Any particle, be it a molecule, an atom or an ion, as a result of the absorption of a light quantum passes to more high level energy state. Most often, the transition from the ground state to the excited state occurs. This causes the appearance of certain absorption bands in the spectra.

The absorption of radiation leads to the fact that when it is passed through a substance, the intensity of this radiation decreases with an increase in the number of particles of a substance that has a certain optical density. This research method was proposed by V. M. Severgin back in 1795.

This method is best suited for reactions where the analyte is able to transform into a colored compound, which causes a change in the color of the test solution. By measuring its light absorption or comparing the color with a solution of known concentration, it is easy to find the percentage of the substance in the solution.

Basic law of light absorption

The essence of photometric determination consists in two processes:

  • transfer of the substance to be determined into a compound absorbing electromagnetic vibrations;
  • measuring the intensity of absorption of these same vibrations by a solution of the substance under study.

Changes in the intensity of the light flux passing through the light absorbing substance will also be caused by light losses due to reflection and scattering. To make the result reliable, parallel studies are carried out to measure the parameters at the same layer thickness, in identical cuvettes, with the same solvent. So the decrease in light intensity depends mainly on the concentration of the solution.

The decrease in the intensity of light passed through the solution is characterized (also called its transmission) T:

T \u003d I / I 0, where:

  • I is the intensity of light passed through the substance;
  • I 0 is the intensity of the incident light beam.

Thus, transmission shows the proportion of unabsorbed light flux passing through the solution under study. Reverse Algorithm transmission values ​​are called the optical density of the solution (D): D \u003d (-lgT) \u003d (-lg) * (I / I 0) \u003d log * (I 0 / I).

This equation shows which parameters are the main ones for the study. These include the wavelength of the light, the thickness of the cuvette, the concentration of the solution, and the optical density.

Bouguer-Lambert-Beer law

It is a mathematical expression that reflects the dependence of the decrease in the intensity of a monochromatic light flux on the concentration of a light-absorbing substance and the thickness of the liquid layer through which it is passed:

I \u003d I 0 * 10 -ε C ι, where:

  • ε is the light absorption coefficient;
  • C is the concentration of the substance, mol/l;
  • ι is the thickness of the layer of the analyzed solution, see

Having transformed, this formula can be written: I / I 0 \u003d 10 -ε·С·ι.

The essence of the law is as follows: different solutions of the same compound at equal concentration and layer thickness in the cell absorb the same part of the light incident on them.

Taking the logarithm of the last equation, we can obtain the formula: D = ε * C * ι.

Obviously, the optical density directly depends on the concentration of the solution and the thickness of its layer. The physical meaning of the molar absorption coefficient becomes clear. It is equal to D for a one molar solution and for a layer thickness of 1 cm.

Restrictions on the application of the law

This section includes the following items:

  1. It is valid only for monochromatic light.
  2. The coefficient ε is related to the refractive index of the medium; especially strong deviations from the law can be observed in the analysis of highly concentrated solutions.
  3. The temperature when measuring optical density must be constant (within a few degrees).
  4. The light beam must be parallel.
  5. The pH of the medium must be constant.
  6. The law applies to substances whose light-absorbing centers are particles of the same type.

Methods for determining the concentration

It is worth considering the calibration curve method. To build it, prepare a series of solutions (5-10) with different concentrations of the test substance and measure their optical density. According to the obtained values, a plot of D versus concentration is plotted. The graph is a straight line from the origin. It allows you to easily determine the concentration of a substance based on the results of measurements.

There is also an additive method. It is used less frequently than the previous one, but it allows you to analyze solutions of complex composition, since it takes into account the influence of additional components. Its essence is to determine the optical density of the medium D x containing the analyte of unknown concentration C x, with repeated analysis of the same solution, but with the addition of a certain amount of the test component (C st). The value of C x is found using calculations or graphs.

Research conditions

In order for photometric studies to give a reliable result, several conditions must be met:

  • the reaction must be completed quickly and completely, selectively and reproducibly;
  • the color of the resulting substance must be stable over time and not change under the action of light;
  • the test substance is taken in an amount sufficient to convert it into an analytical form;
  • optical density measurements are carried out in the wavelength range at which the difference in the absorption of the initial reagents and the analyzed solution is greatest;
  • the light absorption of the reference solution is considered to be the optical zero.

