Optical design of the polarimeter. Ministry of Agriculture of the Russian Federation federal state educational institution

The SM circular polarimeter consists of the following main components:

Analyzer head ( A);

Polarizing device ( P);

Illuminator ( S);

Chamber for a cuvette with test solutions ( T);

The analyzer is made of Polaroid film. The angle of rotation of the analyzer is measured from the dial in degrees and using two verniers, the division value of each vernier is 0.05°.

The polarizing device consists of a polarizer lighting lens and a quartz plate; the quartz plate is 1 mm thick and is positioned symmetrically relative to the polarizer.

Operating principle of the device

The SM circular polarimeter uses the principle of equalizing the color of a field of view divided into three parts. The field of view is divided into three parts by introducing a quartz plate into the optical system of the device, which occupies only the middle part of the field of view. Field equalization occurs near complete field darkening, which corresponds to almost complete crossing of the polarizer and analyzer.

Light from a frosted electric lamp, having passed through a capacitor and a polarizer, passes through the middle part of the beam through a quartz plate, orange protective glass and the analyzer, and with the two extreme parts of the beam only through the glass and the analyzer.

View of the field of view of the SM device (Fig. 4). The triple field of view is equalized by rotating the analyzer around a horizontal axis.

Type of field of view

If you initially equalize the triple field of view (there is no division of the circle into sectors by color), and then introduce a cuvette with an optically active solution (rotating the plane of polarization) between the analyzer and the polarizer, then the equality of color of all parts of the field of view will be violated. It can be restored by rotating the analyzer through an angle equal to the angle of rotation of the polarization plane by the solution.

Consequently, the difference between two readings corresponding to the photometric equilibrium of the field with and without an optically active solution determines the angle of rotation of the plane of polarization by a given solution or liquid.

Work order

Place an empty cuvette in the polarimeter, close the polarimeter tube with a curtain, and turn on the illuminator. Using a clutch, smoothly rotate the analyzer to obtain a triple field in Fig. 4a or 4b. By moving the coupling along the axis of the eyepiece, achieve a sharp image of the lines dividing the fields of view. Position A– corresponds to the crossed arrangement of the oscillation planes of the analyzer and polarizer; position b– corresponds to the parallel arrangement of the oscillation planes of the analyzer and polarizer.

Place one of the filters (red) in front of the illuminator and observe the triple field through the eyepiece (by moving the coupling along the axis of the eyepiece to achieve a sharp image of the lines dividing the fields).

Smooth rotation of the analyzer using a clutch from the position A or from position b achieve a uniform field of view between positions A And b. Note the zero position of the analyzer along the vernier (take it as the reference point for further measurements), taking it as the average of three measurements.

Place a cuvette containing an aqueous sugar solution into the polarimeter. This will disrupt the uniformity of the color of the field of view.

Smoothly rotate the analyzer using the clutch to again achieve a uniform coloration of the field of view.

Determine the angle of rotation of the plane of polarization, taking it as the difference between the second reading, corresponding to a uniform color, with an aqueous solution of sugar and the first without a solution (zero position).

Sequentially installing light filters in front of the illuminator (install light filters in the direction of decreasing wavelength) to achieve uniform coloring of the field by smoothly rotating the analyzer using a friction clutch. Determine the angle of rotation of the plane of polarization for all indicated colors.

Enter the data into a table (the table is drawn arbitrarily) and draw a graph φ = f(λ).

Draw an appropriate conclusion.

Currently, in the technochemical control of fermentation plants, two main methods for determining carbohydrate content are used: polarimetric and chemical. Colorimetric, chromatographic and polarographic methods for the determination of carbohydrates are also known, described in the following sections of this chapter.

Polarimetric method for determining carbohydrate content

Light is electromagnetic vibrations propagating from a light source in all directions along straight lines (rays). There are natural and polarized rays. A beam whose vibrations occur in all planes perpendicular to its direction is called a natural beam (Fig. 27). A polarized beam is a beam that oscillates only in one plane. The plane in which the beam oscillates is called the plane of oscillation of the polarized beam, and the plane perpendicular to it is called the plane of polarization.

The ability of substances and solutions to change (rotate) the plane of polarization of light is called optical activity. Substances that can rotate the plane of polarization of light are optically active. In contrast, substances that are unable to change the plane of polarization of light are optically inactive. Carbohydrates are optically active substances. The optical activity of carbohydrates is due to the presence of asymmetric carbon atoms in their molecule, i.e. such, all four valence bonds of which are connected to different atoms or groups of atoms. Carbohydrates, like other organic substances containing asymmetric carbon, exhibit optical activity in the dissolved state. The polarimetric method for their determination is based on the property of the optical activity of carbohydrates.

There are substances that change the polarization plane of light clockwise - right-handed - and those that change it counterclockwise - left-handed. Dextrorotatory substances include glucose, sucrose, raffinose, starch, and levorotatory substances include fructose. If a polarized beam passes through a solution of an optically active substance, it rotates the plane of polarization. The plane of polarization of the emerging beam turns out to be rotated by a certain angle, called the angle of rotation of the plane of polarization. The magnitude of this angle depends on the nature of the substance, the thickness of the solution layer (beam path length), the concentration of the solution, the wavelength of the polarized light and temperature.

To compare the optical activity of various optically active substances and to use this phenomenon in analytical practice, the concept of specific rotation was introduced. Specific rotation is the angle through which the plane of polarization rotates under the action of a solution containing 100 g of a substance in 100 ml of solution with a layer thickness of this solution of 1 dm (100 mm); We agreed to measure the specific rotation at a temperature of 20° C in the yellow light of a sodium flame and designate it with the index [a]20D. Each optically active substance is characterized by a certain value of specific rotation when dissolved in a certain solvent. Below are the specific rotation values ​​of some carbohydrates [a]20D

The “+” sign means right rotation, the “-” sign means left rotation.

