant weight.
The moisture content of the product x (%) is calculated using the formula
where A is the initial sample of the product, g;
B is the mass of the sample after drying, g.
Volumetric analysis- a method of quantitative analysis, where the desired substance is determined by the volume of a reagent with a precisely known concentration spent on the reaction with this substance.
When determining by volumetric method, a reagent with a precisely known concentration is added in small portions (drop by drop) to a known volume of a solution of the analyte until its amount is equivalent to the amount of the analyte. A solution of a reagent with an accurately known concentration is called a titrated, working or standard solution.
The process of slowly adding a titrated solution to a solution of the analyte is called titration. The moment when the amount of the titrated solution is equivalent to the amount of the substance being determined is called the equivalence point or the theoretical end point of the titration. To determine the equivalence point, indicators are used that undergo visible changes near it, expressed in a change in the color of the solution, the appearance of turbidity, or the formation of a precipitate.
The most important conditions for correct performance of volumetric analytical determinations: 1) the ability to accurately measure volumes of solutions; 2) the availability of standard solutions with precisely known concentrations; 3) the ability to accurately determine the moment of completion of the reaction (correct choice of indicator).
Depending on the reaction on which the determination is based, the following types of volumetric method are distinguished:
neutralization method
· oxidation-reduction method
· precipitation and complexation method.
At the core neutralization method lies the reaction of interaction between H + and OH - ions. The method is used to determine acids, bases and salts (that react with acids or bases) in solution. To determine acids, titrated solutions of alkalis KOH or NaOH are used, and to determine bases, solutions of acids HC1, H 2 SO 4 are used.
To determine, for example, the acid content in a solution, a precisely measured volume of an acid solution with a pipette in the presence of an indicator is titrated with an alkali solution of precisely known concentration. The equivalence point is determined by the change in color of the indicator. Based on the volume of alkali consumed for titration, the acid content in the solution is calculated.
Method oxidation - reduction is based on redox reactions occurring between a standard solution and the analyte. If the standard solution contains an oxidizing agent (reducing agent), then the substance to be determined must contain a corresponding reducing agent (oxidizing agent). The oxidation-reduction method is divided, depending on the standard solution used, into the permanganatometry method, the iodometry method, etc.
The basis of the method deposition there are reactions accompanied by precipitation. Unlike the gravimetric method, the sediment is not processed here; the mass of the substance under study is determined by the volume of the reagent consumed for the precipitation reaction.
Objectives of Quantitative Analysis. Quantitative analysis methods. Chemical methods of analysis. Gravimetric and titrimetric methods of analysis.Instrumental methods of analysis. Photometry and spectrophotometry. Atomic absorption spectroscopy. Atomic emission spectroscopy.AabsorptionBut-spectral method. Nephelometric method for determining substance. Emission flame photometry. Luminescent method. Chromatographic analysis.Electrochemical methods.Potentiometry. FieldsRography. Conductometry.
Quantitative analysis is a branch of analytical chemistry whose task is to determine the quantity (content) of elements (ions), radicals, functional groups, compounds or phases in the analyzed object.
Quantitative analysis makes it possible to establish the elemental and molecular composition of the object under study or the content of its individual components. Depending on the object of study, inorganic and organic analysis are distinguished. In turn, they are divided into elementary analysis, the task of which is to establish the quantity of elements (ions) in the analyzed object, into molecular and functional analyzes, which give an answer about the quantitative content of radicals, compounds, as well as functional groups of atoms in the analyzed object.
Quantitative analysis is carried out in a certain sequence, which includes sampling and preparation of samples, analysis, processing and calculation of analysis results.
Quantitative analysis is widely used to study the composition of ores, metals, inorganic and organic compounds. In recent years, special attention has been paid to determining the content of toxic substances in the air, water bodies, soils, and in products: food, various goods.
Classification of quantitative analysis methods. All methods of quantitative analysis can be divided into two large groups: chemical and instrumental. This division is conditional, since many instrumental methods are based on the use of chemical laws and properties of substances.
Classic methods of chemical quantitative analysis are gravimetric (weight) analysis And titrimetric (volumetric) analysis.
Gravimetric method. The essence of the method is to obtain a sparingly soluble compound that contains a certain component. To do this, a sample of the substance is dissolved in one solvent or another, usually water, and precipitated using a reagent that forms a poorly soluble compound with a low PR value with the analyzed compound. Then, after filtering, the precipitate is dried, calcined and weighed. Based on the mass of the substance, the mass of the component being determined is determined and its mass fraction in the analyzed sample is calculated.
There are variations of the gravimetric method. In the distillation method, the analyzed component is isolated in the form of a gas that reacts with the reagent. The change in the mass of the reagent is used to judge the content of the component being determined in the sample. For example, the carbonate content of a rock can be determined by exposing the sample to acid, which releases CO 2 . The amount of CO 2 released can be determined by the change in the mass of the substance, for example CaO, with which CO 2 reacts.
