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Fuel and Physical Properties of Biodiesel Components 157

of esters of oleic, linoleic, or linolenic acids, which influence the oxida-
tive stability of the fuels. The species formed during the oxidation
process cause the fuel to eventually deteriorate.
A European standard (EN 14112; Rancimat method) for oxidative
stability has been included in the American and European biodiesel
standards (ASTM D6751 and EN 14214). Both biodiesel standards call
for determining oxidative stability at 110 C; however, EN 14214 pre-
scribes a minimum induction time of 6 h by the Rancimat method while
ASTM D6751 prescribes 3 h. The Rancimat method is nearly identical
to the OSI method, which is an AOCS (American Oil Chemists’ Society)
method.
Besides preventing exposure of the fatty material to air, adding antiox-
idants is a common method to address the issue of oxidative stability.
Common antioxidants are synthetic materials such as tert-butylhydro-
quinone (TBHQ), butylated hydroxytoluene (BHT), butylated hydrox-
yanisole (BHA), and propyl gallate (PG) as well as natural materials
such as tocopherols. Antioxidants delay oxidation but do not prevent it,
as oxidation will commence once the antioxidant in a material has been
consumed.

5.4.1 Iodine value
The iodine value (IV) has been included in the European biodiesel stan-
dards to purportedly address the issue of oxidative stability and the
propensity of the oil or fat to polymerize and form engine deposits. The
IV is a measure of the total unsaturation of a fatty material measured
in grams of iodine per 100 g of sample when formally adding iodine to
the double bonds. An IV of 120 has been specified in EN 14214 and 130
in EN 14213, which would largely exclude vegetable oils such as soybean
and sunflower oils as biodiesel feedstock. Thus the IV has not been
included in biodiesel standards in the United States and Australia, and
is limited to 140 in the South African standard (which would permit sun-
flower and soybean oils); the provisional Brazilian standard requires
that it only be noted.
The IV of a vegetable oil or animal fat is almost identical to that of the
corresponding methyl esters; however, the IV of alkyl esters decreases
with higher alcohols used in their production since the IV is molecular
weight dependent. For example, the IV of methyl, ethyl, propyl, and
butyl linoleate is 172.4, 164.5, 157.4, and 150.8, respectively [64].
The use of the IV of a mixture for such purposes does not take into
consideration that an infinite number of fatty acid profiles can yield the
same IV and that different fatty acid structures can give the same IV,
although the propensity for oxidation can differ significantly [64]. Other,