Metrohm 873 Biodiesel Rancimat Manual - Manual (Page 10)

1.2 Rancimat method ￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
2 ￭￭￭￭￭￭￭￭ 873 Biodiesel Rancimat
a GLP test set to carry out these tests (see Optional accessories, page
180).
1.2 Rancimat method
Biodiesel, also referred to as RME (rape seed methyl ester) or FAME (fatty
acid methyl esters), is usually obtained from oil seeds and used in the motor
vehicle area 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 progressing slowly at
ambient temperatures are referred to as autoxidation. They start with rad-
ical reactions on unsaturated fatty acids and undergo a process involving
multiple stages resulting in diverse decomposition products, in particular
peroxides as primary oxidation products and alcohols, aldehydes and car-
boxylic 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 increas-
ing the conductivity. The time until occurrence of these secondary reaction
products is referred to as the induction time or induction 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 induc-
tion time of fats and oils. This method has become established over the
course of time, also being used for derivatives of oils and fats. It has mean-
while become an integral element of various national and international
standards for biodiesel and biodiesel mixtures, e.g. EN 14112 and
ASTM 6751.