Principle

Hydrolysis of fat with an alkali results in the formation of salts of fatty acids (also called soap) and glycerol. This process is called saponification. From the amount of potassium hydroxide utilized during hydrolysis, the saponification value of a given fat sample can be calculated. The saponification value is defined as mg of KOH required to saponify 1g of the given fat.

image

 

 

 

From the above reaction, it may be noted that 3 molecules of KOH are consumed for saponification of each molecule of triacylglycerol irrespective of chain length of fatty acid. Evidently, each g of a triacylglycerol with shorter chain fatty acids will contain larger number of molecules of the triacylglycerol and will thus require much more KOH. The saponification value is thus an indication of average molecular weight of the fatty acids in an aclyglyceride.

The procedure involves refluxing of known amount of fat or oil with a fixed, but an excess of alcoholic KOH. The amount of KOH remaining after hydrolysis is determined by back titrating with standardized 0.5N HCl and the amount of KOH utilized for saponification can thus be calculated.

Procedure

1. Weight accurately 1g of the fat sample in a conical flask and dissolve it in about 3 ml of the fat solvent (Reagent No. 5).

2. Add 25 ml of 0.5N alcoholic KOH, attach a reflux condenser to it and reflux the contents on boiling water bath for 30 minutes.

3. Cool it to room temperature and add a few drops of phenolphthalein into the flask.

4. Titrate the contents of the flask with 0.5N HCl till the pink color disappears.

5. Similarly, run a blank by refluxing 25 ml of 0.5N alcoholic KOH without any fat sample.

Calculations

0.5 N KOH in blank = x ml

0.5 N KOH in test sample = y ml

Titre value for sample = (x – y) ml

28.05 x titre value

Saponification value = ^{______________________________________}

weight of sample (g)

The multiplication factor of 28.05 in the above equation is included since 1 ml of 0.5N KOH contains 28.05 mg of KOH.