ISO 684:1974

The soap is dissolved in an ethanolic solution, the free alkali is neutralized with a known excess of sulphuric acid solution and the excess acid is titrated with ethanolic potassium hydroxide solution. The method is not applicable if the soap cantains additives (alkali, silicates, etw.) which can be decomposed by sulphuric acid by the procedure specified. The method is also not applicable to coloured soaps if the colour interferes with the phenolphthalein end point.