The formula for calculating the specific rotation. Determination of optical rotation and specific rotation

The polarimetric method of analysis is based on the ability of substances to deflect the plane of polarization when polarized light passes through them.

Substances that deflect the plane of polarization of light to the right or to the left are called optically active.

If the rotation of the plane of polarization occurs to the right (clockwise), then the substance is called right-rotating and the index d or the + (plus) sign is placed in front of its name; if the rotation of the plane of polarization occurs to the left (counterclockwise), then the substance is called left-rotating and the index 1 or the sign - (minus) is put in front of its name.

The deviation of the plane of polarization from the initial position, expressed in angular degrees, is called the angle of rotation and is denoted by the Greek letter a.

The value of the angle of rotation depends on the nature of the optically active substance, the thickness of its layer, temperature, the nature of the solvent and the wavelength of light.

As a rule, the determination of optical rotation is carried out at 20 ° C and at the wavelength of the D line of the sodium spectrum (589.3).

The optical activity of a substance is characterized by specific rotation, i.e., the rotation of the plane of polarization caused by a layer of substance (/) 1 dm thick at a concentration of C equal to 1 g of substance in 1 ml of volume at 20 ° C. The specific rotation is designated by the sign (a] r> 20.

The specific rotation of solutions is calculated by the formula:

where: a - measured angle of rotation, degrees; / - thickness of the solution layer, dm; WITH- solution concentration,%.

Knowing the specific rotation of a substance, which is constant in a certain concentration range, it is possible to calculate its content in the solution as a percentage (C) by the formula:

For liquid individual substances, the specific rotation is determined by the formula:

where: a - measured angle of rotation, degrees; / is the thickness of the substance layer, dm; p is the density of the liquid, g / cm 8.

The method of polarimetry is widely used in pharmaceutical analysis to establish the optical activity of medicinal substances, to assess them qualitatively and quantitatively.

To measure the angle of rotation of the plane of polarization, devices called polarimeters are used.

In practical work, polarimeters of various systems are used, based on the same principle of their operation.

The polarimeter device is shown in Fig. 4.

Optical system of the device. Light from a radiation source through a light filter (or frosted glass) falls on a polarizer prism, which forms two separated polarized beams at the output, and the fluxes in each of them are equal. The polarizer is installed so that the polarization planes of both beams make the same angle with the analyzer polarization plane. If a cuvette with a solution is installed in the path of both beams, then the polarization planes will be reversed and one of the beams will be more attenuated by the analyzer than the other. Rotating the expansion joint will compensate for this change in flow. At the same time, the scale rotates, which is illuminated through a prism and observed through a magnifying glass. Coloring of the fields is observed through the telescope.

Operating procedure. 1. The eyepiece of the telescope and the magnifying glass of the scale are set (by rotating their frames) to the maximum sharpness of the image so that the vertical line dividing after sight into two halves is clearly visible, and the strokes and numbers of the lower scale are clearly visible in the field of view of the magnifying glass. and vernier (upper scale).

2. Setting the device to 0. For this, complete homogeneity of both halves of the field of view is achieved using the transmission handle. In this case, the zero divisions of the scale and the vernier must coincide. Otherwise, using the key, move the vernier until its zero division is aligned with the zero division of the scale.

3. Filling the polarimetric cuvette. Before filling, the cuvette is washed with the test solution two times; the liquid is poured so that it protrudes over the edges of the tube. Wait a while for the gas bubbles to rise up. Cover the cuvette with clean glass, as if cutting off the protruding liquid.

4. The polarimetric cuvette with the test solution is inserted into the chamber of the device, while the uniformity of both halves of the field of view changes. By rotating the gear knob, their illumination is equalized.

5. Read off the readings with an accuracy of 0.01.

6. Then the equalization of the illumination of both halves of the field of view is repeated, and the readings are counted again, repeating 5 times. Take the arithmetic mean and take it as the result. Choice of a light filter. 1. If, in the study of colorless or weakly colored solutions, no difference is observed in the shades of color of both halves of the field of view, then the rotary clip is placed in the position corresponding to the designation "M". In this position, frosted glass is introduced into the optical system.

2. If, during the polarization of colorless or weakly colored solutions, there is some difference in the shades of color of both halves of the field of view, which makes it difficult to bring the field of view to uniformity, then the rotary holder is placed in the position corresponding to the designation "C". In this position, a light filter is introduced into the optical system.

3. In the case of working with dark-colored solutions, the clip is placed in the unmarked position, which corresponds to the maximum illumination intensity of the field of view.

Specific rotation of the plane of polarization by an optically active substance is defined as the angle of rotation per unit thickness of the translucent material:

If the angle of rotation is measured in angular degrees, and the thickness of the layer l- in mm, then the dimension of the specific rotation will be [deg / mm].

Accordingly, the specific rotation of an optically active liquid (not a solution) with a density c [g / cm 3] is determined by the expression

Since the optical activity of liquids is much less than the optical activity of solids, and the thickness of the liquid layer is measured in decimeters, the specific rotation of liquids has the dimension [deg · cm-3 / (dm · g)].

