The characterisation of the purity of oils, waxes, fats and other by-products of the raffination process of mineral oils is easily done with an ABBE-refractometer. This historic instrument standard is easy to use in the laboratory but has some difficulties when it comes to emulsions or darker samples.
Nowadays the standard instrument should be an automatic refractometer with a minimum resolution of 0,0001 nD. The measurement according to ASTM D 1218, ASTM D 5006, DIN51521 and DIN51423-2 can be better and more effectively performed with an automatic instrument.
Temperature Dependence
One very important topic when the refractive index is measured is the specific temperature effect of every single pure substance. For example: At 20°C cyclohexane has the same refractive index as a 52.9% TS sucrose solution, while at 25°C cyclohexan has the same refractive index as a 52.1% TS sucrose solution.
Another example: The temperature related refractive index change of Paraffin oil:
| Paraffin Oil |
| Temperature |
Refractive index |
| 20.0°C |
1.48001 |
| 25.0°C |
1.47825 |
| 30.0°C |
1.47644 |
| Refractive index change: 0.00036 / °C |
It is possible to determine the individual temperature correction of a substance and to programme a refractometer to always calculate the same values for it independent from the temperature. It is also possible to program a temperature-corrected refractive index but this only makes sense if it is known to which sample the correction applies. The user has to be aware that all temperature corrections are substance specific.
Taking the temperature effect into account a refractometer in the petrochemical laboratory is only as precise as the temperature measurement is.
The Automatic Alternative
Using the KRÜSS refractometer model DR 6300-T the temperature is controlled through a built-in peltier-thermostate which controls in the range 10 - 80°C with an accuracy of 0.1°C.
The temperature stability is 0,05°C. This temperature control allows a resolution of the refractive index of 0.00001. For our above example the accuracy still is 0.00004.
All KRÜSS refractometers of the DR model range are compliant to ASTM D1218 and can be used with the small external Peltier thermostate PT31 or in the T-version have a built-in Peltier thermostate. Due to these thermostatisation possibilities the KRÜSS refractometers can be used not only for standard temperatures but also for applications with samples that are only fluid at higher temperatures like fats, wax, paraffins and other by-products.
All instruments can be calibrated with a 1-, 2- or 3-point calibration. The measuring time is about 4 sec. The stainless steel sample compartment and the sapphire prism are easy to clean and are IP65 protected. All models are equipped with a serial interface, a printer connection and an USB-port.
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The Laboratory Benefit
Using an automatic instrument means:
- faster results
- better result processing
- GLP conformity
- less human uncertainties (manual optical measurements are dependent on the user)
The world-wide used and well known KRÜSS instruments are of reliable quality and will be a real help in your laboratory.
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This information has been sourced, reviewed and adapted from materials provided by A.KRÜSS Optronic GmbH.
For more information on this source, please visit A.KRÜSS Optronic GmbH.