Optical density of the solution. Optical density purpose

Optical density

D, a measure of the opacity of a layer of substance to light rays. Equal to the decimal logarithm of the radiative flux ratio (See Radiative flux) F 0 incident on the layer, to a flow weakened as a result of absorption and scattering F passed through this layer: D=log( F 0 /F), otherwise, O.P. is the logarithm of the reciprocal of the Transmittance coefficient of a layer of matter: D= log(1/τ). (In the definition of natural op., which is sometimes used, the decimal logarithm lg is replaced by natural ln.) The concept of op. was introduced by R. Bunsen; it is used to characterize the attenuation of optical radiation (See Optical radiation) (light) in layers and films of various substances (dyes, solutions, colored and milky glasses, etc.), in light filters and other optical products. O.P. is used especially widely for the quantitative assessment of developed photographic layers in both black-and-white and color photography, where the methods of its measurement form the content of a separate discipline—densitometry. There are several types of optical radiation depending on the nature of the incident radiation and the method of measuring the transmitted radiation fluxes ( rice. ).

The operating frequency depends on the set of frequencies ν (wavelengths λ) characterizing the original flow; its value for the limiting case of one single ν is called monochromatic O. Regular ( rice. , a) monochromatic O.P. of a layer of a non-scattering medium (without taking into account corrections for reflection from the front and rear boundaries of the layer) is equal to 0.4343 k ν l, Where k ν - natural absorption indicator of the environment, l- layer thickness ( k ν l= κ cl- exponent in the equation Bouguer - Lambert - Beer law a; if scattering in the medium cannot be neglected, kν is replaced by the natural Attenuation indicator). For a mixture of non-reacting substances or a collection of media located one after the other, the opacities of this type are additive, that is, equal to the sum of the same opacities of individual substances or individual media, respectively. The same is true for regular nonmonochromatic radiation (radiation of a complex spectral composition) in the case of media with nonselective (independent of ν) absorption. Regular non-monochromatic The O.P. of a set of media with selective absorption is less than the sum of the O.P. of these media. (For instruments for measuring O. p. see the articles Densitometer, Microphotometer, Spectrozonal aerial photography, Spectrosensitometer, Spectrophotometer, Photometer.)

Lit.: Gorokhovsky Yu. N., Levenberg T. M., General sensitometry. Theory and practice, M., 1963; James T., Higgins J., Fundamentals of the Theory of the Photographic Process, trans. from English, M., 1954.

L. N. Kaporsky.

Types of optical density of a medium layer depending on the geometry of the incident radiation and the method of measuring the transmitted radiation flux (in the sensitometric system adopted in the USSR): a) regular optical density D II is determined by directing a parallel flux to the layer perpendicular to it and measuring only that part of the transmitted flux , which retained the original direction; b) to determine the integral optical density D ε, a parallel flow is directed perpendicular to the layer, and the entire transmitted flow is measured; c) and d) two measurement methods used to determine two types of diffuse optical density D ≠ (incident flux - ideally diffuse). The difference D II - D ε serves as a measure of light scattering in the measured layer.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

OPTICAL DENSITY

density D, a measure of the opacity of a layer of matter to light rays. Equal to the decimal logarithm of the ratio of the radiation flux F0 incident on the layer to the flux F attenuated as a result of absorption and scattering passing through this layer: D lg (F0/ F), otherwise, O.p. is the logarithm of the reciprocal of the transmittance coefficient layer of matter: D lg (1/t). (In the definition of the sometimes used natural op., the decimal logarithm lg is replaced by the natural ln.) The concept of op. was introduced by R. Bunsen; it is used to characterize the attenuation of optical radiation (light) in layers and films of various substances (dyes, solutions, colored and milk glasses and much more), in light filters and other optical products. O.P. is used especially widely for the quantitative assessment of developed photographic layers in both black-and-white and color photography, where the methods of its measurement form the content of a separate discipline—densitometry. There are several types of optical radiation depending on the nature of the incident radiation and the method of measuring the transmitted radiation fluxes (Fig.).

