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Updated: July 4, 2025
Considerable light is thrown on the composition of a fixed oil by ascertaining how much alkali is required to saponify it. Phenol-phthalein solution being added, the excess of potash is estimated by carefully titrating with standard hydrochloric acid solution. It was thus found that 1,000 grammes of oil would require 195.5 grammes of caustic potash to convert it entirely into potash soap.
The oil was found to contain free acid in small quantity, which was estimated by agitating a weighed quantity with alcohol, in which the free acid dissolves while the neutral fat does not, and titrating the alcoholic liquid with decinormal alkali, using solution of phenol-phthalein as an indicator.
A few drops of alcoholic tincture of phenol-phthalein are then added, and as soon as a faint red color appears, addition of soda is stopped. This shows that the fatty acids have been over-saturated. Addition of a little more fat renders them perfectly neutral, and the soap is then ladled out into wooden moulds, lined with loose sheets of zinc.
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