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 polarization plane during the passage of any polarized light.
Substances deflecting the plane of the polarization of light in addition or left are called optically active.
If the rotation of the polarization plane is right (along the clockwise movement), the substance is called the right-rotating and before the title it is put in the index d or sign + (plus); If the rotation of the polarization plane occurs to the left (counterclockwise), the substance is called the left-rotating and before the title it is put index 1 or a sign - (minus).
The magnitude of the deviation of the polarization plane from the initial position, expressed in the angular degrees, is called an angle of rotation and denote the Greek letter a.
The magnitude of the rotation angle depends on the nature of the optically anximal 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 sodium spectrum D (589.3).
The optical activity of the substance is characterized by a specific rotation, i.e., the rotation of the polarization plane caused by a layer of substance (/) with a thickness of 1 dm at a concentration of C, equal to 1 g of substance in 1 ml of volume at 20 ° C. Denote the specific rotation sign (A] g\u003e 20.
The specific rotation of the solutions is calculated by the formula:
where: A - measured rotation angle, degrees; / - the thickness of the layer of the solution, DM; FROM- Concentration of solution,%.
Knowing the specific rotation of the substance, permanent in a certain interval of concentrations, one can calculate its content in the solution in percent (C) by the formula:
For liquid individual substances, the specific rotation is determined by the formula:
where: A - measured rotation angle, degrees; / - the thickness of the substance layer, DM; P is the density of the liquid, r / cm 8.
The polarimetry method is widely used in pharmaceutical analysis to establish the optical activity of drugs, high-quality and quantitative assessment of them.
To measure the angle of rotation of the polarization plane, the devices called polarmeters are changed.
Practical work uses polarimeters of different systems based on the same principle of their work.
The polarimeter device is represented in Fig. four.
Optical instrument system. The light from the radiation source through the light filter (or matte glass) falls on a polarizer prism, which forms two separated polarized beam at the output, and the streams in each of them are equal. The polarizer is established so that the plane of the polarization of both beams is the same angle with the plane of the polarization of the analyzer. If the cuvette cuvette is installed on the path of both beams, the polarization planes will be represented and one of the beams will be more loosen by the analyzer than the other. Rotate the compensator will allow you to compensate for the specified flow change. At the same time, the scale rotates, which is highlighted through the prism and is observed in the magnifying glass. Through the visual tube there is painting fields.
Operating procedure. 1. The eyepiece of the visual pipe and the magnifying glass of the scales are installed (by rotating their frames) to the maximum sharpness of the image so that the vertical line, dividing after two halves, was clearly visible, and in the field of view of the magnifying glass, the touches and numbers of the lower scale and nonius (top scales).
2. Installation of the device for 0. For this, it is achieved by the complete homogeneity of both halves of the field of view with the help of handle transfer. At the same time, zero divisions of the scale and nonioce must coincide. Otherwise, with the help of the key, the nonius is re-coined before combining its zero division with zero division of the scale.
3. Filling the polarimetric cuvette. Before filling, the cuvette is washed with a test solution twice; The liquid bones pour so much so that it performs on top of the edges of the tube. Some time will wait for gas bubbles under-up. Clean the cuvette with clean glass, as if the speaking of the protruding liquid.
4. The polarimetric cuvette with the test solution on-hole into the appliance chamber, while the homogeneity of both halves of the field of view changes. The rotation of the handle of the transmission equalize their illumination.
5. Record indications up to 0.01.
6. Then repeat the elevation of the illumination of both half of the field of view, and again conduct the readings again, repeating 5 times. Take the medium-immetic and accept for the result. Selection of light filter. 1. If, in the study of soft-colored or weakly-headed solutions, it is not observed in the shades of the coloration of both half of the field of view, then the voor clip is set to a position corresponding to the denotation "M". At the same time, the optical system introduces matte glass.
2. If, with the polarization of colorless or weakly-headed solutions, there is some difference in shades of the color of both half of the field of view, which makes it difficult to bring the field of view to homogeneity, then the turning clip is put in the position corresponding to the designation "C". At the same time, a light filter is introduced into the optical system.
3. In the case of working with dark-painted solutions, the clip is set to a position without notation, which corresponds to the maximum intensity of the field illumination.
The specific rotation of the polarization plane by optically active substance is defined as an angle of rotation, referred to a unit of thickness of the translucent material:
If the rotational angle is measured in angular degrees, and the layer thickness l. - in mm, then the dimension of the specific rotation will be [hail / mm].
Accordingly, the specific rotation of the optically active liquid (no solution) with a density with [g / cm 3] is determined by the expression
Since the optical activity of liquids is much less than the optical activity of solid bodies, and the thickness of the liquid layer is measured in decimeters, the specific rotation of the liquids has dimension [hail · cm - 3 / (dm · d)].
