Abbe refractometer

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Abbe refractometer seen from the right
Abbe refractometer seen from the left
Abbe refractometer (wood engraving 1897)

The Abbe refractometer is an optical device for determining the refractive index of liquids. As a total reflectometer , its measuring principle is based on the fact that the critical angle α g of the total reflection at an interface depends on the refractive index of the optical media involved . The total reflection takes place here at the interface of the glass used with a known and high refractive index n 'to the measuring and optically thinner liquid with the lower refractive index n . The optical structure in this refractometer developed by Ernst Abbe enables precise critical angle determination with a thin layer of liquid that is applied between the two glass prisms. About the relationship

the refractive index n of the liquid can thus be calculated.

The refraction of light depends on the wavelength of the incident light. In the past, a sodium vapor lamp was usually used as a monochromatic light source . Today an incandescent lamp is mostly used in combination with an optical filter. The filter is designed so that it only lets light with a wavelength of 589 nm through to the sample. This is the wavelength of the sodium D-line produced by sodium vapor lamps. This gives the same measurement result with less effort.

Depending on the liquid, the refractive index can be strongly temperature-dependent. Therefore, the prisms are brought to a constant, known temperature during the measurement. The usual measuring temperature is 20 ° C. However, for various reasons, such as the melting, boiling and decomposition temperature of the material to be measured, it may be necessary to measure at a different temperature.

The wavelength and temperature used are given as subscripts and superscripts. "D" is the indicator for the light of the sodium D-line (589.00 and 589.59 nm). In older tables, α and β are also given as indexes , which refer to lines of the Balmer series of the hydrogen spectrum (656.28 and 486.13 nm, respectively). The temperature is given in degrees Celsius .

Example: = Sodium D line, 20 ° C

Determination of n

First, the liquid is poured onto the opened double prism (1). Then you look through the right eyepiece, whereby the reticle must be clearly visible. The limit of total reflection is now set to the line cross (2). Now you can read off the angle in the reading microscope, here it is 7 ° 46 '(3).

  • 1

  • 2

  • 3

See also

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