Specific rotation of a solution of an optically active substance in an optically inactive solvent with a concentration WITH(g / 100 ml) of the solution is determined by the formula

In organic chemistry, the magnitude of molar rotation is also used as a kind of specific rotation.

Determination of the concentration of dissolved optically active substances from the results of measuring the angle of rotation b [deg] at a given layer thickness l[dm] for a certain wavelength [nm] is produced according to Biot's equation (1831):

Biot's law is almost always fulfilled in the region of low concentrations, while at high concentrations there are significant deviations

Interfering Factors in Polarimetric Measurements

With each refraction and reflection from a surface that is not perpendicular to the direction of the light, there is a change in the polarization state of the incident light. It follows from this that any kind of turbidity and bubbles in the test substance due to the multitude of surfaces greatly reduces the polarization, and the measurement sensitivity may decrease below the permissible level. The same applies to dirt and scratches on the cuvette windows and on the protective glass of the light source.

Thermal and mechanical stresses in protective glasses and cuvette windows lead to birefringence and, therefore, to elliptical polarization, which is superimposed on the measurement result in the form of an apparent rotation. Since these phenomena are in most cases uncontrollable and not constant over time, care should be taken to ensure that mechanical stresses do not appear in the optical elements.

The strong dependence of optical activity on wavelength (rotational dispersion), which, for example, for sucrose is 0.3% / nm in the visible light region, forces the use of extremely narrow spectral bands in polarimetry, which is usually required only in interferometry. Polarimetry is one of the most sensitive optical measurement methods (the ratio of the sensitivity threshold to the measurement range is 1/10000), therefore, for full-fledged polarimetric measurements, only strictly monochromatic light, i.e., isolated spectrum lines, can be used. High-pressure burners, which provide high light intensity, are unsuitable for polarimetry due to the broadening of spectral lines with a change in pressure and an increased proportion of the background of continuous radiation in this case. The use of wider spectral bands is possible only for devices that provide for compensation of rotational dispersion, such as devices with compensation using a quartz wedge (saccharimeter with a quartz wedge) and devices with compensation for the Faraday effect. In devices with a quartz wedge, the compensation options for sucrose measurements are limited. With Faraday compensation, the rotational dispersion can be subordinated to various requirements by appropriate material selection; however, it is not possible to achieve universality of the methods used.

When measuring with a finite width of the spectral band near the absorption absorption bands, under the influence of absorption, a shift of the effective center of gravity of the wavelength distribution arises, distorting the measurement results, from which it follows that when studying absorbing substances it is necessary to work with strictly monochromatic radiation.

When monitoring fast-flowing continuous flows of solutions, the elliptical polarization arising due to the double refraction of light by the flow can impair the sensitivity of polarimetric measurement methods and lead to gross errors. These difficulties can be eliminated only by careful shaping of the flow, for example, providing a laminar parallel flow in the cuvettes and reducing its speed. polarization light rotation optical

Polarimetry is an optical research method based on the ability of optically active compounds to rotate the plane of oscillation of linearly polarized light (see Isomerism).

The atoms and molecules of luminous bodies emit electromagnetic waves. With complete disorder in the arrangement of these particles, the bodies emit the so-called natural light, in which the oscillation of the vectors of the electric (or magnetic) field strength occurs in all planes passing through the direction of propagation of the light wave. The order in the direction of field fluctuations is called the polarization of light. Such light, in which fluctuations in the intensity of electric (magnetic) fields occur in one plane, is called plane polarized light, and the plane in which the intensity of the magnetic field of light rays fluctuates is called the plane of polarization. Polarized light can be produced by passing natural light through polarizing prisms made from special crystals. These crystals include crystals of Icelandic spar, from which polarizing prisms (Nicolas prisms) are usually prepared. When polarized light passes through a solution of an optically active substance, the plane of polarization rotates, but it can be detected only with the help of a second same polarizing prism (analyzer). The study of the rotation of the plane of polarization is used to study the structure of optically active compounds, as well as for their quantitative determination. Optical activity is characterized by the value of the specific rotation [α], that is, by the angle of rotation of the plane of polarization with a solution containing 1 g of an optically active compound in 1 ml at a liquid layer thickness of 1 dm.

Specific rotation is calculated from the amount of rotation of a solution of a given compound with a known percentage concentration:

[α] = α100 / l · C
where α is the rotation angle in degrees, C is the concentration in%, l is the thickness of the solution layer in dm. Specific rotation changes with temperature and wavelength of light. Therefore, the determination is carried out in monochromatic light at a certain temperature. Wavelength and temperature are labeled at values of [a]. Knowing the specific rotation of a given compound from the lookup tables and determining the angle of rotation of the solution of this compound, it is easy to calculate the concentration:

C = α100 / [α] l
The solution should not contain other optically active compounds.

