According to the law of refraction, the refractive index n Is the angle of incidence of light i Sine and refraction angle of r The ratio of the sine of, that is:
Where, n Is the refractive index, sin i Is the sine of the incident angle of light, sin r Is the sine of the refraction angle of the ray [2] 。
The refractive index of matter is related to temperature and incident light wavelength. The refractive index decreases with the increase of the temperature of the transparent material; The shorter the wavelength of the incident light, the greater the refractive index. Refractive index n Indicates that D is the D line of sodium spectrum, t Is the temperature at the time of measurement.
Definition of refractive index: n ＝ c / v 。 Among them, v Represents the speed of light in different media. For example, the speed of light in the glass is vacuum 0.5 times, then the refractive index of glass relative to vacuum is 2. The refractive index of a substance varies with temperature or wavelength of light. The refractive index decreases with the increase of the temperature of the transparent material; The shorter the wavelength of light, the greater the refractive index. As a standard for purity of liquid substances, refractive index is more reliable than boiling point. The refractive index can be used to identify unknown compounds and determine the composition of liquid mixtures. So the concentration should also be measurable. In fact, there are a lot of empirical data, which can be compared with the corresponding table.
When light enters from one transparent medium to another, the propagation speed of light changes due to different densities of the two mediums, that is refraction In general, the refractive index refers to the ratio of the speed of light in the air to the speed of the test object. According to the law of refraction, the refractive index is sine The ratio to the sine of the broken angle, namely: n ＝sin i /sin r 。 Where, n Is the refractive index, sin i Is the sine of the incident angle of light, sin r Is the sine of the refraction angle. The refractive index is Organic compound One of the most important physical constants, it can be measured accurately and conveniently. As a standard for the purity of liquid substances, it is more reliable than the boiling point. Using refractive index, unknown compounds can be identified. If a compound is pure, then other compounds under consideration can be excluded according to the measured refractive index to identify the unknown. The refractive index is also used to determine the composition of the liquid mixture. When distilling two or more liquid mixtures and Components When the boiling points of are close to each other, the refractive index can be used to determine the composition of the fractions. Because when the composition structure is similar to Polarity , refractive index and Amount of substance Between components linear relationship 。 For example, from 1mol carbon tetrachloride And 1mol toluene The composition of the mixture is 1.4822, while that of pure toluene and pure carbon tetrachloride at the same temperature is 1.4944 and 1.4651 respectively. Therefore, this linear relationship can be used to obtain the composition of the fraction when fractionating the mixture [3] 。 The refractive index of a substance is not only related to its structure and light wavelength, but also affected by temperature pressure And other factors. Therefore, the expression of refractive index must indicate the light used and the temperature at the time of measurement n express. D is based on D line of sodium lamp (5893A0) As light source, t Is the temperature corresponding to the refractive index. For example, it indicates the refractive index of the medium to the D line of the sodium lamp at 20 ℃. Since the atmospheric pressure usually has no significant effect on the refractive index, the pressure effect is only considered in very precise work. Generally speaking, when the temperature increases by 1 ℃, the refractive index of liquid organic compounds decreases by 3.5 × 10 -4 ~5.5×10 -4 。 The temperature coefficient of some liquids can reach 7 × 10, especially when the temperature measured for refractive index is close to its boiling point -4 。 In practical work, the refractive index measured at one temperature is often converted into the refractive index at another temperature. For the convenience of calculation, 4 × 10 is generally used -4 Is the temperature change constant. The value obtained from this rough calculation may be slightly wrong, but it is of reference value. Fundamentals
Refractive index tester for petroleum products Abbe refractometer and operation method
When light enters medium B from medium A, if medium A is a sparse substance to medium B, that is n (A)＜ n (B)＝1/sin r It is also a constant, and its relationship with refractive index is sinA/sinB=n (B)/n (A). Obviously, under certain wavelength and conditions, it can be seen that critical angle , you can get the refractive index, which is usually used Abbe (Abbe) Basic optical principle of refractometer. In order to measure the value, Abbe refractometer adopts the method of "half bright and half dark", that is, let the monochromatic light shoot from medium A to medium B from all angles of 0 °~90 °. At this time, the whole area within the critical angle of medium B has light passing through, so it is bright; All areas beyond the critical angle are dark because no light passes through, and the boundary between light and dark areas is very clear. If an eyepiece is used to observe above medium B, a half bright and half dark image with very clear boundary can be seen.
