Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

IODINE AND SAPONIFICATION VALUES 151
the material on the filter is then washed with 50 c.c. of alcohol.
The alcoholic solution thus obtained is concentrated as far as possible
on the water bath and is then washed into a Claisen flask with a
little alcohol. When the contents of the flask are distilled in
vacuo alcohol and water, and then glycerol pass over. The bulk of
the glycerol is collected between 180° and 195°/13 mm. Yield
about 35 g.
In order to obtain the glycerol quite pure and anhydrous the
distillation must be repeated.
Analysis of Fats.—The " iodine value " of a fat or oil is a quantitative
measure of the number of carbon-carbon double bonds which it
contains. This number is the amount of iodine in grammes which combines
chemically with 100 g. of the fat or oil. Nowadays the number
of double bonds in organic compounds is usually determined with perbenzoic
acid (cf. p. 111).
Determination of the Iodine Value.—Pure iodine (2-5 g.) and mercuric
chloride (3 g.) are each dissolved in 50 c.c. of pure spirit and the
clear solutions are mixed. After twelve hours the iodine titre of the
mixture is determined in a 10 c.c. portion by adding 10 o.c. of 10 per
cent potassium iodide solution and titrating with 0-1 N-thiosulphate
solution.
The fat to be examined (0-5-0-7 g.) is dissolved in 15 c.c. of chloroform
in a dry conical flask (capacity 500 c.c.) and 25 c.c. of the standardised
iodine solution are added. If, after a short time, the colour of the
solution diminishes to a light brown, it is necessary to add a further
10 c.c. of the iodine solution. After four hours the colour of the solution
should still be dark brown. Potassium iodide solution (20 c.c. of 10
per cent solution) is now added and the uncombined iodine still present
titrated as above. The calculation is made in accordance with the
definition of " iodine value". Lard, olive oil, or linseed oil should be
examined.
To determine the saponification value x of a fat 0-5-1-0 g. is boiled
under reflux condenser for half an hour with 10 c.c. of O-SiV-alcoholic
KOH solution and the excess of alkali is then titrated with 0-5 N-HCl,
and phenolphthalein as indicator.
The method is a general one and can be applied to esters to determine
the equivalent of an acid combined in them. Ester equivalent =
a. 1000/6 where a is the weight of ester taken (in grammes) and b is
the number of c.c. of N-alkali used up.
Linseed oil is the most important of the so-called " drying " oils.
These oils contain highly unsaturated acids, such as linolenic C17H29,
CO2H and linolio acid C17H31.CO2H, and are therefore able to combine
1 The number of milligrammes of KOH which is used up by 1 g. of fat.