Colorimetry

From optical methods analysis in the practice of analytical laboratories, colorimetric methods are most widely used (from lat. color- color and Greek. μετρεω - I measure). Colorimetric methods are based on measuring the intensity of the light flux passing through a colored solution.

The colorimetric method uses chemical reactions accompanied by a change in the color of the analyzed solution. By measuring the light absorption of such a colored solution, or by comparing the color obtained with that of a solution of known concentration, the content of the colored substance in the test solution is determined.

There is a relationship between the color intensity of the solution and the content of the colored substance in this solution. This dependence, called the basic law of light absorption (or the Bouguer-Lambert-Beer law), is expressed by the equation:

I = I 0 10 - ε c l

where I is the intensity of light passing through the solution; I 0 - the intensity of the light incident on the solution; ε is the coefficient of light absorption, a constant value for each colored substance, depending on its nature; C is the molar concentration of the colored substance in the solution; l is the thickness of the light-absorbing solution layer, see

physical meaning This law can be expressed as follows. Solutions of the same colored substance at the same concentration of this substance and the thickness of the solution layer absorb an equal amount of light energy, i.e., the light absorption of such solutions is the same.

For a colored solution enclosed in a glass cuvette with parallel walls, it can be said that as the concentration and thickness of the solution layer increase, its color increases, and the intensity of light I transmitted through the absorbing solution decreases compared to the intensity of the incident light I 0 .



Fig.1 Passage of light through a cuvette with a test solution.

The optical density of the solution.

If we take the logarithm of the equation of the basic law of light absorption and reverse the signs, then the equation becomes:

The value is very important characteristic colored solution; it is called the optical density of the solution and is denoted by the letter A:

A = ε C l

It follows from this equation that the optical density of the solution is directly proportional to the concentration of the colored substance and the thickness of the solution layer.

In other words, with the same layer thickness of a solution of a given substance, the optical density of this solution will be the greater, the more it contains a colored substance. Or, conversely, at the same concentration of a given colored substance, the optical density of the solution depends only on the thickness of its layer. From this, the following conclusion can be drawn: if two solutions of the same colored substance have different concentrations, the same color intensity of these solutions will be achieved with the thicknesses of their layers inversely proportional to the concentrations of the solutions. This conclusion is very important, since some methods of colorimetric analysis are based on it.



Thus, in order to determine the concentration (C) of a colored solution, it is necessary to measure its optical density (A). To measure the optical density, the intensity of the luminous flux should be measured.

The color intensity of solutions can be measured various methods. There are subjective (or visual) methods of colorimetry and objective (or photocolorimetric).

Visual methods are such methods in which the assessment of the color intensity of the test solution is done with the naked eye.

With objective methods of colorimetric determination, photocells are used instead of direct observation to measure the color intensity of the test solution. The determination in this case is carried out in special devices - photocolorimeters, from which the method was called photocolorimetric.

Visual Methods

Visual methods include:

1) standard series method;

2) duplication method (colorimetric titration);

3) adjustment method.

Standard series method. When performing analysis by the standard series method, the color intensity of the analyzed colored solution is compared with the colors of a series of specially prepared standard solutions (with the same thickness of the absorbing layer).

Solutions in colorimetry usually have an intense color, so it is possible to determine very small concentrations or amounts of substances. However, this may be accompanied by certain difficulties: in this way, samples for preparing a series of standard solutions can be very small. To overcome these difficulties, standard solution A is prepared at a sufficiently high concentration, for example 1 mg/ml. After that, by dilution from solution A, a standard solution B of a much lower concentration is prepared, and from this, in turn, a series of standard solutions is prepared.

To do this, the required volumes of reagent solutions in desired sequence. It is advisable to add portions of solutions of the analyte from the burette, because their volumes will be different to provide different concentrations in a series of standard solutions. In this case, the initial solution must contain all components, except for the analyte. (zero solution). Solutions of the necessary reagents are added to the test solution. All solutions are brought to a constant volume, and then the color intensity of the test solution is visually compared with the solutions of a series of standard solutions. It is possible to match the color intensity with any solution of the series. Then it is considered that one hundred test solution has the same concentration or contains the same amount of the analyte. If the color intensity seems to be intermediate between neighboring solutions of the series, the concentration or content of the analyte is considered the arithmetic mean between the solutions of the series.