Freshly prepared solutions of some sugars do not immediately exhibit their characteristic specific rotation. The rotational capacity of such solutions changes slowly in the cold, and quickly under certain conditions (heating, slight addition of alkali). This phenomenon of gradual change in specific rotation is called mutarotation and is explained by the presence of a- and b-forms of sugar molecules. For example, a-d-glucose has a specific rotation [a]20D = +110°, and a-d-glucose +19°. A freshly prepared solution of one of these forms gradually changes rotation until its value reaches an average value corresponding to a specific rotation of +52.5°, at which both forms of glucose are in equilibrium.

The specific rotation of an optically active substance in solution is expressed by the formula

where a is the observed angle of rotation of the plane of polarization; C is the concentration of the optically active substance, g/100 ml of solution; l is the thickness of the solution layer, dm.

Using this formula, you can use the angle of rotation of the plane of polarization a to find the concentration of the optically active substance C. A device that can be used to measure the angle of rotation of the plane of polarization produced by an optically active substance is called a polarimeter.

Polarimeter device

The main parts of a polarimeter are a polarizer and an analyzer. The polarizer is used to obtain polarized light, the analyzer is used to study and detect it. Nicolas prisms are usually used as a polarizer and analyzer (Fig. 28). Such a prism is cut from an Iceland spar crystal and consists of two parts abd and bcd, glued along the bd plane. A ray of light l, entering the crystal, is divided into two polarized rays mp and mo. The ray mo, which has a high refractive index, undergoes total internal reflection from the adhesive layer bd and goes towards or. The mpqs beam with a lower refractive index passes through the prism. Thus, the first Nicolas prism (polarizer) makes it possible to obtain polarized light. A Nicolas prism transmits only light vibrations lying in one specific plane; it does not transmit vibrations lying in a perpendicular plane at all. Therefore, if a beam of light is passed sequentially through two Nicolas prisms located one after the other, then different phenomena can be observed depending on how the second of the prisms is rotated. When the polarizer and analyzer are installed mutually parallel, the light rays pass through both prisms (Fig. 29, a). If the analyzer is rotated 90° (Fig. 29, b), then it will not transmit the rays received in the polarizer; in this case, no light will be observed after the analyzer. This position is called setting the nicoles “in the dark.”

Optical activity can be observed in the simplest polarimeter (Fig. 30) as follows. An optically active substance R is placed between polarizer P and analyzer A, placed “in the dark.” The polarized beam, after passing through this substance, will turn at an angle corresponding to the optical activity of the substance and approach the analyzer not at an angle of 90°, but at a different angle. A light will be visible after the analyzer. To extinguish it, you will have to rotate the analyzer through a certain angle equal to the angle of rotation of the plane of polarization when passing through the substance R. In this way, you can determine the angle of rotation of the plane of polarization. However, such a polarimeter cannot be used for precise work, since the human eye is not able to clearly distinguish complete darkness from very weak light. The eye easily and accurately distinguishes the difference in illumination intensity of two adjacent dimly lit planes. To do this, the polarimeter must have a so-called “penumbral” device; A polarimeter with such a device is called penumbral. You can obtain a penumbral polarimeter by using a Cornu polarizer instead of a conventional polarizer.

The structure of this polarizer is as follows. The Nicolas prism is sawn lengthwise in half along line AB (Fig. 31); then the sharp wedge Aba and Abc are removed from each half, and the two remaining halves are glued together again. The polarized rays emerging from the right and left halves of the prism will not be parallel to one another, but will be located at a certain angle. By turning the analyzer, only one of the beams of these rays can be extinguished, while the other will pass through the analyzer and the field of view will consist of two halves - light and dark (Fig. 32, a and c). If we place the analyzer at the same angle (close to 90°) to both halves of the Cornu prism, we will get the same weak illumination - “penumbra” (Fig. 32, b).

The Cornu prism is not entirely convenient, since the line along which the halves of the prism are glued together is visible, which interferes with observation. This drawback is eliminated in the Lippich polarizer (Fig. 33), which consists of two Nicolas prisms - large P and small H, located so that the smaller one covers half the field of view and is rotated at a small angle relative to the large prism. Moreover, if the analyzer is set “to darkness” relative to a large prism, then one half of the field will be illuminated, and the second will be dimly illuminated. If you set it “to darkness” with a relatively small prism, then the first half of the field will be illuminated, and the second will be darkened. Between these two positions of the analyzer, you can find one in which both fields will be weakly and evenly illuminated (see Fig. 32, b).

In the control of fermentation production, polarimeters are used to determine sucrose - the so-called saccharimeters. In polarimeter-saccharimeters, the analyzer is installed motionless and instead of rotating the analyzer, quartz compensators are used. Quartz is an optically active substance; There are two varieties of quartz - right- and left-handed. If two quartz wedges are placed between the polarizer and the analyzer - one dextrorotatory and the other levorotatory - so that the thickness of the layer of one is equal to the thickness of the layer of the other, then their rotational ability will be zero.

The quartz compensator consists of a dextrorotatory quartz plate P and two left-handed wedges K1 and K2 (Fig. 34, a), of which the longer one, K2, can move parallel to the wedge K1. If both wedges are folded tightly, they form a plate with parallel sides, rotating to the left. The thickness of this plate can be changed by pushing in more or less wedge K2: if you push it in more, the left-handed quartz layer will become thicker than the right-handed quartz plate P, and the entire quartz system (as a whole) will rotate to the left, which will make it possible to compensate for the right rotation of the object under study sugar solution. If wedge K2 is gradually pushed back, then at first we will get a system that does not rotate either to the right or to the left (the sum of the thicknesses K1 and K2 will become equal to the thickness P). Then, with further movement of the wedge, the right rotational ability of the plate P will outweigh and you will get a dextrorotatory system capable of compensating for left rotation.

Another quartz compensation system is also used (see Fig. 34, b), which consists of two wedges K1 and K2. Wedge K2 made of left-handed quartz is movable, wedge K1 made of dextrorotatory quartz is stationary. The wedges with their thinner ends are directed in one direction. The light beam passes through the large thickness of the wedge K2 and through the small thickness of the wedge K1; in this case, the wedge system rotates to the left and can compensate for the rotation of the dextrorotatory solution. If the movable wedge K2 is moved so that a thin part of it is in the path of light, then the right rotation of the wedge K1 will outweigh and the wedge system will rotate to the right, compensating for the rotation of any solution of a levorotatory substance.

