Metrohm 893 Professional Biodiesel Rancimat - Manual (Page 31)

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4 Operation
893 Professional Biodiesel Rancimat
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4 Operation
4.1 Rancimat method
Biodiesel, also referred to as RME (rapeseed methyl ester) or FAME (fatty
acid methyl esters), is usually obtained from oil seeds and used in the
automotive industry either in pure form or as a mixture with conventional
diesel fuel. As is the case with the natural oils, the fatty acid methyl esters
obtained from them also exhibit a relatively low level of storage stability,
as they are oxidized by atmospheric oxygen. These oxidation processes,
which progress slowly at ambient temperatures, are referred to as autoxi-
dation. They start with radical reactions on unsaturated fatty acids and
undergo a process involving multiple stages resulting in diverse decompo-
sition products, in particular peroxides as primary oxidation products and
alcohols, aldehydes and carboxylic acids as secondary oxidation products.
In the Rancimat method, the sample is exposed to an air flow at a con-
stant temperature of 110 °C. Highly volatile, secondary oxidation products
(for the most part formic acid and acetic acid) are transferred into the
measuring vessel with the air flow, where they are absorbed in the mea-
suring solution (distilled water). Here the conductivity is continuously reg-
istered. The organic acids can thus be detected by an increase in the con-
ductivity. The time until these secondary reaction products occur is refer-
red to as the induction time or induction period, which is a good charac-
teristic for the oxidation stability.
The Rancimat method has been developed as an automated version of the
extremely complex AOM (active oxygen method) for determining the
induction time of fats and oils. This method has established itself over
the course of time and has also been applied for derivatives of oils and
fats. It has become an accepted method for biodiesel and biodiesel blends
in various national and international standards, e.g. EN 15751, EN 14112
and ASTM 6751.