Brix, Refractive Index and Optical Rotation

Refractive Index / °Brix

When light passes from one medium to another, the speed at which the light travels will change depending on the parameters of the materials. This principle can be seen when looking at a straw in a glass or an oarsman on the river, where the straw or oar appears to be bent at the water/air interface.

The ratio or change in the speed of light is called refractive index and instruments that measure this are called refractometers.

The refractive index of a liquid is related to its concentration and so a refractometer can display the concentration in suitable units, such as °Brix (sucrose), total dissolved solids (TDS), Baume, probable alcohol, sodium chloride, urea and urine specific gravity to name just a few.

Refractometers are simple optical instruments used throughout industry for over 150 years and have now developed in to sophisticated digital instruments capable of measuring the % of dissolved solids to 3 decimal places.

When plane-polarised light passes through an optically active or substance or chiral compound, the plane of polarisation will rotate by an amount that is specifically related to the product through which it travelled.

As many chemical compounds display this chiral characteristic, the measurement of optical rotation using a polarimeter is commonplace within the sugar, food, chemical and pharmaceutical manufacturing industries as a production control and quality assurance tool.

Optical rotation is relative to the concentration of the dissolved sample and the length over which the measurement is taken and so it is common to quote chemicals as a specific rotation based on all the affecting factors, including temperature and wavelength. Concentration and purity may also be calculated by transformation.