Colorimetric titration (duplication method). This method is based on comparing the color of the analyzed solution with the color of another solution. - control. To prepare a control solution, prepare a solution containing all components of the test solution, with the exception of the analyte, and all reagents used in the preparation of the sample, and add the standard solution of the analyte from the burette to it. When so much of this solution is added that the color intensities of the control and analyzed solutions become equal, it is considered that the analyzed solution contains the same amount of the analyte as it was introduced into the control solution.

Equalization method. This method is based on equalizing the colors of the analyzed solution and a solution with a known concentration of the analyte - a standard solution. There are two options for performing a colorimetric determination by this method.

According to the first option, the equalization of the colors of two solutions with different concentrations of the colored substance is carried out by changing the thickness of the layers of these solutions at the same strength of the light flux passing through the solutions. In this case, despite the difference in the concentrations of the analyzed and standard solutions, the intensity of the light flux passing through both layers of these solutions will be the same. The ratio between the thicknesses of the layers and the concentrations of the colored substance in the solutions at the time of equalization of the colors will be expressed by the equation:

l 1= C2

where l 1 is the thickness of the solution layer with the concentration of the colored substance C 1 , and l 2 is the thickness of the solution layer with the concentration of the colored substance C 2 .

At the moment of equality of colors, the ratio of the thicknesses of the layers of the two compared solutions is inversely proportional to the ratio of their concentrations.

Based on the above equation, by measuring the thickness of the layers of two identically colored solutions and knowing the concentration of one of these solutions, one can easily calculate the unknown concentration of the colored substance in the other solution.

To measure the thickness of the layer through which the light flux passes, glass cylinders or test tubes can be used, and at more precise definitions special devices - colorimeters.

According to the second option, to equalize the colors of two solutions with different concentrations of the colored substance, pass through layers of solutions of the same thickness light streams varying intensity.

In this case, both solutions have the same color when the ratio of the logarithms of the intensities of the incident light fluxes is equal to the ratio of the concentrations.

At the moment of achieving the same color of the two compared solutions, with an equal thickness of their layers, the concentrations of the solutions are directly proportional to the logarithms of the intensities of the light incident on them.

According to the second option, the determination can be performed only with a colorimeter.

Optical density

D, a measure of the opacity of a layer of matter to light rays. Equal to the base 10 logarithm of the radiant flux ratio (See radiant flux) F 0 incident on the layer to a stream weakened as a result of absorption and scattering F passing through this layer: D=lg( F 0 /F), otherwise, O. p. is the logarithm of the reciprocal of the Transmission coefficient of the substance layer: D= lg(1/τ). (The decimal logarithm lg is replaced by the natural logarithm logarithm logarithm logarithm lg, which is sometimes used.) The concept of a natural limit 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 milky glasses, and many others), in light filters and other optical products. Densitometry is especially widely used for the quantitative evaluation of developed photographic layers in both black-and-white and color photography, where methods for measuring it 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 fluxes of radiation ( rice. ).

The O.P. depends on the set of frequencies ν (wavelengths λ) that characterizes the initial flow; its value for the limiting case of one single ν is called monochromatic op. rice. , a) the 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 0.4343 k ν l, Where k ν - natural absorption index of the environment, l- layer thickness ( k ν l= κ cl- indicator in the equation of Bouguer - Lambert - Beer law a; if scattering in the medium cannot be neglected, kν is replaced by the natural Weakening index). For a mixture of non-reacting substances or a set of media arranged one after the other, the OD of this type is additive, i.e., it is equal to the sum of the same OD of individual substances or individual media, respectively. The same is true for regular nonmonochromatic optical radiation (radiation of a complex spectral composition) in the case of media with nonselective absorption (independent of ν). Regular non-monochromatic The opp of a set of media with selective absorption is less than the sum of the opp of these media. (For devices for measuring O. p., see the articles Densitometer, Microphotometer, Spectrozonal aerial photography, Spectrosensitometer, Spectrophotometer, Photometer.)

Lit.: Gorohovsky 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 the medium layer depending on the geometry of the incident and the method of measuring the transmitted radiation flux (in the sensitometric system adopted in the USSR): , which retained the original direction; b) to determine the integral optical density D ε, a parallel flow is directed perpendicular to the layer, the entire past flow is measured; c) and d) two measurement methods used to determine two types of diffuse optical density D ≠ (incident flux - ideally scattered). 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 .

