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5.7 SAPONIFICATION VALUE

 ​           The saponification value is the number of mg of potassium hydroxide required to neutralize the free acids and saponify the esters contained in 1.0 g of the substance.

Procedure

 ​           Place about 2 g of the substance, accurately weighed, or the quantity specified in the monograph, in a 250-ml, conical, borosilicate glass flask. Add 25.0 ml of 0.5 M ethanolic potassium hydroxide and a few glass beads, attach a reflux condenser and heat in a boiling water-bath for 30 minutes to 1 hour until the substance is completely saponified. This is shown by the formation of a clear, homogeneous solution which does not change on addition of water. Add 1 ml of phenolphthalein TS and immediately titrate (while the solution is still hot) the excess potassium hydroxide with 0.5 M hydrochloric acid VS. At the same time, perform a blank test in exactly the same manner (Residual Titrations, Appendix 6.17).

 ​           Calculate the saponification value from the expression:

where 56.11 = the molecular weight of potassium hydroxide,

 ​           b = the volumes, in ml, of 0.5 M hydrochloric acid VS consumed by the blank,

 ​           a = the volumes, in ml, of 0.5 M hydrochloric acid VS consumed by the actual test,

 ​           M = the exact molarity of hydrochloric acid VS, and

 ​           W = the weight, in g, of the substance.