Metrohm 743 Rancimat - Manual (Page 10)

1.2 Rancimat method ￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
2 ￭￭￭￭￭￭￭￭ 743 Rancimat
offers a GLP test set to carry out these tests (see Optional accessories,
page 185).
1.2 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 alterations caused by the effect of
atmospheric oxygen. These oxidation processes progressing 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.
With the Rancimat method, the sample is exposed to an air flow at a
constant temperature between 50…220 °C (see Figure 1, page 3).
Highly volatile, secondary oxidation products (for the most part formic
acid) are transferred into the measuring vessel with the air flow, where
they are absorbed in the measuring solution (distilled water). Here the
conductivity is continuously registered. The organic acids can thus be
detected by increasing the conductivity. The time until occurrence of these
secondary reaction products is referred to as the induction time or induc-
tion period, which is a good indicator for the oxidation stability.
The Rancimat method has been developed as an automated variant to 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.