How to measure the refractive index
When light enters medium B from medium A, if medium A is a hydrophobic substance to medium B, that is, nA< nP=1/sin is also a constant, and its relationship with the refractive index is: Indicates. Obviously, under a certain wavelength and certain conditions, it can be seen that the refractive index can be obtained by measuring the critical angle, which is the basic optical principle of the commonly used Abbe refractometer.
In order to measure the value, the Abbe refractometer adopts the "half-light and half-dark" method, which is to let the monochromatic light enter the medium B from the medium A from all angles of 0-90°, at this time, the entire critical angle in the medium B is Light passes through all areas, so it is bright; while all areas outside the critical angle do not pass light, so it is dark, and the boundary between the light and dark areas is very clear. If you observe with an eyepiece above the medium B, you can see a semi-dark image with a very clear boundary.
The medium is different, the critical angle is also different, and the boundary position of the light and dark areas in the eyepiece is also different. If a "cross" line is engraved in the eyepiece, change the relative position of the medium B and the eyepiece, so that each time the boundary between the light and dark areas always coincides with the intersection of the "cross line", by measuring its relative position ( Angle) and after conversion, the refractive index can be obtained. The reading engraved on the scale of the Abbe refractometer is the converted refractive index, so it can be read directly. At the same time, the Abbe refractometer has a dispersion-eliminating device, so it can directly use sunlight, and the measured numbers are the same as those measured by sodium light. These are the advantages of the Abbe refractometer.
How to use the Abbe refractometer: first connect the refractometer to the constant temperature tank, after constant temperature, separate the right-angle prism, and gently wipe the upper and lower mirror surfaces with a small amount of ethanol or acetone on silk or lens cleaning paper. After the ethanol or acetone evaporates, add a drop of distilled water to the mirror below, close the prism, and adjust the mirror to make the field of view inside the mirror bright.
Turn the prism until a boundary line or a colored light band appears in the mirror; if a colored light band appears, adjust the dispersion to make the boundary between light and dark clear, and then turn the right-angle prism so that the boundary line happens to pass through the intersection of the "cross". Record the reading and temperature, repeat twice to measure the average refractive index of pure water and compare it with the standard value of pure water (=1.33299), the calibration plant of the refractometer can be obtained, and then the refractive index of the liquid sample to be tested can be measured in the same way . The correction value is generally small, and if the value is too large, the entire instrument must be recalibrated. The following points should be paid attention to when using the refractometer:
(1) Abbe's measuring range is from 1.3000 to 1.7000, and the precision is ±0.0001; when measuring, attention should be paid to whether the temperature of the insulation jacket is correct. If you want to measure to ±0.0001, the temperature should be controlled within the range of ±0.1°C.
(2) The instrument should not be exposed to sunlight during use or storage, and should be covered with a black cloth when not in use.
(3) The prism of the refractometer must be protected to avoid scratches on the mirror surface. When adding liquid, the end of the dropper must not touch the prism.
(4) The mirror surface should be cleaned before each drop of the sample; after use, the mirror surface should also be cleaned with acetone or 95% ethanol, and the prism should be closed after drying.
(5) Liquids that corrode or dissolve the prism glass, insulation metal and the adhesive between them should be avoided.
Finally, it should be pointed out that the Abbe refractometer cannot be used at higher temperatures; it is difficult to measure volatile or water-absorbing samples; in addition, the requirements for the purity of the samples are also high. Generally speaking, when the temperature increases by one degree, the refractive index of the liquid organic compound decreases by 3.5×10-4—5.5×10-4. Some liquids, especially when the temperature whose refractive index is to be obtained is close to its boiling point, have a temperature coefficient of up to 7×10-4. In practical work, the refractive index measured at a certain temperature is often converted into the refractive index at another temperature. For the convenience of calculation, 4.5×10-4 is generally used as the temperature change constant. The value obtained by this rough calculation may have a slight error, but it is of reference value. In other words, the refractive index decreases with the increase of temperature, and the refractive index changes by about 0.00045 for every 1 degree Celsius change. We can calculate the refractive index corrected to 20°C by the following formula: nD(t) = nD(20) - 0.00045(t-20°C)
where nD(t) is the experimentally measured refractive index at temperature t. This shows that when the experimental temperature is higher than 20°C, nD(20) is larger than nD(t); while when the experimental temperature is lower than 20°C, nD(20) is smaller than nD(t).
Example: Given nD(t) =1.3667, t=25.2℃, calculate nD(20).
nD(t)=nD(20) - 0.00045(t-20℃)
nD(20)=1.3667+0.00045(25.2℃-20℃)
=1.3667+0.00045 × 5.2
=1.36904 Effect of light wavelength
The refractive index of a substance varies with the wavelength of light, with longer wavelengths having a smaller refractive index and shorter wavelengths having a larger refractive index. The light source used for measurement is usually white light. When the white light is refracted by the prism and the sample liquid, due to the different wavelengths of each color light, the degree of refraction is also different. After refraction, it is decomposed into a variety of color lights. This phenomenon is called dispersion. The dispersion of light will make the dividing line between light and dark in the field of vision unclear, resulting in measurement errors. In order to eliminate dispersion, a dispersion compensator is installed at the lower end of the observation tube of the Abbe refractometer.
Effect of temperature
The refractive index of a solution changes with temperature, the refractive index decreases as the temperature rises, and increases as the temperature decreases. The scale on the refractometer is engraved at the standard temperature of 20°C. Therefore, it is best to measure the refractive index at 20°C. Otherwise, temperature correction should be performed on the measurement results. When it exceeds 20°C, add the correction number; when it is lower than 20°C, subtract the correction number
