Determination of specific optical rotation
Some organic compounds, especially many natural organic compounds, appear spiral in molecular structure. Such a configuration enables the plane of vibration of the polarized light to be rotated by a constant angle α, wherein the polarized light is rotated to the left by a left-handed substance, and the polarized light is rotated to the right by a right-handed substance.
The angle α is the corresponding optical rotation. The degree of optical rotation is proportional to the length of the sample tube (ie, the thickness of the solution). Within a certain range, it is approximately proportional to the concentration of the solution. In addition, the optical rotation is closely related to factors such as solvent, temperature and wavelength.
Therefore, it is necessary to use a standard to compare the optical rotation of optically active compounds. This standard is the specific optical rotation, which is used to examine the optical density per unit concentration and unit thickness.
Specific optical rotation is one of the important physical constants of optically active substances, and is often used to indicate the optical rotation of optically active compounds. By measuring the optical rotation, the purity of the optically active substance can be checked and its content can be determined. The basic structure for measuring the optical rotation and its measurement principle are as follows.
When the light passes through the polarizer from the light source and then passes through the optical tube containing the optically active substance, the polarized light of the material cannot pass through the second prism and must be rotated to pass the analyzer. The angle of rotation of the analyzer is indicated by the angle of movement of the scale. This reading is the optical rotation α of the substance at this concentration.
In addition to the nature of the sample itself, the optical rotation a is related to the concentration of the sample solution, the solvent, the length of the optical tube through which the light passes, the temperature, and the wavelength of the light. In general, temperature has little effect on the optical rotation measurement, and it is usually not necessary to place the sample in a thermostat. Therefore, the specific optical rotation [α] λ t is often used to indicate the optical rotation of each substance. The specific optical rotation [α] λ t at a certain wavelength and temperature can be expressed by the following relationship:
Specific optical rotation of pure liquid = [α] λ t = α / (d · l)
Specific optical rotation of solution = [α] λ t =100·α/(c·l)
α: angle reading of the rotation of the scale disc (ie, optical rotation), measured by a polarimeter;
λ: the wavelength of light of the light source;
d: density of pure liquid (unit: g/cm3);
l: length of the optical tube (unit: dm);
c: concentration of the solution (mass of the sample contained in the 100 mL solution, g);
t: Temperature at the time of measurement (°C).
operating:
1. Preparation of solution samples:
Accurately weigh 0.1 to 0.5 g of pure sample on the analytical balance, dissolve it, and place it in a 25 mL volumetric flask. The solvent is usually selected from water, ethanol, chloroform, and the like. After the solution is prepared, it must be transparent and free of solid particles, otherwise it must be filtered through dry filter paper. When the optical rotation is directly measured with a pure liquid, if the optical rotation angle is too large, a shorter sample tube can be used.
2. Loading of samples:
Seal one end of the sample tube with a glass cover and a copper cap, then erect the tube, open the opening, inject the prepared solution or pure liquid sample into the sample tube, and make the solution form a convex surface due to surface tension. The center is raised above the top of the tube, and the glass cover on the sample tube is covered. No air bubbles can be introduced, and then the copper cap is covered so that it does not leak.
Note: There is direct contact between the glass cover and the glass tube, and a rubber gasket is required between the copper cap and the glass cover. Do not screw too tightly between the copper cap and the glass cover, as long as the liquid does not flow out. If the screw is too tight, the glass cover will generate a torque, which will cause a gap in the sample tube, which will affect the rotation.
3. Correction of the polarimeter zero point Correct the zero point of the polarimeter before measuring the sample. Wash the sample tube, install distilled water, make the liquid surface protrude from the nozzle, and cover the lid. Dry the sample tube, put it into the polarimeter, cover the lid; then turn on the sodium light, then adjust the focus of the instrument's eyepiece so that the three parts of the field of view are bright and dark, and the boundary is clear (as shown in Figure 2.30). , write down its readings.
4. Determination of optical rotation Before the measurement, the sample tube must be washed 2 to 3 times with the test solution to avoid any other substances. According to the above method, the sample is placed in an optical tube to measure the optical rotation, and the difference between the obtained reading and the zero point is the optical rotation of the substance. Make a note of the length of the sample tube and the temperature of the solution at this time, and then calculate the specific optical rotation according to the formula.
5. After the experiment, wash the optical tube and fill it with distilled water.
note:
1. The minimum scale of the dial of the general polarimeter is 0.25°, plus the upstream mark, which can be read to 0.01°.
2. When measuring the zero point (or the optical rotation of the optically active compound), the operation must be repeated at least five times and the average value is taken. If the zero points differ greatly, they should be recalibrated.
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