Water content determination of vegetable oils by Karl Fischer
Titration
Natural or simply filtered vegetable oils can also be used
as a fuel for diesel motors. In contrast
to biodiesel, which is obtained by chemically altering the
vegetable oil and which can be used in standardized quality
with existing motor technology, natural vegetable oils require
adaptation of the diesel motor. As the presence of water favors
the cleavage of fatty acids in the vegetable oil, standard
DIN V 51 605 limits the permitted water content
to 750 ppm.
The EN ISO standard 12 937
«Petroleum products – Determination of water
– Coulometric Karl Fischer titration method»
describes the coulometric Karl Fischer titration for determining
the water content. It is also stipulated as a test method
for vegetable oils. Because of the poor solubility of oils
in methanol the use of a solubility promoter is also recommended
here.
Coulometric Karl Fischer Titration by Metrohm
The 756 and 831 KF Coulometers from Metrohm
easily meet all the specifications stipulated in the standard
and are outstandingly suitable for this application. In most
cases the sample can be injected directly into the reaction
solution. In order to improve the solubility of the samples,
xylene (dimethylbenzene) is added to the KF reagent.
Get rid of side-reactions and contaminations
Many biofuels contain additives that could participate in
side-reactions during the direct coulometric
Karl Fischer titration. In such cases Metrohm recommends that
the biodiesel sample is not injected directly into the reaction
solution. Instead the water contained in the biodiesel should
be driven off in a Karl Fischer oven. The
832 KF Thermoprep oven is suitable for this.
The water is driven off at 120 °C and transported to the
titration cell of the KF Coulometer in a stream of carrier
gas (dry air or inert gas). This process can be completely
automated with the 774 Oven Sample Processor.
An additional option is reagent exchange using an automated
dosing device. The KF vessel does not have to be opened during
this procedure. This means that no hazardous reagents have
to be handled, which results in increased operator safety.
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Biofuel
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