The operating frequency depends on the set of frequencies n (wavelengths l) characterizing the original flow; its value for the limiting case of one single n is called monochromatic OP. Regular (Fig. , a) monochromatic OP of a layer of a non-scattering medium (without taking into account corrections for reflection from the front and rear boundaries of the layer) is equal to 0.4343 k n l, where k n is the natural absorption index of the medium, l is the thickness of the layer (k n l k cl is the index in the Bouguer-Lambert-Beer law equation; if scattering in the medium cannot be neglected, k n is replaced by the natural attenuation index). For a mixture of non-reacting substances or a set of media located one after the other, the opacities of this type are additive, that is, equal to the sum of the same opacities of individual substances or individual media, respectively. The same is true for regular nonmonochromatic radiation (radiation of a complex spectral composition) in the case of media with nonselective (independent of n) absorption. Regular non-monochromatic The O.P. of a set of media with selective absorption is less than the sum of the O.P. of these media. (For instruments for measuring O. p., see the articles Densitometer, Microphotometer, Spectrozonal aerial photography, Spectrosensitometer, Spectrophotometer, Photometer.)

Lit.: Gorokhovsky Yu. N., Levenberg T. M., General sensitometry. Theory and practice, M., 1963; James T., Higgins J., Fundamentals of the Theory of the Photographic Process, trans. from English, M., 1954.

L. N. Kaporsky.

Great Soviet Encyclopedia, TSB. 2012

I. Terms and definitions

Standard solution (sr)- this is a solution containing per unit volume a certain amount of the test substance or its chemical analytical equivalent (GOST 12.1.016 - 79).

Test solution (ir) - this is a solution in which it is necessary to determine the content of the test substance or its chemical analytical equivalent (GOST 12.1.016 - 79).

Calibration chart- graphical expression of the dependence of the optical density of the signal on the concentration of the test substance (GOST 12.1.016 - 79).

Maximum permissible concentration (MPC)) harmful substance - this is a concentration that, with daily (except for weekends) work for 8 hours or other working hours, but not more than 40 hours a week throughout the entire working experience, cannot cause diseases or health problems detected by modern research methods, in the process of work or in the long term of the life of the present or subsequent generations (GOST 12.1.016 - 79).

Colorimetry - This is a method for quantitative analysis of the content of an ion in a transparent solution, based on measuring the intensity of its color.

II. Theoretical part

The colorimetric method of analysis is based on the relationship between two quantities: the concentration of the solution and its optical density (degree of coloration).

The color of the solution can be caused either by the presence of the ion itself (MnO 4 -, Cr 2 O 7 2- ), and the formation of a colored compound as a result of the chemical interaction of the ion under study with the reagent.

For example, the weakly colored Fe 3 ion + gives a blood-red compound when reacted with thiocyanate ions SCH -, copper ion Cu 2+ forms a bright blue complex ion 2 + when interacting with an aqueous solution of ammonia.

The color of the solution is due to the selective absorption of light rays of a certain wavelength: the colored solution absorbs those rays whose wavelength corresponds to the complementary color. For example: blue-green and red, blue and yellow are called complementary colors.

Iron thiocyanate solution appears red because it absorbs predominantly green rays (  5000Á) and transmits red ones; on the contrary, a green-colored solution transmits green rays and absorbs red ones.

The colorimetric method of analysis is based on the ability of colored solutions to absorb light in the wavelength range from ultraviolet to infrared. Absorption depends on the properties of the substance and its concentration. With this method of analysis, the substance under study is part of an aqueous solution that absorbs light, and its amount is determined by the light flux passing through the solution. These measurements are carried out using photocolorimeters. The action of these devices is based on changes in the intensity of the light flux when passing through the solution, depending on the thickness of the layer, degree of color and concentration. The measure of concentration is optical density (D). The higher the concentration of a substance in a solution, the greater the optical density of the solution and the lower its light transmittance. The optical density of a colored solution is directly proportional to the concentration of the substance in the solution. It must be measured at the wavelength at which the substance under study has maximum light absorption. This is achieved by selecting light filters and cuvettes for the solution.

