Refractometer

 

This is an Abbe Refractometer, a device first developed by Ernst Abbe (1840-1905) in 1871 when he was an Associate Professor at the University of Jena in Germany. Refractometers measure the refractive index of substances, which is determined by how light scatters through them. A refractometer is an easy device to use, and allows the user to quickly identify the purity and/or concentration of a substance.

The Abbe Refractometer has an illuminating prism (upper prism) and a refracting prism (lower prism) inside the cube-shaped part of the instrument. The cube can be opened to allow a sample to be placed between the prisms, and when light is shone across it, both illuminated and dark areas form which can be observed by looking down the cylindrical scope at the top. By measuring the angle between the illuminated area and the dark area, the refractive index of the sample can be calculated.

This instrument can be used to identify liquids, resins and solids. The small bottle in the door of the refractometer’s storage case contains 1-Bromonaphthalene, a substance with a high refractive index that is used as an embedding agent for solids. As refractive indexes change with temperature, measurements are typically undertaken at 20°C, or a further calculation is applied to normalise it if the temperature is different. The thermometer that is also located inside the case’s door is used to record the temperature at the time of measurement; to do this, it is inserted into an opening on the cube-shaped part of the instrument.

Abbe Refractometers were first used over one hundred years ago at the University of Canterbury, by undergraduates, postgraduates and academic staff. In undergraduate laboratories, students were tested on their ability to accurately measure the refractive indexes of given substances. These days, however, refractometers are rarely used at the University, as other analytical techniques that identify the purity and/or concentration of substances have been developed.

Text by: Harrison Bowman

 

Abbe Refractometer