A ray of light, passing through wedges K1 and K2, directed with their narrowed ends in one direction, will, of course, be refracted and change its direction and, in addition, will be decomposed into a spectrum. To prevent this from happening, install an additional compensating glass prism C, which is directed with its thin end in the opposite direction compared to the wedges K1 and K2 and therefore restores the previous direction of the light beam (see Fig. 34, b).

The described wedge quartz compensation is called single. Polarimeters with double wedge compensation are often used. Double compensation has two pairs of wedges (Fig. 35). One pair of so-called control wedges K is made of dextrorotatory quartz and serves to measure the rotation of levorotatory substances; the second pair of wedges, the so-called working wedges A, is made of left-handed quartz and serves to measure the rotation of dextrorotatory substances. The advantage of polarimeters with a quartz compensator is the increase in reading accuracy, since the thickness of the quartz wedge when changing its position can be measured more accurately than the angle of rotation of the analyzer.

Light filter. When polarizing colorless or slightly colored solutions, one half of the field of view of the saccharimeter has a slightly yellowish tint, and the other half is bluish. To absorb and thereby eliminate the possibility of the appearance of different colors, a light filter is installed. A tube with a solution of potassium dichromate (K2Cr2O7) or yellow glass is used as a light filter. When polarizing colored solutions, such as molasses, which themselves are yellow in color and absorb rays from the undesirable part of the spectrum, it is not necessary to use a light filter. Therefore, when working with colored solutions, in order to improve illumination of the field of view, a light filter is sometimes removed from the optical system of the polarimeter.

Polarimeter illumination. When using a polarimeter with a moving analyzer, it is necessary to use monochromatic (single-color) light, such as yellow sodium flame light. In this case, it is impossible to use complex white light, since rays of different wavelengths rotate at different angles and rotational dispersion is obtained: for rays with a short wavelength, for example violet, the plane of polarization rotates at a larger angle than for rays with a long wavelength, for example red. Therefore, when using complex white light in such a polarimeter, it is impossible to achieve weak, uniform illumination of both halves of the field of view by rotating the analyzer. The presence of a quartz compensator in the saccharimeter makes it possible to use ordinary white rather than monochromatic light. The rotational dispersion for quartz is almost the same as for sugar solutions. Therefore, white polarized light, decomposed when passing through a sugar solution into component rays with different rotations of the plane of polarization, upon further passage through a quartz compensator again turns into the original white light, and the decomposed rays are again folded into the original beam. Matte incandescent lamps of 100 W are used as a light source for saccharimeters; Currently, saccharimeters are produced in which the lamp is inserted into the device.

Polarimeter scales. There are polarimeters with a circular and linear (empirical) scale. The circular scale is graduated in angular degrees; the linear scale is graduated in percentage of sucrose. Polarimeters with a linear scale are used in the fermentation industry. This scale gives a reading of 100 if 100 ml of an aqueous solution contains 26.00 g of pure sucrose and the solution is polarized in a tube 200 mm long; all operations are performed at 20° C. A sample of 26.00 g is called normal. Thus, if the normal weight x. parts of sucrose dissolved in water and bring the volume of the solution to the 100 ml mark in the flask, then such a solution in a tube 200 mm long will give a reading on the scale equal to 100.0%. If you take a normal sample of a product (for example, molasses or sugar syrup) containing n% sucrose, then obviously the scale will read n%. Therefore, in order to obtain the percentage of sucrose in the product under study directly on the polarimeter scale, the following conditions must be met: 1) the portion of the product under study must be exactly 26.00 g; 2) this sample must be dissolved to a volume of 100 ml; 3) polarization of the solution is carried out in a tube 200 mm long.

The linear scale of the polarimeter makes it possible to count with an accuracy of 0.1 division. A vernier is used to count tenths. In Fig. 36a shows the position of the scale relative to the vernier, corresponding to a reading of +12.7. In this case, the vernier zero is located after 12 full scale divisions, and the seventh division of the vernier coincides with one of the scale divisions. In Fig. 36b shows the position of the vernier, corresponding to the reading -3.2. In this case, the vernier zero is located to the left of the scale by three full scale divisions, and the second division of the vernier coincides with the scale division.

Polarimetric tubes and their use. For polarimetric determinations, the test solution is poured into a polarimetric tube (Fig. 37). The tubes are made of metal (brass, copper) and glass. When studying acidic solutions, use only glass tubes. Tube lengths 100, 200 and 400 mm. A tube 200 mm long is considered normal. The length of the tubes is checked with special calipers that give readings accurate to 0.1 mm. The tubes are covered with cover slips, pressing them to the ends of the tubes with nuts; To seal, rubber rings are placed between the cover slips and the nuts. Before use, cover slips should be washed and wiped dry. The tube must be clean and dry. Dry the tube by pushing a filter paper swab through it with a wooden stick. If the tube was not dried before filling, then it is rinsed 2 times with the test solution. The tubes are filled as follows: the tube is closed at one end with glass and a nut, taken with two fingers, held at an angle (so that air bubbles do not get carried into the tube) and so much liquid is poured into it so that it protrudes over the edges of the tube in the form of a drop. Then they close the tube from above with a cover glass, moving it from one side in a horizontal direction along the side of the tube, as if cutting off a protruding drop of liquid; The tube must be closed quickly and carefully so that no air bubble remains under the cover glass. If this could not be done immediately, then after wiping the glass dry and refilling the tube, this operation should be repeated. Cover slips should not be pressed too hard as this may cause them to become optically active.