The critical angle varies with the medium boundary line The location is also different. If on eyepiece A cross line is engraved in the middle, and the relative position of medium B and the eyepiece is changed so that the boundary between the light and dark areas always coincides with the intersection point of the cross line. The refractive index can be obtained by measuring its relative position (angle) and converting it. and Abbe refractometer The reading engraved on the scale is the converted refractive index, so it can be read directly. At the same time, Abbe refractometer has an anti dispersion device, so it can directly use daylight, and the measured figure is the same as that of sodium light. These are the advantages of Abbe refractometer. How to use Abbe refractometer: first connect the refractometer to the constant temperature bath. After the constant temperature, separate the right angle prism, and gently scrub the upper and lower mirror surfaces with silk or lens cleaning paper dipped in a small amount of ethanol or acetone. After ethanol or acetone volatilizes, add a drop of distilled water to the mirror below, and close Prism , adjust the reflector to make it inside field Bright, turn the prism until a boundary or colored light band is observed in the mirror; If there are colored light bands, adjust the dispersion to make the light dark boundary clear, and then rotate the right angle prism to make the boundary just pass through the intersection of the "cross". Record the reading and temperature, and compare the average refractive index of pure water measured twice with the standard value of pure water (=1.33299) to obtain the calibration of the refractometer, and then measure the refractive index of the liquid sample to be measured in the same way. The calibration value is usually very small. If the value is too large, the whole instrument must be recalibrated. matters needing attention
Pay attention to the following points when using refractometer:
Refractometer (2) The instrument shall not be exposed to sunlight during use or storage, and shall be covered with black cloth when not in use.
(3) The prism of the refractometer must be protected to avoid nicks on the mirror surface. When dropping liquid, the end of the dropper must not touch the prism.
(4) The mirror surface shall be cleaned before dropping the sample each time; After use, also apply acetone Or 95% ethanol wash the mirror surface, and then close the prism after drying. (5) The liquid that corrodes or dissolves the prism glass, insulation sleeve metal and the adhesive between them should be avoided.
Finally, it should be pointed out that Abbe refractometer cannot be used at higher temperatures; It is difficult to measure volatile or absorbent samples; In addition, the requirements for the purity of samples are also high.
Temperature correction of refractive index
Generally speaking, when the temperature increases by 1 ℃, the refractive index of liquid organic compounds decreases by 3.5 × 10 -4 ~5.5×10 -4 。 The temperature coefficient of some liquids, especially when the temperature of the refractive index to be calculated is close to its boiling point, can reach 7 × 10 -4 。 In practical work, the refractive index measured at one temperature is often converted into the refractive index at another temperature. In order to facilitate calculation, 4.5 × 10 -4 As the temperature change constant. The value obtained from this rough calculation may be slightly wrong, but it is of reference value. In other words, the refractive index decreases with the increase of temperature. For every 1 ℃ change, the refractive index changes about 0.00045. We can calculate the refractive index corrected to 20 ℃ by the following formula:
n D (t) = n D (20)－0.00045( t －20)。
Among them, n D ( t ）Is at a temperature of t The refractive index measured experimentally at ℃. This shows that when the experimental temperature is higher than 20 ℃, n D (20) Ratio n D ( t ）Large; When the experimental temperature is lower than 20 ℃, n D (20) Then compare n D ( t ）Small.
Example: known n D ( t )＝1.3667， t ＝ 25.2 ℃, calculated n D (20)。
By formula n D ( t )＝ n D (20)－0.00045( t - 20), then
n D (20)＝1.3667＋0.00045(25.2－20)
=1.3667＋0.00045 × 5.2
=1.36904。
Factors affecting refractive index
Influence of light wavelength
The refractive index of the material varies with the wavelength of the light. The longer the wavelength, the smaller the refractive index, and the shorter the wavelength, the larger the refractive index. The light source for determination is usually white light. When white light is refracted through prism and sample solution, due to different wavelengths of colored light, the degree of refraction is also different. After refraction, it will be decomposed into a variety of colored light. This phenomenon is called dispersion. The dispersion of light will make the light dark boundary of the field of vision unclear, resulting in measurement errors. In order to eliminate dispersion, the lower end of the observation lens tube of Abbe refractometer is installed Dispersion compensator 。 Effect of temperature
The refractive index of the solution changes with temperature, and decreases with the increase of temperature; The refractive index increases when the temperature decreases. The scale on the refractometer is engraved at the standard temperature of 20 ℃, so it is better to measure the refractive index at 20 ℃. Otherwise, the measured results shall be temperature corrected. When the temperature exceeds 20 ℃, add a positive number; When the temperature is lower than 20 ℃, subtract the correction number.