One of the main disadvantages of the gravimetric method is its labor intensity and relatively long duration. Less labor-intensive is the electrogravimetric method, in which the metal to be determined, such as copper, is deposited on a cathode (platinum mesh)
Сu 2+ + 2е = Cu
Based on the difference in the mass of the cathode before and after electrolysis, the mass of the metal in the analyzed solution is determined. However, this method is only suitable for the analysis of metals that do not produce hydrogen (copper, silver, mercury).
Titrimetric analysis. The essence of the method is to measure the volume of a solution of a particular reagent consumed in the reaction with the analyzed component. For these purposes, so-called titrated solutions are used, the concentration of which (usually the titer of the solution) is known. A titer is the mass of a substance contained in 1 ml (1 cm3) of a titrated solution (in g/ml and g/cm3). The determination is carried out by titration, i.e. gradual addition of a titrated solution to a solution of the analyte, the volume of which is accurately measured. Titration stops when the equivalence point is reached, i.e. achieving equivalence of the reagent of the titrated solution and the analyzed component.
There are several types of titrimetric analysis: acid-base titration, precipitation titration, complexometric titration, and redox titration.
At the core acid-base titration lies the neutralization reaction
H + + OH - ↔ H 2 0
The method allows you to determine the concentration of acid or cations that are hydrolyzed to form hydrogen ions by titration with an alkali solution or to determine the concentration of bases, including anions that are hydrolyzed to form hydroxide ions by titration with acid solutions. The equivalence point is established using acid-base indicators that change color in a certain pH range. For example, using the acid-base titration method, you can determine the carbonate hardness of water, i.e. concentration of HCO 3 - in water by titrating its solution with HCl in the presence of a methyl orange indicator
HCO 3 - + H + →H 2 0 + C0 2
At the equivalence point, the yellow color of the indicator turns into pale pink. The calculation is made using the equation of the law of equivalents/
Cec, HC O3, V 1 = Cec, HCl V 2,
where V 1 and V 2 - volumes of analyzed and titrated solutions; With eq HCl is the normal concentration of equivalents of the substance HCl in a titrated solution, with eqHCl is the determined molar concentration of equivalents of HCl ions in the analyzed solution.
At precipitation titration the analyzed solution is titrated with a reagent that forms a poorly soluble compound with a component of the titrated solution. The equivalence point is determined using an indicator that forms a colored compound with the reagent, for example, a red precipitate of Ag 2 Cr0 4 when the K 2 Cr0 4 indicator interacts with an excess of Ag + ions when titrating a chloride solution with a silver nitrate solution.
Complexometric titration. In complexometric titration, the component being determined in a solution is titrated with a solution of a complexone, most often ethylenediaminetetraacetic acid (EDTA, complexone II) or its disodium salt (complexon III or Trilon B). Complexons are ligands and form complexes with many cations. Indicators of the equivalence point are usually ligands that form a colored complex compound with the analyzed ion. For example, the indicator chromogen black with calcium and magnesium forms complexes [Ca Ind] - and - red. As a result of titration of a wine-red solution containing calcium, magnesium ions and an indicator with a solution of complexone III, calcium binds into a more stable complex with the complexone; at the equivalence point, the indicator anions are released and give the solution a blue color. This complexometric titration method is used, for example, to determine the total hardness of water.
Redox titration. This method consists of titrating a reducing agent solution with a titrated oxidizing solution or titrating an oxidizing solution with a titrated reducing solution. Solutions of potassium permanganate KMn0 4 (permanganatometry), potassium dichromate K 2 Cr 2 0 7 (dichromatometry), and iodine I 2 (iodometry) have been used as titrated solutions of oxidizing agents.
During permanganatometric titration in an acidic medium, Mn (VII) (raspberry color) turns into Mn (II) (colorless solution). For example, permanganometric titration can determine the nitrite content in a solution
2KMn0 4 + 5KN0 2 + 3H 2 S0 4 = 2MnS0 4 + K 2 S0 4 + 5KN0 3 + ZN 2 0
In dichromatometric titration, the indicator is diphenylamine, which turns the solution blue when there is an excess of dichromate ions. In iodometric titration, starch serves as an indicator. Iodometric titration is used to analyze solutions of oxidizing agents, in which case the titrated solution contains iodide ion. For example, copper can be determined by titrating its solutions with an iodide solution
2Ci 2+ + 4G = 2CuI +I 2
The resulting solution is then titrated with a titrated solution of sodium thiosulfate Na 2 S 2 0 3 with a starch indicator added at the end of the titration
2Na 2 S 2 0 3 +I 2 = 2NaI + Na 2 S 4 0 6