Diagram of a saccharimeter. The SU-1 and SU-2 saccharimeters currently produced by the Kyiv Instrumentation Plant have the following diagram (Fig. 38). Light from electric lamp 1 passes through frosted glass 2 or light filter 3, then through condenser lens 4 and enters polarizer 5. The polarized beam from the polarizer passes through two protective glasses 6 and 7, between which a polarimetric tube with the test solution is placed. Behind the protective glass 7 there is a quartz compensator installed, consisting of three wedges: a movable quartz wedge 8, a glass counter wedge 9 and a fixed quartz wedge 10. Next, an analyzer 11 and a telescope are installed, consisting of a two-lens objective 12, 13 and an eyepiece 14. From the electric lamp 1 light also falls into the reflective prism 15 and, reflected, falls on the protective glass 16. This glass scatters light, which then illuminates the scale 17 and vernier 18. The numbers and divisions on the scale and vernier are viewed in an enlarged form using an eyepiece consisting of two lenses 19 and 20. Scale 17 is connected to a movable quartz wedge 8. Thus, the displacement of the movable quartz wedge 8, proportional to the angle of rotation of the plane of polarization, is transmitted to scale 17 and counted using the eyepiece of scale 19-20.

Installation of a saccharimeter. The saccharimeter should be installed on a table in a dark room about 2 m long and 1.2 m wide with walls painted black. If there is no such room, a plywood cap is installed above the polarimeter. The length of the cap is 1.2, width 0.9 and height 0.8 m. The inside of the cap is painted black. A curtain made of dark and dense material is hung over the opening of the cap facing the observer. For ease of operation, the table with the device should be positioned so that the polarizer sits with his back to the window. This eliminates the penetration of daylight into the observer's eye and reduces eye fatigue during observation. The table on which the saccharimeter is installed must have two switches: one to the electric lamp illuminating the polarimetric room, and the second to the electric lamp of the device.

Practice using a saccharimeter. Polarization is carried out as follows. The analyzer eyepiece 1 (Fig. 39) is set for clear visibility and by rotating screw 2, the same intensity of illumination is achieved in both halves of the field of view; The saccharimeter readings should be zero. Then a polarimetric tube filled with the test solution is placed in the saccharimeter chamber 3. The field of view of the saccharimeter is divided along a vertical line into two halves (see Fig. 32, a) - dark and light. Then, by rotating screw 2, the same intensity of illumination of both halves of the field of view is again achieved, after which a countdown is carried out. For greater accuracy, polarization should be carried out 2-3 times in a row (without removing the tube) and the average should be derived from the resulting readings.

The saccharimeter should be kept absolutely clean. The polarimetric tube placed in the saccharimeter must be completely dry and clean. The accuracy of the saccharimeter readings is checked using special control tubes.

Clarifiers

Solutions of the studied products for polarization should be completely transparent and as little colored as possible. The more intense the color of the solution, the more difficult it is to conduct sugar, since the difference in the intensity of illumination of both halves of the field of view is less noticeable. Therefore, colored products are lightened before polarization. Clarification also removes other optically active substances, such as proteins. So, when studying molasses, it is clarified with the Herles reagent. This reagent consists of two solutions: Herles I and Herles II. Gerles I is a solution of lead nitrate, Gerles II is a solution of caustic soda. When studying sugar beets and other sugar-containing products, basic lead acetate is used as a clarifier; for starch-containing products, ammonium molybdate is used.

Automatic saccharimeter

Currently, the Kiev Instrumentation Plant produces a photoelectric automatic polarimeter of type SA designed by V. I. Kudryavtsev. This polarimeter automatically compensates for the rotation of the plane of polarization by the solution and gives a reading of the percentage of sugar. The basic diagram of the saccharimeter designed by Kudryavtsev (Fig. 40) is as follows. Light from the electric lamp 1 through the condenser 2 enters the polarizer 3. Polarized light, the polarization plane of which is vibrated by the magneto-optical modulator 4, passes through the light filter 5, the polarimetric tube with the test solution 6, the diaphragm 7, the quartz compensator 8, 10, the glass counter wedge 9, analyzer 11 and hits the photocell 12. The photocell converts fluctuations in light intensity into alternating electric current.

Unlike a conventional polarimeter, the role of a polarizer and analyzer is performed not by Nicolas prisms, but by polaroids, consisting of a plate coated with a layer of organic iodine compounds; Polaroids are installed in the “cross” position. If there is no tube with a solution of an optically active substance, no light comes out of the analyzer. When a tube with the test solution is placed between the polarimeter and the analyzer, light falls on the photocell, the intensity of which depends on the angle of rotation of the plane of polarization. The rotation of the plane of polarization by the solution under study is compensated by the movement of the movable wedge 8 of the quartz compensator, and this movement is proportional to the angle of rotation of the plane of polarization, and therefore proportional to the concentration of the solution.

The readings of the device are taken on a scale 19, connected to a movable wedge 8 of the quartz compensator and equipped with a vernier 18. For the convenience of reading the division readings, both the scale and vernier numbers are projected onto the translucent screen 21 of the optical projection system, consisting of an illuminator 16, a condenser 17 and a lens 20. The movable wedge and the associated scale are moved by a reversible two-phase motor 13 through a gearbox 14 and a ratchet gear 15. One of the electric motor windings is powered through a step-down transformer 26 and a voltage stabilizer 27 from an alternating current network with a frequency of 50 Hz. The second winding is powered by an alternating current amplifier 22, at the input of which a photocell 23 is switched on. The current to the amplifier is supplied through rectifiers 24 and 25. The electric motor rotates when an alternating voltage is applied to the windings with a frequency of 50 Hz.

Determination of sucrose content in molasses

The sucrose content in molasses is determined as follows. A normal sample of molasses (26.00 g) is transferred with warm water (hereinafter, where not specifically stated, distilled water is meant) into a 100 ml volumetric flask, cooled to 20 ° C, and 8-10 ml is added for clarification. Herles reagent solutions. Herles solutions are added in 4-5 doses; after each addition of a solution of lead nitrate, the same amount of sodium hydroxide solution is added, the mixture is stirred by gently rotating the flask for 1.5-2 minutes, then the clarifier is added again in the same order. The contents of the flask are brought to the mark with water (at a temperature of 20 ° C), shaken and after standing for 2-5 minutes, filtered and polarized in a tube 200 mm long. The polarimeter reading directly gives the percentage of sucrose in the molasses being tested.