Specific rotation of the optically active substance solution in an optically inactive solvent with concentration FROM (g / 100 ml) solution is determined by the formula
In organic chemistry as a variety of specific rotation, the magnitude of molar rotation is also used.
Determination of the concentration of dissolved optically active substances according to the results of measuring the angle of rotation B [hail] with this thickness of the layer l. [DM] for a certain wavelength [nm] is made according to the Bio equation (1831):
The BIO law is almost always performed in the field of low concentrations, while at high concentrations there are significant deviations.
Minding factors in polarimetric measurements
With each refraction and reflection from the surface, not perpendicular to the direction of light, there is a change in the state of polarization of the incident light. It follows from this that any type of turbidity and bubbles in the studied substance due to the plurality of surfaces strongly reduces polarization, and the sensitivity of the measurement may decrease below the permissible level. The same applies to pollution and scratches on the windows of the cuvette and on the protective glasses of the light source.
Thermal and mechanical stresses in protective glasses and windows of the cuvette lead to double refraction and, therefore, to elliptical polarization, which is superimposed on the measurement result in the form of apparent turn. Since these phenomena in most cases are uncontrollable and not constant in time, should be carefully monitored so that the mechanical stresses in the optical elements do not appear.
The strong dependence of the optical activity on the wavelength (rotational dispersion), which, for example, for sucrose is 0.3% / nm in the visible light, it makes it use in polarimetry extremely narrow spectrum bands, 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), so only strictly monochromatic light can be used for full-fledged polarimetric measurements, i.e. isolated spectrum lines. High pressure burners that provide high light intensity are unsuitable for polarimetry due to the expansion of spectral lines when the pressure changes and increased the share of the background of solid radiation. The use of broader spectral bands is possible only for instruments in which the rotational dispersion compensation is provided, as, for example, in appliances with compensation with the help of a quartz wedge (a saccharimeter with a quartz wedge) and appliances with compensation for the Faraday effect. In the instruments with a quartz wedge, the possibility of compensation when measuring sucrose is limited. When compensating for the Faraday effect by appropriate selection of the material, the rotational dispersion can be subordinate to various requirements; However, it is not possible to achieve universality of the methods used.
When measured with a finite width of the spectral strip near the absorption absorption bands under the action of absorption, a displacement of an effective center of gravity of the wavelength distribution arises, distorting measurement results, from which it follows that during the study of absorbent substances you need to work with strictly monochromatic radiation.
Under the control of fast-flowing continuous streams of solutions, the elliptical polarization arising due to the double refraction of light may worsen the sensitivity of polarimetric measurement methods and lead to rough errors. These difficulties can be eliminated only by thorough streaming, for example, the provision of a laminar parallel stream in cuvettes and a decrease in its speed. polarization light rotation optical
Polarimetry is an optical research method based on the ability of optically active compounds to rotate the oscillation plane of linearly polarized light (see isomeria).
Atoms and molecules of luminous bodies emit electromagnetic waves. With complete disorder in the location of these body particles, the so-called natural light is emitted, in which the oscillation of the vectors of the electric (or magnetic) fields occurs in all planes passing through the direction of propagation of the light wave. Organization in the direction of oscillations of fields is called the polarization of light. Such a light in which the fluctuations of the electric (magnetic) fields occur in the same plane, is called a flat-polarized light, and the plane in which the magnetic field tension fluctuates the light beams is the polarization plane. Polarized light can be obtained, passing natural light through polarizing prisms made of special crystals. Such crystals include crystals of Icelandic spam, of which the polarizing prisms (prisms of Nicolas) are usually prepared. When the polarized light passes through the solution of an optically active substance, the polarization plane is rotated, but it is possible to detect it only with the second same polarizing prism (analyzer). The exploration of rotation of the polarization plane is used to study the structure of optically active compounds, as well as for quantifying them. The optical activity is characterized by the value of the specific rotation [α], i.e. the angle of rotation of the polarization plane with a solution containing in 1 ml of 1 g of an optically active compound with a thickness of the liquid layer in 1 dm.
Specific rotation is calculated by the magnitude of the rotation of the solution of this compound with a known percentage concentration:
[α] \u003d α100 / L · C
where α is the rotation angle in degrees, C is a concentration in%, L - the thickness of the layer of the solution in DM. Specific rotation changes with a change in temperature and wavelength of light. Therefore, the definition is carried out in the monochromatic light of the PRB of a certain temperature. The wavelength and temperature are marked at the values \u200b\u200b[A]. Knowing from reference tables The specific rotation of this compound and determining the angle of rotation of the solution of this compound, it is easy to calculate the concentration:
C \u003d α100 / [α] l
The solution should not contain other optically active compounds.
