question_answer 75)

  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 ?

Answer:

                  Acid-base titration (i.e., titrating a solution of soap with standard hydrochloric acid using phenolphthalein as indicator) can be used to determine the excess amount of alkali present in soap. The source of this excess alkali is the alkali left unused- when the soap is prepared by hydrolysis of oils.