Determination of starch content in grain

The starch content in grain is determined using the Evers method, which involves converting insoluble grain starch into soluble starch by heating with dilute hydrochloric acid. A 5.0000 g sample of ground grain (i.e., with an accuracy of 0.0001 g) is transferred quantitatively (through a funnel with the end cut off) into a dry 100 ml volumetric flask, add 25 ml of 1.124% hydrochloric acid, rinsing the glass with it , in which they weighed. The next 25 ml of acid washes away grain particles from the walls of the flask. The mixture is stirred and the flask is placed in a boiling water bath for 15 minutes, and during the first three minutes the contents of the flask are stirred with smooth circular movements. It is necessary to ensure that the water in the bath covers the entire flask, and that the boiling is vigorous and does not stop when the flask is immersed.

After 15 minutes, remove the flask, pour 40 ml of water into it, shake and quickly cool to 20 ° C. To clarify the solution and precipitate proteins, add 4-6 ml of ammonium molybdate solution, add water to the mark, shake and filter through a dry filter into clean dry flask. To prevent evaporation, the funnel is covered with glass. The first 20 ml of the filtrate is poured out and the subsequent ones are immediately polarized in a 200 mm long glass tube.

When examining starch-containing products (grains, potatoes), the polarimeter will not directly show the starch content. In order to calculate the starch content, proceed as follows. From the specific rotation formula we find C:

When using a polarimeter with a linear scale, the formula takes the following form:

where P is the readings of a polarimeter with a linear scale; 0.3468 is the coefficient of transition from the linear scale of the polarimeter to the circular one.

To determine the starchiness of the grain, use a 5 g sample and dissolve the starch to a volume of 100 ml with dilute hydrochloric acid. Using the above formula, the starch content is obtained in 100 ml of solution or (which is the same) in 5 g of sample. The percentage of starch in grain is found by multiplying the calculation result by 20 (100:5 = 20).

Therefore, grain starchiness K can be calculated using the formula

In the indicated formula, all quantities, except P (polarimeter readings), are constant. Therefore, we can write K = kP, where k is a constant coefficient. The coefficients k for different types of starch are somewhat different, since the values ​​of the specific rotation of starch of individual grain crops are different. The k coefficients were calculated by Evers and are called Evers coefficients. These coefficients were calculated for a 5 g sample using a 100 ml volumetric flask and a 200 mm long polarimetric tube.

We present the values ​​of specific rotation and Evers coefficient for various types of starch.

The percentage of starch is obtained by multiplying the polarimeter scale reading by the corresponding Evers coefficient.

Example. When analyzing a corn sample, the polarimeter reading was 28.4. The starch content will be 28.4 * 1.849 = 52.51%.

A. N. Bondarenko and V. A. Smirnov believe that the specific rotation of starches isolated from grain and cereal crops when dissolved in 1.124% hydrochloric acid and determined by the Evers method is the same and equal to 181.0 °. Accordingly, the Evers coefficient will be the same, equal to 1.910.

Polarimetry is a method of physical and chemical analysis based on measuring the rotation of the plane of linearly polarized light by optically active substances. Most optically active substances are organic compounds with an asymmetric carbon atom, that is, one whose affinity units are saturated with four different substituents. Compounds of asymmetric atoms of tetravalent tin, sulfur, selenium, silicon and pentavalent nitrogen are also optically active.

Terms and symbols

If a polarized beam of light is passed through a layer of an optically active substance or its solution, then the plane of polarization of the emerging beam turns out to be rotated by a certain angle, called the angle of rotation of the polarization plane or rotation angle a.

Depending on the nature of the substance, the rotation of the plane of polarization can have a different direction and magnitude.

If the plane of polarization rotates to the right of the observer to whom the light passing through the optically active substance is directed (clockwise), then the substance is called dextrorotatory and a “+” or D sign is placed before the name; if the rotation of the plane of polarization occurs to the left, then the substance is called levorotatory and the sign “-” or L is placed before the name.

The magnitude of the rotation angle depends on the nature of the optically active substance, the thickness of the layer of substance through which the light passes, the temperature and wavelength of the light. The rotation angle is directly proportional to the layer thickness. The effect of temperature is mainly associated with changes in the density of solutions and, in most cases, is insignificant. Typically, the determination of optical rotation is carried out at 20 °C and at a wavelength closest to the yellow line D of the sodium spectrum (589.3 nm).

For a comparative assessment of the ability of various substances to rotate the plane of polarization, the so-called specific rotation of the plane of polarization of monochromatic light, caused by a layer of a substance 1 dm thick, is calculated at a temperature t and a concentration of the active substance of 1 g/cm3. If the determination of specific rotation is carried out at 20 °C and the wavelength of line D of the sodium spectrum, then it is designated by the sign [a]D20.

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

where a is the measured angle of rotation, degrees; l is the thickness of the liquid layer, dm; d is the relative density of the liquid.

For solutions, specific rotation is determined by the formula:

where C is the concentration of the solution, g of dissolved active substance per 100 cm3 of solution.

The specific rotation depends on the nature of the solvent and the concentration of the solution. When replacing the solvent, not only the magnitude of the rotation angle may change, but also its direction. In many cases, the specific rotation is constant only within a certain concentration range. Therefore, when giving the specific rotation value, it is necessary to indicate the solvent and concentration of the test solution.

In the concentration range in which the specific rotation is constant, the concentration of the substance in the solution can be calculated from the angle of rotation using the formula:

It is useful to plot the dependence of the concentration of the active substance on the angle of rotation of the plane of polarization with standard solutions of various concentrations. Using this graph, the concentration of the solution is subsequently determined from the obtained value of the rotation angle.

Polarimetric measurements have wide practical applications. Based on determining the sign and magnitude of rotation of the plane of polarization, one can judge the chemical structure and spatial configuration of optically active substances, and sometimes draw conclusions about the reaction mechanism.

The polarimetric method has long been the main method of control in the sugar industry - the sugar content in the solution is determined by the angle of rotation of the plane of polarization of light.

Polarimeters and saccharimeters

Device and principle of operation. The SM polarimeter is shown schematically in Fig. 191. Light from source 9 passes sequentially through a polarizing device 7, a polarimetric tube 6, an analyzer with a device 5 that rotates the plane of the polarized beam, and enters the telescope 8.

