Question

purpose of creating standard curve for refractive index in smple and fractional distillation experiment?

Answer 1

A refractometer is used in this experiment to measure mole fractions. To obtain mole fraction information from refractive index measurements, the refractometer must be calibrated (i.e., construct a graph of refractive index versus composition) using solutions of known composition. This calibration curve can then be used "in reverse" to find the composition of unknown mixtures from an experimental measurement of the refractive index.

            One of the most common uses of the refractive index is to compare the value you obtain with values listed in the literature. This comparison is used to help confirm the identity of the compound and/or assess its purity.

            Determining the concentration of a solute in a solution is probably the most popular use of refractometry. For example, refractometer-based methods have been developed for determining the percentage of sugar in fruits, juices, and syrups, the percentage of alcohol in beer or wine, the salinity of water, and the concentration of antifreeze in radiator fluid.

            Additional information

                        1. Refractive indices

The refractive index (n) is a physical constant that can be used to characterize liquids. It can be used to determine the optical performance, purity, concentration and dispersion etc. Therefore, refractometer is widely used as one of the indispensable tools in petrology, oil and fat analysis, pharmaceutical, paint, food, chemical and sugar making industries as well as in universities and industrial research centers for R&D and quality control (QC).

Refractive index is the ratio of the velocity of light in air to the velocity of light in the liquid. It is also equal to the ratio of the sine of the angle of incidence (θ₂) to the sine of the angle of refraction (θ₁).

          Velocity in air          sin θ₂

             n = ——————— = ———

        Velocity in liquid        sin θ₁                  

                                               

Fig 1. Refraction of light

The angle of refraction is also a function of temperature and the wavelength of light. Because the velocity of light in air is always greater than that through a liquid, the refractive index is a number greater than 1.

The measurement is made on a refractometer using a few drops of liquid. Temperature correction must be applied to the observed reading by adding 0.00045 for each degree above 20 °C:

                 n²⁰_D = n^t_D + 0.00045 (t-20 °C)

The superscript 20 indicates that the refractive index was measured at 20 °C and the subscript D refers to the yellow D-line from a sodium vapor lamp, light with a wavelength of 589 nm.

PROCEDURE

There are many different designs of refractometers. The most common is the Abbé design shown in Fig. 2.

Fig. 2. Abbé refractometer

To determine the refractive index, 2 -3 drops of the sample are placed on the open prism using a Pasteur pipette (please be careful not to scratch the prism). The prism is closed and the light is turned on and positioned for maximum brightness as seen through the eyepiece. The knob is turned so that the line separating the dark and light areas is at the crosshairs, as in Fig. 3 (b). Then the chromatic adjustment is turned until the demarcation line is sharp and colorless. At this point the value of the refractive index can be read on the dial. Read the temperature on the thermometer attached to the refractometer, and make the appropriate temperature correction to the observed index of refraction.

Fig. 3. (a) View when index knob is not adjusted. (b) View into refractometer when properly adjusted (c) View when chromatic adjustment is correct.

                       

Fig. 4. Refractometer reading scheme