Refractometry

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Refractometry

INTRODUCTION

Refractometry is the method of measuring the refractive index of substances. Refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction or refractive index (n) for the substance.

Refraction phenomenon

PRINCIPLE AND THEORY

The main principle involved in refractometry is the refraction based on the speed of the light that passes in the different mediums.

Light enters into the light denser medium to high denser medium at an angle, that is, with bent. The bent in the light ray is known as the refraction. The relation between the refraction of the light between the air and the medium is given by Snell's law:

n = sin i/sin r

where n = refractive index; sin i = angle of incidence; sin r = angle of refraction.

Samples with different refractive index will produce different angles of refraction. This helps in the assessment of the compounds’ composition and the purity of the compounds.

There are different methods for the determination of the refractive index as follows:

• Becke line method: This method was first proposed by Mittchell and is mainly used for the determination of facetted stones. It is comprised of the microscope with the light field illumination. By using the known substance, the refractive index of the unknown substance is measured.
• Immersion contrast method: This method involves the determination of the relief and appearance of the girdle and the facet edges when immersed in the known liquid.
• Direct measurement method: This method involves the measurement of the refractive index by using the microscope with a verneir scale. This method is more effective for the single refracting stones.
• Minimum deviation method: This method involves the use of the table spectrometer and is the most accurate method for the measurement of the refractive index. It requires skill and ideal conditions.

INSTRUMENTATION

The instrument used for the determination of the refractive index is known as the refractometer. There are different refractometers used for the determination of the refractive index. They are as follows:

• Traditional handheld refractometer: The main principle involved in this refractometer is the measurement of the critical angle. It is comprised of the lenses and prisms to project the black line on the glass when the sample is placed between the measuring prism and the plate.
• Digital hand held refractometer: The principle is the same as the traditional handheld refractometer. The main difference is that the light from the LED light source is focused on the prism. This creates the black line due to the refection of the photodiode arrays.
• Abbes refractometer: This is a benchtop refractometer, designed by Ernst Abbe, which provides high accuracy. In this refractometer, the sample is held between the illuminating prism and the refracting prism. A light source is allowed through the illuminating prism and the detector is placed behind the refracting prism.
• Inline process refractometer: This is mainly used for the continuous measurement of a fluid flowing through a pipe. This refractometer consists of a sensor placed in the flow of the fluid. This is connected to the control box which gives the digital readout.

FACTORS AFFECTING THE REFRACTIVE INDEX

• Temperature: It is inversely proportional to the refractive index.
• Viscosity: It is inversely proportional to the refractive index.
• Wavelength: The sodium D-line at 595 nm is the appropriate wavelength for the determination of the refractive index.

APPLICATIONS

• Used in the determination of the refractive index of the compounds
• Used in the determination of the concentration of the compounds
• Used in the determination of the structure
• Used in the determination of the critical micelle concentration of the compounds

REVIEW QUESTIONS

1. What is refraction?
2. Explain Snell's law.
3. Explain the different methods used in the refractometric measurements.
4. What are the different types of instruments used in refractometry?
5. What are the factors affecting refractometry?
6. What are the applications of refractometry?