The polarizing device consists of an illuminating lens, a polarizer and a quartz plate located symmetrically relative to the polarizer. The polarizer and quartz plate are in a certain position and are rigidly attached to the frame. The main working part of the device is the analyzer head, consisting of a fixed dial 1, a simultaneously rotating clutch 5 and two verniers 4, an analyzer and a telescope 8.

When measuring, a polarimetric tube 6 with the test substance (solution) is inserted into tube 8. To prevent the penetration of extraneous light, the cutout in the pipe is closed with a rotating curtain. The telescope is used to observe a triple field of view and consists of a lens and an eyepiece. By moving clutch 3, the eyepiece is adjusted to the sharpness of the image of the triple field of view. In the eyepiece shell there are two magnifying glasses 2, which allow, without changing the position of the head, to count the angle of rotation of the vernier relative to the dial scale. On dial 1 there is a degree scale from 0 to 360°. Inside the dial, on a movable sleeve connected to the analyzer, there are two verniers 4 located diametrically. Verniers have 20 divisions with a value of 0.05°. At large angles of rotation, both verniers are used and the measurement result is calculated as the average value of the obtained readings for the first and second verniers.

The polarimetric tube 6 is made of glass. The tube has a bulge necessary to collect air bubbles. At the ends of the tube there are metal tips, onto which caps are screwed, pressing the coverslips. There are rubber gaskets between the lids and cover slips that prevent tension from forming in the glass when the lids are screwed on.

The illuminator 9 consists of a cartridge attached to a bracket. To adjust the lighting, the socket can be moved along the bracket, and the bracket itself can be moved up, down and around the stand. The light source is a 25 W matte incandescent lamp. The light from the light bulb passes through a specially selected filter and polaroids, resulting in the maximum spectral distribution of the beam corresponding to the yellow sodium line.

Carrying out measurements. An empty polarimetric tube is inserted into the telescope, closed with a shutter, the illuminator is turned on and the illumination of the triple field is observed through the eyepiece. If the outer fields are unevenly illuminated, then by moving the illuminator, they are achieved to be illuminated evenly. After installing the illuminator, determine the initial position of the analyzer. By moving the coupling along the axis, a sharp image of the dividing line of the triple field, observed through the eyepiece, is achieved. By smoothly rotating the analyzer using the clutch, we achieve equal darkness in the image of the triple field (Fig. 192), visible through the eyepiece, which determines the initial position of the analyzer. After setting the triple field to equal darkness, a count is made through a magnifying glass using a vernier dial. The initial position does not have to coincide with the zero division of the dial degree scale.

Setting the initial position of the analyzer and counting the degree scale divisions of the dial should be repeated at least 5 times and the average value of the received readings should be taken as the instrument reading.

After this, the polarimetric tube is filled with the test solution, for which, by unscrewing the cap from one end of the tube, it is filled in a vertical position with a transparent solution (the cloudy solution is filtered) until a convex meniscus appears at the upper end of the tube. Then a cover glass is slid over the side, a rubber gasket is applied, and the lid is screwed on. In this case, it is necessary to ensure that there are no air bubbles left in the tube. The outer sides of the cover slips should be transparent and free of any traces of liquid, which can be removed with filter paper.

The filled polarizing tube is inserted into the telescope and covered with a shutter. By moving clutch 3, the dividing lines of the triple field are set to sharpness. Then, smoothly rotating the analyzer using clutch 5, achieve uniform darkness of the triple field image and make a count.

The counting order is as follows: determine how many full degrees the vernier zero is rotated relative to the dial, then determine the number of divisions from the vernier zero to the vernier stroke that coincides with the degree stroke of the dial, and multiply the resulting number of divisions by 0.05. The resulting result is added to the first. The difference in counts corresponding to the photometric equilibrium of the field with and without an optically active substance is equal to the angle of rotation of the plane of polarization of a given solution.

Setting the triple field to equal darkness and counting must be done at least 5 times.

If the polarimeter has a casing for thermostatting the polarizing tube, then before testing the solution, water from the thermostat is passed through the casing for 10 minutes at a temperature of 20 ± 0.1 ° C. If there is no casing, you should work indoors at 20 ± 3 ° C. When determining the specific rotation of an individual liquid, it is kept in a thermostat at 20 ± 0.1 ° C for 30 minutes.

Universal saccharimeter SU-3. In saccharimeters, in contrast to polarimeters, to compensate for the rotation of the plane of the polarized beam with the solution being studied, special quartz wedges are used, according to the position of which the value of the rotation angle is read on the scale.

The saccharimeter scale is graduated in degrees of the International Sugar Scale (°S). One degree of the sugar scale corresponds to 0.26 g of sucrose in 100 cm3 of solution, if the measurement is made in a 200 mm long polarimetric tube at 20 °C. One hundred degrees on the sugar scale (100°S) corresponds to 34.62° angular.

Device and principle of operation. The optical design of the SU-3 universal saccharimeter is shown in Fig. 193.

Light from source 1 passes through frosted glass 2, designed to scatter light (instead, a light filter can be introduced into the optical system). Next, the light flux passes through the condenser lens 3, enters the polarizer 4 and leaves it plane-polarized. Behind the polarizer there are two protective glasses 5 and 6, between which a polarimetric cuvette with the test solution is installed. The movable quartz wedge 7, the glass counter wedge 8 and the fixed quartz wedge 9 form a quartz compensator that compensates for the rotation of the polarization plane. Behind the analyzer 10 there is a telescope, consisting of a two-lens objective 11 and an eyepiece 12, which is focused on the output edge of the polarizer 4. Using the telescope, you can view the dividing line of the field of view of the device in an enlarged form.

The light from the electric lamp also illuminates the scale 15 and the vernier 14 with the help of a reflective prism 17 and protective glass 16, which scatters the light. The numbers and divisions of the vernier and scale are viewed in an enlarged form under a magnifying glass 13, consisting of two lenses. Based on the zero division of the vernier, the scale value corresponding to the state of equal illumination of both halves of the field of view is recorded.