The preliminary selection of cuvettes is made visually according to the color intensity of the solution. If the solution is intensely colored (dark), use cuvettes with a short working wavelength. In the case of weakly colored solutions, cuvettes with a longer wavelength are recommended. A solution is poured into a pre-selected cuvette, its optical density is measured by turning on a light filter in the path of the rays. When measuring a series of solutions, the cuvette is filled with a solution of average concentration. If the obtained optical density value is approximately 0.3-0.5, this cuvette is selected to work with this solution. If the optical density is more than 0.5-0.6, take a cuvette with a shorter working wavelength, if the optical density is less than 0.2-0.3, choose a cuvette with a longer working wavelength.

The accuracy of measurements is greatly influenced by the cleanliness of the working edges of the cuvettes. During work cuvettes are grasped with hands only by the non-working edges, and after filling with solution carefully monitor the absence of even the smallest air bubbles on the walls of the cuvettes.

According to the law Bouguer-Lambert-Baer, the proportion of absorbed light depends on the thickness of the solution layer h, solution concentration C and intensity of incident light I 0

where I - intensity of light passing through the analyzed solution;

I is the intensity of the incident light;

h is the thickness of the solution layer;

C is the concentration of the solution;

The absorption coefficient is a constant value for a given colored compound.

Taking logarithm of this expression, we get:

(2)

where D is the optical density of the solution and is a constant value for each substance.

Optical density D characterizes the ability of a solution to absorb light.

If the solution does not absorb light at all, then D = 0 and I t =I, since expression (2) is equal to zero.

If the solution absorbs the light rays completely, then D equals infinity and I = 0, since expression (2) equals infinity.

If the solution absorbs 90% of the incident light, then D = 1 and

I t =0.1, since expression (2) is equal to one.

For accurate colorimetric calculations, the change in optical density should not exceed the range of 0.1 - 1.

For two solutions of different layer thicknesses and concentrations, but the same optical density, we can write:

D = h 1 C 1 = h 2 C 2,

For two solutions of the same thickness, but different concentrations, we can write:

D 1 = h 1 C 1 and D 2 = h 2 C 2,

As can be seen from expressions (3) and (4), in practice, to determine the concentration of a solution using the colorimetric method, it is necessary to have a standard solution, that is, a solution with known parameters (C, D).

The definition can be done in different ways:

1. You can equalize the optical densities of the test and standard solutions by changing their concentration or the thickness of the solution layer;

2. You can measure the optical density of these solutions and calculate the desired concentration using expression (4).

To implement the first method, special devices are used - colorimeters. They are based on a visual assessment of the intensity of transmitted light and therefore their accuracy is relatively low.

The second method - measuring optical density - is carried out using much more accurate instruments - photocolorimeters and spectrophotometers, and it is this method that is used in this laboratory work.

When working with a photocolorimeter, the technique of constructing a calibration graph is often used: they measure the optical density of several standard solutions and construct a graph in coordinates D = f(C). Then the optical density of the test solution is measured and the desired concentration is determined from the calibration graph.

The equation Bouguer-Lambert-Baer This is true only for monochromatic light, so accurate colorimetric measurements are carried out using light filters - colored plates that transmit light rays in a certain wavelength range. For work, select a light filter that provides maximum optical density of the solution. Light filters installed on a photocolorimeter transmit rays not of a strictly defined wavelength, but in a certain limited range. As a result, the measurement error on the photocolorimeter is no more than ±3 % on the weight of the analyte. Strictly monochromatic light is used in special devices - spectrophotometers, which have higher measurement accuracy.

The accuracy of colorimetric measurements depends on the concentration of the solution, the presence of impurities, temperature, acidity of the solution medium, and determination time. This method can only analyze dilute solutions, that is, those for which the dependence D = f(C)-straight.

When analyzing concentrated solutions, they are first diluted, and when calculating the desired concentration, a correction for dilution is made. However, the measurement accuracy decreases.

Impurities can affect the accuracy of measurements by either producing a colored compound with the added reagent or by hindering the formation of a colored compound of the ion being studied.

The colorimetric analysis method is currently used to carry out analyzes in various fields of science. It allows accurate and rapid measurements using negligible amounts of substance, insufficient for volumetric or gravimetric analysis.