Ensuring sufficient optical density (fill) of characters and images on the page is an important factor in the subjective assessment of print quality. Irregularities in the electrophotographic process can cause unwanted dark variations (fills) in the image. These deviations may be within acceptable limits or out of them. The value of these allowable deviations is set in specifications for consumables for a specific device and may vary significantly for different devices. Objective assessment fill density characterizes the heterogeneity of the process and is defined as the limit and standard deviation of the reflection coefficient of the printed character across the page.

The term optical density is used to characterize the measure of light transmission - for transparent objects and reflection - for opaque. It is quantified as the decimal logarithm of the reciprocal of the transmittance (reflection). In electrography, this term is used to evaluate the quality of image elements on copies obtained under certain development conditions (using a certain type of toner, estimating the contrast value of an electrostatic latent image, copy quality using a particular development method, etc.). In the printing industry, this characteristic is used to evaluate publishing originals, intermediate images and prints.

Optical density is denoted OD (Optical Density) or simply D. The minimum value of optical density D=0 corresponds to white color. The more light absorbed by the medium, the darker it is, i.e., for example, black has a higher optical density than gray.

Reflectance is related to optical density and contrast density as follows:

D = lg (1/R pr) and D c ​​= R pr / R pt

where D is the optical density of the image;

R pt - reflection coefficient at the measurement point;

D c - contrast density;

R pr - paper reflectance.

The values ​​of the image optical density on copies for black in electrography for different devices (as noted above) are significantly different. Generally, according to toner manufacturers' specifications for laser printers these values ​​(the minimum allowed in the normal state of the equipment) lie in the range from 1.3D to 1.45D. For quality toners, optical density takes values ​​in the range from 1.45D to 1.5D and does not exceed 1.6D. In specifications, it is customary to set limits on the lower permissible limit with a standard deviation in optical density of 0.01.

The value of optical density is measured with a special device - a densitometer, the principle of operation of which is based on measuring the flow reflected from the imprint and converting this indicator into units of optical density.

In electrography, the optical density of images is used to characterize the developer (toner) in order to determine the required values ​​​​of the optical density of lines of a set width under certain conditions for developing or characterizing an electrophotographic image on copies in the mode of nominal operation of the equipment

concept optical density(Optical Density) refers primarily to the original being scanned. This parameter characterizes the ability of the original to absorb light; it is designated as D or OD. The optical density is calculated as the logarithm of the ratio of the intensities of the incident and reflected (in the case of opaque originals) or transmitted (in the case of transparent originals) light intensities. The minimum optical density (D min) corresponds to the lightest (transparent) area of ​​the original, and the maximum density (D max) corresponds to the darkest (least transparent) area. The range of possible optical density values ​​is between 0 (perfectly white or completely transparent original) and 4 (black or completely opaque original).

Typical optical density values ​​for some types of originals are shown in the following table:

The dynamic range of a scanner is determined by the maximum and minimum values ​​of optical density and characterizes its ability to work with various types originals. The dynamic range of a scanner is related to its bit depth (color bit depth): the higher the bit depth, the greater the dynamic range and vice versa. For many flatbed scanners, mainly for office work, this parameter is not specified. In such cases, the optical density value is considered to be approximately 2.5 (typical value for office 24-bit scanners). For a 30-bit scanner, this parameter is equal to 2.6-3.0, and for a 36-bit scanner - from 3.0 and higher.

As the dynamic range increases, the scanner better reproduces the gradation of brightness in very bright and very dark areas of the image. On the contrary, if the dynamic range is insufficient, image details and smoothness of color transitions in dark and light areas are lost.

Permission

Resolution or scanner resolution- a parameter that characterizes the maximum accuracy or degree of detail in the representation of the original in digital form. Resolution is measured in pixels per inch(pixels per inch, ppi). Often, resolution is indicated in dots per inch (dpi), but this unit is traditional for output devices (printers). Speaking of resolution, we will use ppi. Distinguish hardware (optical) and interpolation resolution of the scanner.

Hardware (optical) resolution

Hardware (optical) resolution (Hardware/optical Resolution) is directly related to the density of placement of photosensitive elements in the scanner matrix. This is the main parameter of the scanner (more precisely, its optical-electronic system). Typically, the horizontal and vertical resolution is specified, for example, 300x600 ppi. You should focus on a smaller value, i.e. on the horizontal resolution. The vertical resolution, which is usually twice the horizontal resolution, is ultimately obtained by interpolation (processing the results of direct scanning) and is not directly related to the density of the sensing elements (this is the so-called double step resolution). To increase the resolution of the scanner, you need to reduce the size of the photosensitive element. But as the size decreases, the sensitivity of the element to light is lost and, as a result, the signal-to-noise ratio deteriorates. Thus, increasing the resolution is a non-trivial technical problem.

