Refractive Index Essay

Published: 2020-01-28 14:41:04
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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 its 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 Snells 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

Snells 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 Abbes 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 Abbes refractometer.
Figure 2. The schematic of the Abbes refractometer

The refraction index depends on the wavelength of light, because the speed of light n this experiment, refractometry is used for the quantitative determination of the I waves depends on their wavelength. Light of different colors (different wavelengths) is bending at different angles even if it comes at the same angle of incident composition of binary compounds, specifically 2-propanolall the wavelengths, produces a and water. Knowledge of physical (dispersion). As a result, the white light, that comprises ainbow after passing through binary compounds droplets of moisture in associated with and rthermodynamic properties of the optical prism (orformed by components the atmosphere).

However, despite the beauty of a rainbow, this is an unwanted effect in hydrogen bonds are important for theoretical aspects of the laboratory. Determination of these refractive index determination. It causes the smearing of an interface between the illuminated and dark regions refractive index and densities increase the precision of a factors is done by measuring thein the Abbes refractometer. To of two components and their measurement, it is therefore preferable to use a monochromatic light (light of a single mixture over several temperature intervals, computing the specific refraction and calibration curves.

Experimental

The determination of the index of refraction is made using the Abbe refractometer shown in Fig. 3. The methods to use the machine are given by Experiments in Physical Chemistry.6

Figure 3. Abbes Refractometer

A pipette is used to fill the space between the prisms of the refractometer with a 2-propanol. The scale knob was turned to get a clear interface between the illuminated and dark regions; the integer was read from the rough scale and decimal from the refined scale. The substance was allowed to equilibrate for thirty seconds before the integer was read each time, then the surface was cleaned and wiped with a chemical wipe. The same method was repeated for water and propanol over five temperature readings on a Celsius scale, 22, 34, 46, 58 and 70. Once each substance had been measured three times, an average was taken to determine the index of refraction of each component at the specified temperatures. The density for 2-propanol of 2propanol and water were taken from literature values, and the mixture was determined by measurement in lab. Once the refractive index and density is obtained, the Lorents-Lorenz equation can be used to determine the specific refraction of each substance: where n is the refractive index, N is the number of molecules per unit volume and alpha is the polarizability. (2)

Finally, the molar composition of 2-propanol and water in the binary mixture were calculated with the following formula.
(1/ ρ mixture)*(η mixture ²-1/ η mixture ²+2) = x1(H2O)*r1(H20) + x2(2-propanol)*r2(2-propanol)

(3)

Results and Discussion

The refractive index for 2-propanol, Water and the Mixture is reported in Table 1. The results showed that the refractive index did not vary much with water, but it does with 2propanol. The densities were taken from literature values (Table 2) of each substance at different temperatures and measured for the mixture (Fig. 4). The relationship between refractive index and temperature is due to the decrease in density and dielectric constant of the medium.8 Density and refractive were proportional to each other (Fig. 5). With refractive index and density of the medium, specific refraction could be obtained using Lorentz-Lorenz equation (Table 3). Finally, the molar composition of water and 2-propanol were calculated and are shown in Table 4. These results yield the composition is the same at each temperature of 2-propanol and Water because composition does not change with temperature. The composition of this mixture was 71% water and 29% 2-propanol.

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