COLORED SOLUTIONS USING CONCENTRATOR

PHOTOELECTRIC CALORIMETER KFK–2

Goal of the work: study the phenomenon of attenuation of light when passing through a substance and the photometric characteristics of the substance, study the device of the concentration photoelectric calorimeter KFK-2 and the method of working with it, determine the optical density and concentration of a colored solution using KFK-2.

Devices and accessories: photoelectric concentration calorimeter KFK - 2, test solution, set of solutions of standard concentration.

Theory of operation

When light falls on the interface between two media, the light is partially reflected and partially penetrates from the first substance to the second. Light electromagnetic waves set into oscillatory motion both free electrons of the substance and bound electrons located on the outer shells of atoms (optical electrons), which emit secondary waves with the frequency of the incident electromagnetic wave. Secondary waves form a reflected wave and a wave penetrating into the substance.

In substances with a high density of free electrons (metals), secondary waves generate a strong reflected wave, the intensity of which can reach 95% of the intensity of the incident wave. The same part of the light energy that penetrates into the metal experiences strong absorption in it, and the energy of the light wave is converted into heat. Therefore, metals strongly reflect the light falling on them and are practically opaque.

In semiconductors, the density of free electrons is lower than in metals, and they absorb visible light less well, and in the infrared region they are generally transparent. Dielectrics absorb light selectively and are transparent only for certain parts of the spectrum.

In general, when light falls on a substance, the incident luminous flux F 0 can be represented as the sum of light fluxes:

Where Ф r– reflected, F a- absorbed, Ф t– light flux passing through a substance.

The phenomenon of interaction of light with matter is described by dimensionless quantities called reflection, absorption and transmission coefficients. For the same substance

r+a +t = 1. (2)

For opaque bodies t= 0; for perfectly white bodies r = 1; for absolute black bodies a = 1.

Magnitude is called the optical density of the substance.

Odds r, a, t characterize the photometric properties of a substance and are determined by photometric methods.

Photometric methods of analysis are widely used in veterinary medicine, animal science, soil science, and materials technology. When studying substances dissolved in a practically non-absorbing solvent, photometric methods are based on measuring the absorption of light and on the relationship between absorption and concentration of solutions. Instruments designed for absorption (absorption - absorption) analysis of transparent media are called spectrophotometers and photocalorimeters. In them, using photocells, the colors of the solutions under study are compared with the standard.

The relationship between the absorption of light by a colored solution and the concentration of the substance obeys the combined Bouguer–Lambert–Beer law:

, (3)

Where I 0 – intensity of the light flux incident on the solution; I- intensity of the light flux passing through the solution; c- concentration of the colored substance in the solution; l- thickness of the absorbing layer in the solution; k- absorption coefficient, which depends on the nature of the solute, solvent, temperature and wavelength of light.

If With expressed in mol/l, and l- in centimeters, then k becomes the molar absorption coefficient and is denoted e l, therefore:

. (4)

Taking logarithms of (4), we get:

The left side of expression (5) is the optical density of the solution. Taking into account the concept of optical density, the Bouguer–Lambert–Beer law will take the form:

that is, the optical density of a solution under certain conditions is directly proportional to the concentration of the colored substance in the solution and the thickness of the absorbing layer.

In practice, cases of deviation from the combined absorption law are observed. This happens because some colored compounds in the solution undergo changes due to the processes of dissociation, solvation, hydrolysis, polymerization, and interaction with other components of the solution.

Dependency graph type D = f(c) shown in Fig. 1.

Colored compounds have selective light absorption, i.e. The optical density of the colored solution is different for different wavelengths of incident light. Measurement of optical density in order to determine the concentration of the solution is carried out in the region of maximum absorption, i.e. at the wavelength

incident light close to l max.

To photometrically determine the concentration of a solution, first construct a calibration graph D = f(c). To do this, prepare a series of standard solutions. Then the values of their optical density are measured and a dependence graph is plotted

D = f(c). To build it you need to have 5 – 8 points.

Having experimentally determined the optical density of the solution under study, find its value on the ordinate axis of the calibration graph D = f(c), and then the corresponding concentration value is counted on the x-axis With X.

