Refractometer

For mixtures, the index of refraction then allows the concentration to be determined using mixing rules such as the Gladstone–Dale relation and Lorentz–Lorenz equation.

The refractometer is used to help identify gem materials by measuring their refractive index, one of the principal properties used in determining the type of a gemstone.

Gemstone refractometers are available both as classic optical instruments and as electronic measurement devices with a digital display.

[3] In marine aquarium keeping, a refractometer is used to measure the salinity and specific gravity of the water.

The automatic measurement of the refractive index of the sample is based on the determination of the critical angle of total reflection.

Due to focusing light to a spot at the prism surface, a wide range of different angles is covered.

This dependence of the reflected light intensity from the incident angle is measured with a high-resolution sensor array.

In the infrared wavelength range several absorption maxima and fluctuations in the refractive index appear.

To guarantee a high quality measurement with an accuracy of up to 0.00002 in the refractive index the wavelength has to be determined correctly.

Therefore, in modern refractometers the wavelength is tuned to a bandwidth of +/-0.2 nm to ensure correct results for samples with different dispersions.

Once an automatic refractometer is equipped with a flow cell, the sample can either be filled by means of a syringe or by using a peristaltic pump.

A peristaltic pump opens the way to monitor batch processes in the laboratory or perform multiple measurements on one sample without any user interaction.

Today's laboratories do not only want to measure the refractive index of samples, but several additional parameters like density or viscosity to perform efficient quality control.

The manufacturers of pharmaceuticals have to follow several international regulations like FDA 21 CFR Part 11, GMP, Gamp 5, USP<1058>, which require a lot of documentation work.