The light source is an A-10 electric lamp (15 W, 12 V), powered from a 220 V AC mains through a 12 V step-down transformer built into the base of the device.

Carrying out measurements. The saccharimeter should be installed on a table in a dark laboratory room with walls painted black, which increases the sensitivity of the observer's eye.

Before starting measurements, the device must be grounded using a grounding screw, plugged into the network and set to zero. Zeroing is performed when there is no polarimetric cell in the chamber. By rotating the handle of the ratchet transmission, the uniformity of illumination of both halves of the field of view is established. In this case, the zero divisions of the scale and vernier must coincide (Fig. 194). Otherwise, move the vernier until its zero division aligns with the zero division of the scale. After checking the zero point of the scale, you can proceed directly to measurements.

A polarimetric cuvette with the test solution is placed into the chamber of the device. This changes the uniformity of illumination of both halves of the field of view. By rotating the handle of the ratchet transmission, the illumination of both halves of the field of view is equalized and the readings are taken with an accuracy of 0.1 scale divisions using a vernier. The reading is repeated 5 times. The result is the arithmetic mean of five measurements.

Readings using a vernier are illustrated by pictures. In Fig. 195 on the left shows the position of the scale and vernier, corresponding to the reading + 11.8 °S (the vernier zero is located to the right of the scale zero by 11 full divisions, and on the right side of the vernier the eighth division of the vernier is combined with one of the scale divisions). The same figure on the right shows the position of the scale and vernier, corresponding to a reading of -3.2 °S (the zero of the vernier is located to the left of the scale zero by three full scale divisions, and on the left side of the vernier, the second division of the vernier coincides with one of the scale divisions).

To determine the mass percentage of sucrose in the test solution, the degrees of the sugar scale measured on the saccharimeter scale should be multiplied by a conversion factor of 0.260 and divided by the density of the test solution.

The SU-3 saccharimeter is equipped with polarimetric tubes 100, 200 and 300 mm long. Tubes 100 mm long are used for colored solutions and when calculating the analysis results, the value obtained on the instrument scale is doubled. Tubes 400 mm long are used when studying solutions of low concentration and when calculating the value on the instrument scale they are halved.

To check the correctness of the readings, a control tube with two normal quartz plates at -40 and +100 °S is attached to the device.

The accuracy of the saccharimeter readings must be checked using a control tube at a steady temperature of 20 °C. If the device is tested at a temperature different from 20 °C, recalculation is carried out using the formula:

where a20 and at are the rotational ability of the quartz plate at 20 °C and the measurement temperature t, respectively, °S.

The discrepancy in the readings of the device being tested and the control tube at the adjusted zero point is the error of this device.

Care of devices and their storage

After finishing work, devices and accessories should be thoroughly wiped with a soft, lint-free cloth; Put a cover on the devices and put the accessories in the case.

The devices should be stored in a dry and clean room at an air temperature of 10-35 ° C and a relative humidity of 30-80%. There should be no harmful impurities in the room air.

It is not allowed to disassemble the devices and clean the optical parts located inside the devices.

Protective glass is cleaned using a wooden stick with a thin layer of absorbent cotton wrapped around it, being careful not to scratch the polished surfaces of the glass.

There are several different types of polarimeters, differing mainly in the nature of the light source and the accuracy of the readings. Let us describe the structure of the device using the example of a circular polarimeter.

Monochromatic light from a source (sodium lamp) located in compartment (1) passes through a polarizer (2), thereby becoming polarized. Next, the beam of polarized light enters the cuvette compartment (3), where the cuvette (4) with the test solution is located. If there is an optically active substance in the cuvette, the plane of polarization of light rotates to the right or left depending on the nature of the substance. Due to the rotation of the plane of polarization, a beam of light can pass through the analyzer (5) and enter the eyepiece (6) only if the analyzer is rotated at the same angle and in the same direction. The analyzer is rotated using the regulator (7).

The readings are taken using scales located on both sides of the eyepiece in the windows (8). The device is turned on and off using the toggle switch (9).

Measurements using a polarimeter are carried out as follows:

1. After turning on, the device warms up for about 10 minutes until a bright yellow light appears in the ventilation holes of the compartment (1).

2. The cuvette (4) is filled without air bubbles with the test solution and placed in the cuvette compartment (3), the lid of the cuvette compartment is closed.

3. Rotate the regulator (10) to set the sharpness of the image in the eyepiece (6). In this case, a yellow circle separated by a vertical line should be visible on a black background; one half of the circle may be darker than the other.

4. By turning the regulator (7), a position is reached in which both halves of the illuminated circle acquire the same brightness and the vertical border of the semicircles disappears.

5. The angle of rotation (“rotation”) of the plane of polarization of the light beam is determined using scales (8). In precision measurements, the angle is counted twice (on the left and on the right scale) with the calculation of the arithmetic mean; for training measurements, you can limit yourself to one reading on the left scale.

The principle of reading is shown in the figure. The moving scale divisions are marked every 0.5 degrees. A fixed vernier allows you to determine the angle with an accuracy of 0.02 o. First, the number of degrees separating the zero of the vernier from the zero of the moving scale is determined. Then, among the divisions of the vernier, there is one that merges into one line with some division of the moving scale. This division of the vernier gives tenths and hundredths of a degree. Both readings add up. So, the image in the figure corresponds to an angle of 3.5 + 0.06 = 3.56 o. Insert a picture.

Polarized light differs from ordinary light in that it oscillates in only one plane, while ordinary light oscillates in all planes of space.

Polarized light can be obtained by passing a beam of ordinary light through a Nicolas prism, the crystal lattice of which stops the vibrations of light in all planes except one, through which it penetrates to the other side of the crystal in the form of polarized light. A Nicolas prism used to produce polarized light is called a polarizer.

If a second Nicolas prism is placed in the path of polarized light, the plane of polarization of which coincides with the first prism, then the polarized light will freely pass through the second prism and illuminate the space behind it. If the second prism is displaced so that the parallel planes of polarization are disrupted, polarized light will not be able to fully pass through the second prism and the space behind it will be partially or completely darkened (depending on the degree of displacement). The second prism, located in the path of polarized light, is called an analyzer.