The photoelectric concentration calorimeter KFK-2 used in this work is designed to measure the ratio of light fluxes in individual sections of wavelengths in the range of 315 - 980 nm, emitted by light filters, and allows you to determine the transmittance and optical density of liquid solutions and solids, as well as the concentration of substances in solutions method of constructing calibration graphs D = f(c).

The principle of measuring the optical characteristics of substances with the KFK-2 photocalorimeter is that light fluxes are sent alternately to the photodetector (photocell) - full I 0 and passed through the medium under study I and the ratio of these flows is determined.

The appearance of the KFK-2 photocalorimeter is shown in Fig. 2. It includes

includes a light source, an optical part, a set of light filters, photodetectors and a recording device, the scale of which is calibrated for light transmittance and optical density readings. On the front panel of the KFK-2 photocalorimeter there are:

1 - microammeter with a scale digitized in the values of the coefficient of pro-

launches T and optical density D;

2 - illuminator;

3 - knob for switching light filters;

4 - switch of cuvettes in the light beam;

5 - photodetector switch “Sensitivity”;

6 - knobs “Setting 100”: “Coarse” and “Fine”;

7 - cuvette compartment.

Work order

1. Connect the device to the network. Warm up for 10 – 15 minutes.

2. With the cuvette compartment open, set the microammeter needle to “0”

on the "T" scale.

3. Set the minimum sensitivity; to do this, turn the “Sensitivity” knob

Move the “Setup 100” “Coarse” knob to the extreme left position.

4. Place a cuvette with a solvent or control solution into the light beam.

rum in relation to which the measurement is made.

5. Close the lid of the cuvette compartment.

6. Use the “Sensitivity” and “Setting 100” knobs to set “Coarse” and “Fine”

reading 100 on the photocalorimeter scale. The “Sensitivity” knob can be in one of three positions “1”, “2”, or “3”.

7. By turning knob “4”, replace the cuvette with the solvent with the cuvette with the test substance

solution.

8. Take a reading on the microammeter scale corresponding to the pro-

release of the test solution as a percentage, on the “T” scale or on the “D” scale - in units of optical density.

9. Carry out measurements 3–5 times and the final value of the measured value is

divide as the arithmetic mean of the obtained values.

10. Determine the absolute measurement error of the desired quantity.

Task No. 1. Study of the dependence of optical density on length

Waves of incident light

1.1. For a standard solution, determine the optical density at different frequencies of incident light.

1.2. Enter the data into table 1.

1.3. Plot the dependence of optical density on wavelength l pa-

giving light D = f(l).

1.4. Define l and filter number for D max .

Table 1

Task No. 2. Checking the dependence of optical density on thickness

Absorbent layer

2.1. For a standard solution, using a filter with l D for cuvettes of various sizes.

2.2. Enter the data into table 2.

table 2

2.3. Build a dependency graph D = f(l).

Task No. 3. Construction of a calibration graph and determination of concentrations

Walkie-talkie of unknown solution

3.1. For a series of standard solutions of known concentration, using light

tofilter with l max (see task No. 1), determine D.

3.2. Enter the measurement data in table 3.

Table 3

3.3. Build a calibration graph D = f(c).

3.4. On schedule D = f(c) Determine the concentration of an unknown solution.

Control questions

1. The phenomenon of attenuation of light when passing through matter, the absorption mechanism

tions for different types of matter.

2. Parameters characterizing the photometric properties of a substance.

3. Explain the essence of photometric methods of analysis.

4. Formulate the combined Bouguer–Lambert–Beer absorption law.

5. What are the reasons for possible deviations of the properties of solutions from the combined

takeover horse?

6. Molar absorption coefficient, its definition and factors on which it depends

7. How to select the wavelength of absorbed radiation during photocaloric

rimetric measurements?

1. How is a calibration graph constructed?

2. Explain the design and operating principle of the KFK-2 photocalorimeter.

3. Where and for what is absorption analysis used?

Literature

1. Trofimova T. I. Physics course. M.: Higher. school, 1994. Part 5, ch. 24, § 187.

2. Savelyev I.V. Course of general physics. M.: Nauka, 1977. Volume 2, part 3, chapter. XX,

3. Grabovsky R.I. Physics course. St. Petersburg: Lan. 2002. Part P, ch. VI, § 50.

LABORATORY WORK No. 4–03

Optical density D, a measure of the opacity of a layer of substance to light rays.