To determine the rotation of the plane of polarization, optical instruments-polarimeters are used. The polarimeter (Fig. 1) consists of two polarizing prisms: a fixed one - a polarizer and a rotating one - an analyzer and a tube with the test solution. The angle of rotation can be determined by setting the analyzer to equal illumination of the entire field of view, first without a solution, and then with a solution of an optically active compound. In this case, the analyzer must be rotated by an angle equal to the angle of rotation of the plane of polarization of the test solution. The angle of rotation is measured in a circle with divisions (limb). If, after installing the tube with a solution, the analyzer is rotated clockwise, then they speak of right (+), if counterclockwise, left (-) rotation. To improve the accuracy, the polarimeters are equipped with additional quartz parts. In some polarimeters, the equalization of the illumination after the installation of the solution and the reading of the concentration of the optically active substance is carried out by linear movement of the quartz wedge. The accuracy of conventional polarimeters is 0.05 °. To obtain monochromatic light, light filters are usually used. The polarimetry method is widely used in laboratories; in clinical laboratories and laboratories of the food industry, the method of polarimetry is used to determine the sugar content. Polarimeters used to measure cane sugar are called saccharimeters (Figure 2).

Rice. 1. Schemes of different types of polarimeters: a - a system with two biquartz plates; b - penumbra with nicole; c - penumbra with two nicols. 1 - polarizer; 1 "and 1" - nicols; 2 - biquartz plate; 3 - a tube with a solution; 4 - analyzer (on the right - illumination schemes of polarimeter fields).

Rice. 2. Wedge polarimeter-saccharimeter SOK (diagram): 1 - illuminator; 2 - light filter; 3 - diaphragm; 4 - lens; 5 - nicole; 6-tube for the test solution; 7 - fixed quartz wedge; 8 - movable quartz wedge; 9 - analyzer; 10-eyepiece; 11 - cover; 12 - screw; 13 - magnifier.

Polarimetry theory

The optical activity of substances is very sensitive to changes in the spatial structure of molecules and to intermolecular interactions.

Study of the optical activity of substances

With the help of optical polarimeters, the amount of rotation of the plane of polarization of light is determined when it passes through optically active media (solids or solutions).

Polarimetry is widely used in analytical chemistry for the rapid measurement of the concentration of optically active substances (see Saccharimetry), for the identification of essential oils, and in other studies.

The amount of optical rotation in solutions depends on their concentration and the specific properties of optically active substances.
Measuring the rotational dispersion of light (spectropolarimetry, determining the angle of rotation when changing the wavelength of light allows you to study the structure of substances.

Literature

Vol'kenshtein M.V., Molecular Optics, M.-L., 1951
Jerassi K., Optical Rotation Dispersion, trans. from English, M., 1962
Terent'ev A.P., Organic analysis, M., 1966

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Specific heat
Specific electrical conductivity

See what "Specific Rotation" is in other dictionaries:

Specific rotation- see Rotational capacity of chemical compounds ...

specific rotation of matter- The angle by which the plane of polarization of optical radiation of a certain wavelength is rotated when it travels a unit length path in a substance. [GOST 23778 79] Topics of optics, optical instruments and measurements EN specific rotation of ... ...

specific rotation of the solution- The ratio of the angle by which the plane of polarization of optical radiation of a certain wavelength is rotated when it travels a unit length path in a solution of a substance to the concentration of this substance. [GOST 23778 79] Topics optics, optical ... Technical translator's guide

Specific rotation of some organic substances- Substance Solvent Specific rotation * Sucrose Water +66.462 Glucose Water +52.70 ... Chemical handbook

relative specific rotation of matter- The ratio of the specific rotation of a substance to the density of this substance. [GOST 23778 79] Topics of optics, optical instruments and measurements EN relative specific rotation of substance DE relative spezifische Materialdrehung FR rotation relative spec ... ... Technical translator's guide

Rotation of the plane of polarization- a transverse wave is a physical phenomenon consisting in the rotation of the polarization vector of a linearly polarized transverse wave around its wave vector when the wave passes through an anisotropic medium. The wave can be electromagnetic, ... ... Wikipedia

ROTATION OF THE POLARIZATION PLANE- ROTATION OF THE POLARIZATION PLANE, changing the direction (plane) of oscillations of the rays of polarized light (see Optical polarization). This property is possessed by: 1. All transparent bodies, if they are placed in a magnetic field (magnetic VF). For… … Great medical encyclopedia

SPECIFIC MAGNETIC ROTATION- the same as (see VERDE CONSTANT). Physical encyclopedic dictionary. M .: Soviet encyclopedia. Chief editor A.M. Prokhorov. 1983 ... Physical encyclopedia

Rotational capacity of chemical compounds- Under the name of the rotational ability of chemical compounds is meant the ability inherent in some of them to deflect the plane of polarization of a light beam from its original direction. Let's say that in a beam of such polarized light ... ... Encyclopedic Dictionary of F.A. Brockhaus and I.A. Efron

Sucrose- (chemical) name derived from the word sucrose, a synonym for cane sugar; systematically used to designate carbohydrates of the general formula С12Н22О11 only in the present Ents. sl. and in volume 1, op. Tollens Handb. der Kohlenhydrate (Bresl. ... ... Encyclopedic Dictionary of F.A. Brockhaus and I.A. Efron