Metrohm 892 Professional Rancimat - Manual (Page 33)

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4 Operation
892 Professional Rancimat
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4 Operation
4.1 Rancimat method
The decay of vegetable and animal fats, which can be perceived in the ini-
tial stage through a deterioration of odor and taste (rancidity), is to a
great extent the result of chemical modifications caused by the effect of
atmospheric oxygen. These oxidation processes, which progress slowly at
ambient temperatures, are referred to as autoxidation. They start with
radical reactions on unsaturated fatty acids and undergo a process involv-
ing multiple stages resulting in diverse decomposition products, in particu-
lar 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 between 100 - 140 °C. Highly volatile, secondary oxida-
tion products (for the most part formic acid and acetic acid) are transfer-
red into the measuring vessel with the air flow, where they are absorbed
in the measuring solution (distilled water). Here the conductivity is contin-
uously registered. The organic acids can thus be detected by an increase
in the conductivity. The time until these secondary reaction products occur
is referred to as the induction time or induction period, which is a good
characteristic for the oxidation stability.
The Rancimat method has been developed as an automated variant of the
extremely complex AOM (active oxygen method) for determining the
induction time of fats and oils. This method has become established
over the course of time and has been incorporated in various national and
international standards, e.g. AOCS Cd 12b-92 and ISO 6886.