, Where

e is the absorption (extinction) coefficient of light flux. Depends on the nature of the substance and the wavelength of light.

C is the concentration of the substance in solution in m/l.

l is the thickness of the light-absorbing solution layer.

The optical density of the solution is directly proportional to the concentration of the light-absorbing substance in the solution and the thickness of the solution layer. In other words, at a certain thickness of the solution layer, the greater the concentration of the substance in the solution, the greater the optical density. It follows that by determining the optical density of a solution, one can directly determine the concentration of a substance in the solution. With modern technology, optical density can be measured very accurately. Increasing layer thickness l very small concentrations of substances can be measured.

Photocolorimeter- an optical device for measuring the concentration of substances in solutions. The action of the colorimeter is based on the property of colored solutions to absorb light passing through them, the more strongly the higher the concentration of the coloring substance in them. Unlike a spectrophotometer, measurements are carried out in a beam not of monochromatic, but of polychromatic narrow spectral light formed by a light filter. The use of various light filters with narrow spectral ranges of transmitted light makes it possible to separately determine the concentrations of different components of the same solution. Unlike spectrophotometers, photocolorimeters are simple, inexpensive, and yet accurate enough for many applications.

Colorimeters are divided into visual and objective (photoelectric) - photocolorimeters. In visual colorimeters, light passing through the solution being measured illuminates one part of the field of view, while another part is illuminated by light passing through a solution of the same substance, the concentration of which is known. By changing the thickness l of the layer of one of the solutions being compared or the intensity I of the light flux, the observer ensures that the color tones of the two parts of the field of view are indistinguishable by eye, after which the concentration of the solution under study can be determined using the known relationships between l, I and c.

Photoelectric colorimeters (photocolorimeters) provide greater measurement accuracy than visual ones; They use photocells (selenium and vacuum), photomultipliers, photoresistors (photoresistors) and photodiodes as radiation receivers. The strength of the photocurrent of the receivers is determined by the intensity of the light incident on them and, therefore, by the degree of its absorption in the solution (the greater the higher the concentration). In addition to the photoelectric colorimeter (photocolorimeter) with direct current reading, compensation colorimeters are common, in which the difference between the signals corresponding to the standard and measured solutions is reduced to zero (compensated) by an electrical or optical compensator (for example, a photometric wedge); In this case, the count is taken from the compensator scale. Compensation allows you to minimize the influence of measurement conditions (temperature, instability of the properties of colorimeter elements) on their accuracy. The colorimeter readings do not immediately give the concentration values of the test substance in the solution; to go to them, calibration graphs are used, obtained by measuring solutions with known concentrations.

Measurements using a colorimeter are simple and fast. Their accuracy in many cases is not inferior to the accuracy of other, more complex methods of chemical analysis. The lower limits of the determined concentrations, depending on the type of substance, range from 10 −3 to 10 −8 mol/l.

21. FEK circuit, which is based on a comparison of 2 light fluxes, where L-lamp, Z-mirrors, Sph-light filters, K-capacitors, A-cuvette with a controlled solution, F1 and F2-photocells, EI-electronic amplifier, IN- zero indicator, OK-optical wedge.

Operating principle: the luminous flux from lamp L is divided into 2 streams and reflected from mirrors Z hits identical photocells F1 and F2. The flux that goes through the upper light channel passes through the light filter Sf, condensate K and the optical wedge OK, and the flux of light that goes through the lower light channel passes through the lower light filter Sf of condensate K and cuvette A, which is filled with the controlled substance. Photodetectors F1 and F2 are connected back-to-back and the electronic amplifier of the EU is connected to their circuit. By changing the position of the OK (optical wedge) we achieve equality of light fluxes in both channels. Then both channels will produce the same photocurrents and the imbalance signal at the input to the electronic amplifier will become zero, and the IN indicator will show zero. After setting the instrument reading to zero, i.e. Having balanced the circuit, we place cuvette A with a controlled solution in the device; as a result of a change in the equality of light fluxes, an imbalance will arise, which will be fed to an electronic amplifier. In order to equalize the light fluxes, it is necessary to move the OK until the imbalance signal ceases to be sent to the amplifier, i.e. the photocurrents will level out and the arrow, which is connected to the optical wedge, will not show the effective value of the concentration of the solution placed in cuvette A.