Interpolation Resolution

Interpolated Resolution - resolution of the image obtained as a result of processing (interpolation) of the scanned original. This artificial resolution upscaling usually does not improve image quality. Imagine that the actually scanned image pixels are moved apart, and “calculated” pixels are inserted into the resulting gaps, similar in some sense to their neighbors. The result of such interpolation depends on its algorithm, but not on the scanner. However, this operation can be performed using a graphics editor, such as Photoshop, and even better than your own. software scanner. The interpolation resolution, as a rule, is several times greater than the hardware one, but in practice this means nothing, although it can mislead the buyer. A significant parameter is precisely the hardware (optical) resolution.

The technical passport of the scanner sometimes simply indicates the resolution. In this case, we mean hardware (optical) resolution. Often, both hardware and interpolation resolution are indicated, for example, 600x 1200 (9600) ppi. Here 600 is the hardware resolution and 9600 is the interpolation resolution.

Line visibility

Line detectability - maximum amount parallel lines per inch, which are reproduced by the scanner as separate lines (without sticking). This parameter characterizes the suitability of the scanner for working with drawings and other images containing many small details. Its value is measured in lines per inch (lines per inch, Ipi).

What scanner resolution should I choose?

This question is most often asked when choosing a scanner, since resolution is one of the most important parameters of a scanner, on which the possibility of obtaining high-quality scan results depends significantly. However, this does not mean at all that one should strive for the highest possible resolution, especially since it is expensive.

When developing requirements for scanner resolution, it is important to understand the general approach. The scanner is a device that converts optical information about the original into digital form and, therefore, performs its sampling. At this stage of consideration, it seems that the finer the discretization (the greater the resolution), the lower the loss of the original information. However, scan results are intended to be displayed using some output device such as a monitor or printer. These devices have their own resolution. Finally, the human eye has the ability to smooth images. In addition, printed originals obtained by printing or by means of a printer also have a discrete structure (printed screen), although this may not be noticeable to the naked eye. These originals have their own resolution.
So, there is an original with its own resolution, a scanner with its own resolution, and a scan result, the quality of which should be as high as possible. The quality of the resulting image depends on the set resolution of the scanner, but up to a certain limit. If you set the scanner's resolution to be greater than the native resolution of the original, then the quality of the scan result, generally speaking, will not improve. This is not to say that scanning at a higher resolution than the original is useless. There are a number of reasons why this should be done (for example, when we are going to enlarge the image when outputting to a monitor or printer, or when we need to get rid of moiré). Here we draw attention to the fact that the improvement in the quality of the resulting image by increasing the resolution of the scanner is not unlimited. You can increase the scan resolution without improving the quality of the resulting image, but increasing its size and scanning time.

We will talk about the choice of scanning resolution more than once in this chapter. Scanner resolution is the maximum resolution that can be set when scanning. So what kind of resolution do we need? The answer depends on what images you are going to scan and on what devices you want to output. Below we give only indicative values.
If you are going to scan images for later display on a monitor screen, then 72-l00ppi is usually sufficient. For output to a regular office or home jet printer- 100-150 ppi, for a high-quality inkjet printer - from 300 ppi.

When scanning texts from newspapers, magazines and books for further processing by optical character recognition (OCR - Optical Character Recognition) programs, a resolution of 200-400 ppi is usually required. For output to the screen or printer, this value can be reduced several times.

For amateur photography, 100-300 ppi is usually required. For illustrations from luxury printed albums and booklets - 300-600ppi.

If you are going to enlarge the image for display on the screen or printer without losing quality (clearness), then the scanning resolution should be set with some margin, i.e. increase it by 1.5-2 times compared to the above values.

Advertising agencies, for example, require high-quality scanning of slides and paper originals. When scanning slides for printing in 10x15 cm format, a resolution of 1200 ppi is required, and in A4 format - 2400 ppi.
Summarizing the above, we can say that in most cases, the scanner's hardware resolution of 300 ppi is sufficient. If the scanner has a resolution of 600 ppi, then this is very good.

