Refractive Index

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  • Topic: Refractive index, Refraction, Snell's law
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  • Published : February 5, 2013
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Refractometry: Determining Composition of Water and 2-Propanol in a Binary Mixture

Holly Ford
Physical Chemistry Lab, Belhaven University, 1500 Peachtree Street, Jackson, MS

Abstract
The purpose of this lab was to learn how to properly use the refractometer and determine the composition of 2-propanol and water in an unknown binary mixture. This was carried about by measuring the refractive index of each substance at different temperatures (22°C, 34°C, 46°C, 58°C and 70°C) using the Abbe refractometer. By obtaining the densities, the information can be plugged in to several equations to determine the unknown composition. The binary mixture was determined to 71% water and 29% 2-propanol.

Introduction
Refraction is the bending of a wave when it enters a medium where it’s speed is different, and this results in the change of direction in which light propagates. When light passes through two isotropic media, some portion of light approaching the interface between them at an incident angle α is reflected back to the first medium while the rest propagates into the second medium at an angle of refraction β (Fig. 1).1 Snell’s law, or the law of refraction, is a formula used to describe

the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between to different isotropic media.2 This law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the speed of light in the original medium to the speed of light in the refraction medium: (1)

where n1 is the refractive of medium 1 and n2 is the refractive index of medium 2.3

Figure 1. Refraction of Light

Snell’s law is often stated in the indexes of refraction of the two media rather than speeds of light in the media.3 The use of measurements of index of refraction have been reported as a quick, convenient, and accurate way to estimate the density of liquid mixtures and the molar refraction of compounds.4 For a transparent medium, the refractive index is the ration of the speed of light in a vacuum to the speed of light in that medium.5 It is a parameter with no units, and is equal to 1 for a vacuum and larger for other materials and is dependent upon temperature, wavelength, pressure and concentrations of species if it s a mixture.6 A refractometer is a laboratory or field device for the measurement of an index of refraction, although instruments also exist for determining the index of refraction of a solid. The refractometer is regulated by a water bath above the ambient temperature to reduce the fluctuation of the refractive index because it is easily biased to change in temperature. For measurement, wavelength is usually that of yellow light (589.6 nm), and temperature specified is 25°C because it is easier to maintain with a constant temperature bath in normal laboratory conditions.6 This is experiment used a prism system called the Abbe’s refractometer, shown in Fig. 2., which has two optical Amici prisms to rotate in opposite directions with a thin space for liquid sample between them. An Amici prism is designed to produce a limited amount of dispersion but no angular deviation of light and are used to obtain the same result with white light that would be using sodium arc illumination. This system is slightly less precise than other refractometers and requires less exact temperature control. Temperature can be maintained by using a thermostat bath by means of a pump passing distilled water.

function of angle "max, which is different for samples with different refractive indices n1. The simple readout from the scale of refractometer then provides the refractive index directly, or it can be readily determined using a conversion table.

Fig. 2. The schematic of the Abbe's refractometer.
Figure 2. The schematic of the Abbe’s refractometer

The refraction index depends on the wavelength of light, because the speed...
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