22. Refractometers are designed to determine the refractive index of the test substance, on the basis of which a conclusion is made about its composition, the presence of impurities, and the percentage composition of dissolved solids is determined. These devices are designed for studying non-aggressive liquids of medium viscosity and solids.

Refractometers are used in the chemical industry,

food industry, for the analysis of products and raw materials, in medicine and veterinary medicine; in the pharmaceutical industry for the study of aqueous solutions of drugs, as well as in many other industries.

Typically, the refractive indices of liquid and solid bodies are determined by refractometry with an accuracy of 0.0001 on refractometers in which the limiting angles of total internal reflection are measured. The most common are Abbe refractometers with prism blocks and dispersion compensators, which make it possible to determine spectral lines in “white” light using a scale or digital indicator. The maximum accuracy of absolute measurements (10 -10) is achieved with goniometers using methods of deflecting rays with a prism made of the material under study. Interference methods are most convenient for measuring the refractive indices of gases. Interferometers are also used for precise (up to 10 -7) determination of differences in the refractive indices of solutions. For the same purpose, differential refractometers are used, based on the deflection of rays by a system of two or three hollow prisms.

Automatic refractometers for continuous recording of refractive indices in liquid flows are used in production for monitoring and automatic control of technological processes, as well as in laboratories for monitoring rectification and as universal detectors of liquid chromatographs.

Refractometry, performed using refractometers, is one of the most common methods for identifying chemical compounds, quantitative and structural analysis, and determining the physical and chemical parameters of substances.

23.

1- illuminator; 2- collimator; 3 - cuvette; 4, 5 -- prisms; 6 - photocells.

The cuvette consists of two chambers separated by a transparent partition, one of which is filled with a standard solution of a given concentration, and the other with a controlled solution. If the refractive indices of the reference are equal P and controlled P" liquids, a beam of light passes through both chambers without deviation, and when the concentration of the controlled medium changes, the indicator P" changes and the light beam is deflected. The more noticeable the difference between the concentrations of the reference and controlled liquids, the greater the beam deflection. The design of the differential cuvette provides temperature compensation, i.e., equality of temperatures at which both liquids are located.

24. When measuring with mass spectrometers, the main physical parameter of a substance is used - the mass of a molecule or atom. This makes it possible to determine the composition of a substance regardless of its chemical and physical properties. The advantage of the mass spectrometric method is the rapid and complete analysis of multicomponent gas mixtures. In this case, negligible amounts of the substance are required for analysis. "

Under high vacuum conditions, the molecules or atoms of the analyte are ionized to form positively charged ions. Ions that are accelerated in an electric field are separated according to their masses in a magnetic field. The sum of the electrical charges of moving ions forms an ion current. Measuring the strength of the ionic current created by particles of a particular mass makes it possible to judge the concentration of particles in the overall composition of the analyzed substance. In a mass spectrometer of any design, the main part is the mass analyzer, in which ionization occurs, the formation of an ion beam, its division into component ion beams corresponding to strictly defined masses, and sequential separate collection of ion beams on a collector. According to these processes, the mass analyzer of any mass spectrometer consists of an ion source, the analyzer itself, and an ion receiver.

According to the configuration and mutual orientation of magnetic and electric fields, as well as the nature of the change in these fields over time, mass spectrometers are divided into four groups: with separation of ions in a uniform magnetic field; with separation of ions in a non-uniform magnetic field; with ion separation by time of flight; radiofrequency.

Mass spectrometers with ion separation in a uniform magnetic field and time-of-flight are predominantly used.

25. Automatic refractometer.

26. Action of a refractometer

pH meter

Optical density of the solution. Optical density purpose

See also interpretations, synonyms, meanings of the word and what OPTICAL DENSITY is in Russian in dictionaries, encyclopedias and reference books:

I. Terms and definitions

II. Theoretical part