DYED SOLUTIONS WITH THE HELP OF A CONCENTRATION

PHOTOELECTRIC CALORIMETER KFK-2

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

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

Theory of work

When light falls on the interface between two media, light is partially reflected and partially penetrates from the first substance into the second. Light electromagnetic waves set in oscillatory motion both the free electrons of the substance and the bound electrons located on the outer shells of atoms (optical electrons), which emit secondary waves with a 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 undergoes strong absorption in it, and the energy of the light wave is converted into heat. Therefore, metals strongly reflect the light incident on them and are practically opaque.

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

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

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

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

r+a +t = 1. (2)

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

Value 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. In the study of 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. Devices designed for absorption (absorption - absorption) analysis of transparent media are called spectrophotometers and photocalorimeters. In them, with the help of photocells, the colors of the studied solutions are compared with the standard.

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

, (3)

Where I 0 is the intensity of the light flux incident on the solution; I is the intensity of the light flux passing through the solution; c is the concentration of the colored substance in the solution; l- the 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 k becomes the molar absorption coefficient and is denoted by e l , therefore:

. (4)

Taking the logarithm 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:

i.e., the optical density of the 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, there are cases of deviation from the combined absorption law. This is because some colored compounds in solution undergo changes due to the processes of dissociation, solvation, hydrolysis, polymerization, and interaction with other components of the solution.

Type of dependency graph D = f(c) shown in fig. 1.

Colored compounds exhibit selective absorption of light, i.e. the optical density of the colored solution is different for different wavelengths of the incident light. The 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 a wavelength

incident light close to l max.

To determine the concentration of a solution photometrically, a calibration graph is first built 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 test solution, find its value on the y-axis of the calibration graph D = f(c), and then the corresponding concentration value is read off on the abscissa axis With X.

The KFK-2 photoelectric concentration calorimeter used in the work is designed to measure the ratio of light fluxes in separate 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 directed alternately to the photodetector (photocell) - full I 0 and passed through the studied medium I and the ratio of these flows is determined.

Appearance photocalorimeter KFK-2 is shown in fig. 2. It includes

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

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

releases T and optical density D;

2 - illuminator;

3 - filter switching knob;

4 - switch cell in the light beam;

5 - switch of photodetectors "Sensitivity";

6 - knobs "Installation 100": "Rough" and "Exact";

7 - cuvette compartment.

Work order

1. Turn on the device in the network. Warm up for 10 - 15 minutes.

2. With the cell compartment open, set the pointer of the microammeter to "0"

on the "T" scale.

3. Set the minimum sensitivity, for this the “Sensitivity-

switch to position “1”, switch “Setting 100” “Coarse” to the leftmost position.

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

rum against which the measurement is made.

5. Close the lid of the cell compartment.

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

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

7. By turning the knob "4" replace the cuvette with the solvent with the cuvette with the test

solution.

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

release of the test solution in percent, on the "T" scale or on the "D" scale - in units of optical density.

9. Take measurements 3–5 times and the final value of the measured value is op-

Determine as the arithmetic mean of the obtained values.

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

Task number 1. Studying 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 the incident light.

1.2. Enter the data in table 1.

1.3. Plot absorbance versus wavelength l pa-

giving light D = f(l).

1.4. Define l and filter number for D max .

Table 1

Task number 2. Checking the dependence of optical density on thickness

absorbing layer

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

2.2. Enter the data in table 2.

table 2

2.3. Plot dependency graph D = f(l).

Task number 3. Building a calibration graph and determining the concentration

Walkie-talkies of an unknown solution

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

tofilter with l max (see task number 1), determine D.

3.2. Enter the measurement data in table 3.

Table 3

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

3.4. On schedule D = f(s) determine the concentration of the unknown solution.

Control questions

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

for different types substances.

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 the possible deviations of the properties of solutions from the combined

takeover horse?

6. Molar absorption coefficient, its definition and the factors from which it

7. How is the choice of the wavelength of absorbed radiation in the case of photocalo-

rimetric measurements?

1. How is a calibration graph built?

2. Explain the device and principle of operation of the KFK-2 photocalorimeter.

3. Where and why is absorption analysis used?

Literature

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

2. I. V. Savelyev, Course of General Physics. M.: Nauka, 1977. Volume 2, part 3, ch. XX,

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

LAB #4–03