To determine the rotation of the polarization plane, optical polarimeters are used. The polarimeter (Fig. 1) consists of two polarizing prisms: a fixed polarizer and a rotating - analyzer and tube with the test solution. The angle of rotation can be determined by establishing an analyzer on an equal illumination of the entire field of view at first without a solution, and then with a solution of an optically active compound. In this case, the analyzer must be rotated at an angle equal to the angle of rotation of the polarization of the under study. The angle of rotation is counted in a circle with divisions (limb). If after installing the tube with a solution and the analyzer is rotated clockwise, then they speak about the right (+), if counterclockwise - about the left (-) rotation. To increase accuracy, polarimeters are equipped with additional parts from quartz. In some polarimeters, leveling the illumination after the installation of the solution and the concentration of the optically active substance is made by linear movement of the quartz wedge. The accuracy of ordinary polarimeters is 0.05 °. For monochromatic light, they usually use light filters. Polarimetry method is widely used in laboratories; In clinical laboratories and food industry laboratories, the polarimetry method is used to determine the content of sugars. Polarimeters used to determine the content of cane sugar are called saccharameters (Fig. 2).
Fig. 1. Schemes of polarimeters of various types: a - a system with two plates from a bicker; b - semi-leaven with Nikole; B - semi-leaven with two nicols. 1 - polarizer; 1 "and 1" - Nicoli; 2 - Plate of the bikvar; 3 - tube with solution; 4 - analyzer (right - field illumination patterns of polarimeters).
Fig. 2. Wedge polarimeter saccharimeter juice (scheme): 1 - illuminator; 2 - light filter; 3 - diaphragm; 4 - lens; 5 - Nicole; 6-tube for the investigated solution; 7 - fixed quartz wedge; 8 - mobile quartz wedge; 9 - analyzer; 10-eyed; 11 - cover; 12 - screw; 13 - Lupa.
Theory of Polarmetry
The optical activity of substances is very sensitive to changes in the spatial structure of molecules and to intermolecular interaction.
Research of optical activity of substances
Using optical polarimeters, the magnitude of the rotation of the plane of the polarization of light when it passes through optical-active media (solids or solutions) is determined.
Polarimetry is widely used in analytical chemistry to quickly measure the concentration of optical-active substances (see saccharimetry), to identify essential oils and in other studies.
- The magnitude of the optical rotation in solutions depends on their concentration and the specific properties of optical-active substances.
- Measurement of the rotational dispersion of light (spectropolarimetry, determination of the rotation angle when the light wavelength change allows you to study the structure of substances.
see also
Literature
- Volkenstein M. V., Molecular optics, M.-L., 1951
- Jrasci K., dispersion of optical rotation, per. from English, M., 1962
- Terentyev A. P., Organic Analysis, M., 1966
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- Specific heat
- Specific electrical conductivity
Watch what is "specific rotation" in other dictionaries:
Specific rotation - See the rotational ability of chemical compounds ...
specific rotation of the substance - The angle on which the plane of the polarization of the optical radiation of a certain wavelength is rotated during the passage of a single length path in the substance. [GOST 23778 79] Optics Topics, Optical devices and measurements EN SPECIFIC ROTATION OF ... ...
specific rotation of the solution - The ratio of the angle that turns the plane of polarization of the optical radiation of a certain wavelength when it passes the path of a single length in the solution of the substance, to the concentration of this substance. [GOST 23778 79] Optics Topics, Optical ... Technical translator directory
Specific rotation of some organic substances - Substance solvent Specific rotation * Sacraosis water +66,462 glucose water +52.70 ... Chemical handbook
relative specific rotation of the substance - The ratio of the specific rotation of the substance to the density of this substance. [GOST 23778 79] Topics Optics, optical instruments and measurements EN Relative Specific Rotation of Substance De Relative Spezifische Materialdrehung FR Rotation Relative Spécifique ... ... Technical translator directory
Rotation of polarization plane - transverse wave The 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 polarization plane - rotation of the polarization plane, changes in the direction (plane) of the oscillations of the rays of polarized light (see polarization optical). This property has: 1. All transparent bodies, if they are placed in a magnetic field (magnetic V. p. P.). For ... ... Big medical encyclopedia
Specific magnetic rotation - The same as (see Verde is constant). Physical encyclopedic dictionary. M.: Soviet Encyclopedia. Editor-in-Chief of A. M. Prokhorov. 1983 ... Physical encyclopedia
Rotational ability of chemical compounds - under the name of the rotational ability of chemical compounds, the ability to inherent in some of them is implied to deflect the polarization plane of the light beam from its initial direction. Suppose that in the beam of such polarized light ... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron
Sucrose - (Chem.) The name produced from the word sucrose is synonym for cane sugar; Systematically used to designate carbohydrates of the general formula C12N22O11 only in the present ENT. sl. and in 1 m Tome Op. Tolens Handb. Der Kohlenhydrate (Bresz. ... ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron
