If soap has high alkali content it irritates skin. How can the amount of excess alkali be determined ? What can be the source of excess alkali ?

Explanation

A solution of soap is titrated with standard hydrochloric acid. It is an acid-base titration. In this titration, phenolpthalein is used as an indicator. During the preparation of soap, fat (i.e., glyceryl ester of fatty acid) is heated with aqueous sodium hydroxide

Thus, the source of this excess alkali (which irritates skin) is the alkali left unused when the soap is prepared by hydrolyis of fat.