If a layer of liquid that does not contain optically active substances, for example distilled water, is placed between the polarizer and the analyzer, installed so that polarized light passes through the analyzer, then the plane of vibration of the polarized light will not deviate and the beam will pass through the analyzer in the same way as in the case when there was no layer of liquid.

The plane of polarized light will shift by a certain amount if, at the initial position of both prisms, a layer of liquid containing an optically active substance, for example glucose, is placed between them. In this particular case, the shift will occur by an angle a and the light will not be able to pass through the analyzer.

In order for the light to pass through the analyzer, the latter must be rotated by the same angle a so that the plane of polarized light again coincides with the plane of the analyzer. By placing a scale graduated in degrees in front of the analyzer, you can measure the angle of deflection, and at the same time the angle of rotation of the plane of polarized light a.

Devices built on the principle described above are called polarimeters and with their help the angle of rotation of polarized light is determined. The schematic structure of the simplest polarimeter is shown in Fig. 95, a, b.

Rice. 95.
a, b - schematic representation of the polarimeter. Explanations in the text.

The mirror (1) serves to direct the light beam into the device, and the orange filter (2), installed in front of the polarizer, transmits only yellow light, since polarimetry is preferably carried out with yellow, and even better, with monochromatic light from a sodium lamp. The polarizer (3) serves to polarize the light beam, and the tube (4) is intended to be filled with the liquid under study. The analyzer (5) and the associated rotary disk (6) are used for rotation at the appropriate angle. The polarimeter eyepiece (7) is needed to view the field of view, and the vernier scale (8) and eyepiece (9) are needed to record the rotation angle.

The field of view of a polarimeter is usually divided into two equal parts (Fig. 96, a). When there is an optically inactive liquid in the tube, both halves of the visual field are illuminated equally, since the analyzer does not block light. When there is an optically active solution in the tube, one half of the visual field is darkened because the plane of polarized light is deflected and the light does not completely pass through the analyzer. By turning the disk to which the analyzer is attached, the latter is rotated through an angle a corresponding to the rotation of the plane of polarized light, while both halves of the visual field are illuminated equally. The angle is determined on the instrument scale.

In some devices, the field of view is divided not into two, but into three parts - a central strip and two side segments on the sides (Fig. 96, b). These devices are more convenient than polarimeters with two parts of the field of view. With an optically inactive liquid, all three parts of the field of view are illuminated equally. With an optically active liquid in the tube, the plane of polarized light is deflected and the central strip of the visual field is darkened.

The disk with the analyzer is rotated until all three parts of the field of view receive the same illumination, after which the angle of rotation is noted.

For an optically active substance, the angle of rotation of polarized light depends on a number of factors:
1) on the nature of the substances, each of which has its own characteristic angle of rotation, which is called “specific rotation” and is designated ;
2) on the concentration of the optically active substance;
3) on the length of the tube in which the liquid under study is placed (layer thickness).

The relationship between these quantities can be expressed by the following equation:

where 1) is the specific rotation of the substance;
2) l - layer thickness;
3) C - concentration of optically active substance. Thus, knowing the specific rotation of the substance and the length
tube, you can determine its concentration in the solution. The length of the tube may vary in different devices. This value is indicated in the instructions for use.
denotes a specific angle of rotation (specific
rotation), i.e. the angle of rotation of polarized light at a concentration of one gram of a substance per 1 ml, with a tube length of 10 cm, a temperature of 20°, with yellow sodium light (D - spectrum line).

Polarimeter P-161 is currently not produced, but is used in many laboratories. It is very easy to use and is designed to determine sugar in urine. The device consists of three main parts: a stand, a polarimeter tube and a cuvette tube.

The cuvette tube is made of ceramic, onto which caps with rubber gaskets and protective glasses are screwed on so that the test liquid does not leak out. The cuvette made of opaque ceramic allows it to be installed in the open bed of a polarimetric tube. The ceramic cuvette tube is unbreakable, acid-resistant, and its walls are less reflective than glass. The length of the ceramic cuvette tube is 94.7 mm, designed in such a way that twice the reading number gives the direct sugar content in 100 ml of urine or, accordingly, the sugar content as a percentage.

A more complex device is circular polarimeter type SM, allowing you to determine the angle of rotation within ±360°. A beam of light from an incandescent lamp passes through a hole in the illuminator casing through a light filter, a lighting condenser lens that produces a beam of parallel rays, and then through a polarizer placed between two protective glasses. Polarized light passes through a diaphragm with a quartz plate positioned so that only the rays from the middle part of the beam pass through it. The plate deflects the plane of polarization of light passing through the polarizer by 5-7°.

By turning the analyzer, the illumination of the photometric field is adjusted, which in the SM polarimeter is divided into three parts (Fig. 96, b). The darkness of the fields is determined through a telescope and recorded either in the absence of a tube with the test solution or with a tube filled with water.

Rice. 96. Field of view of a polarimeter.
a - with two fields; b - with central and lateral segments.

A complex and high-precision productive device is a polarimeter manufactured by Perkin-Elmer.

Polarimeter of this company model 241 MS has a monochromator. Monochromatic light passes through a polarizer, sample cell, and analyzer and enters a photomultiplier tube. The device operates on the principle of optical zero reference. The polarizer and analyzer are installed in the zero position on the vertical optical axis. When an optically active sample is placed in the light beam, the analyzer is rotated using a servo system until the optical zero is again established. The rotation angle is measured on a scale and the result is indicated on a digital display.

Measurement of the angle of rotation of the substances under study can be carried out in the rays of a high-pressure mercury lamp, and also, if necessary, in the light of a sodium lamp with a wavelength of 589 nm, a deuterium lamp with a wavelength of 250-420 nm, or an iodine-quartz lamp (350-650 nm). The last three lamps are mounted in a separate block, which is easily installed in the device and allows you to quickly switch the required light source.

For studying small volumes of solutions there is a special microcell of 0.2 ml. Instrument accuracy: ±0.002° for rotational quantities<1°.

Dimensions of the device: length - 950 mm, width - 280 mm, height - 350 mm. Weight - about 50 kg.