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Refractometry
Refractometry
Refractometry is generally speaking the measurement of refractive index and its interpretation under dif-
ferent starting points. Refractometric measurements can be used for example for purity investigations,
sample recognition, dilution control or composition. It's also used for characterization of substances or for
determination of substance data, especially of organic liquids.
An important area of application is food analysis. For example the investigation of oils or fat, sugar and
sugar containing substances. Also beer and spirits belong to the material class, which are investigated
by refractometry. The liquid-chromatography also relies on the capabilities of the refractometric measure-
ment, providing quality control for process optimization.
A refractometer measures refractive index based on the index of refraction. After the index of refraction is
determined, the value can be converted to more concrete and useful units like ...
• Brix value
• Dry substance (DS)
• Oechsle scale
• Moisture content of honey
The dry substance doesn't depend on the temperature, but the refractive index changes with the tem-
perature. Because of this it is always necessary to measure the temperature as well if you determine the
refractive index. The same applies for almost every special scale that has a concrete meaning. A scale that
considers the temperature is called "temperature corrected".
Unfortunately the temperature dependency of the refractive index is substance-specific. This means, that
for a certain temperature e.g. cyclohexane shows the same measuring value like a treacle with 53% DS
and at another temperature the same like a treacle with 52% DS. Thus all temperature corrections apply
only for certain substance groups. The Brix scale is defined with the concentration of sucrose in water and
that's why the temperature correction of this scale is correct for sucrose dilutions. The temperature behav-
ior is very similar to, for example, marmalade or orange juice and the temperature corrected brix scale can
be used for other sucrose substances as well. The temperature correction of the brix scale applies for the
field where the scale is used and, for example, comestible goods industry.
Calculating of a special scale
Generally you need two values to calculate a scale value of a special scale: the refractive index and the
temperature. Because of this the conversion occurs in two steps. In the first step only the refractive index
is considered and the temperature behavior at the second one. So the result of the first step is correct for
only one specific temperature. This temperature is called reference temperature.
The Reichert refractometers accomplish the first step with the help of a polynom and the second one is
executed by using a matrix. This will be explained in the following:
Normally you generate the scale value at the reference temperature out of the refractive index in the first
step. But you can also cascade a scale, that means that instead of the refractive index a scale value can
input the value of another scale as well. This is why we generally call this input parameter "Input".
At first you define the auxiliary quantity "r", which results of the difference of the input parameter and the
value 1.33. Thereupon the polynom is computed with this auxiliary quantity. This method isn't straight
forward, but it helps to handle the polynom itself easier, in any case if the input parameter is the refractive
index. Take a look at the second example note below.
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(continued)
1400700-101 Rev. A
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