ISO 456 Surfactants - Analysis of Soaps - Determination of Free Caustic Alkali

Material Testing

ISO 456 Surfactants - Analysis of Soaps - Determination of Free Caustic Alkali

The ISO 456 standard, developed by the International Standards Organization (ISO), can be performed in two different ways, with the Detection ethanol method or the barium chloride method. In the first case, the soap is dissolved in neutralized ethanol and the free caustic alkali is titrated with a solution of ewtanol hydrochloric acid. The second way involves precipitating soap and carbonate with barium chloride and determining the residual alkalinity of the solution, which represents the alkalinity of free potassium hydroxide.

ISO 456 Surfactants - Analysis of Soaps - Determination of Free Caustic Alkali

Ethanol Method:


It should only be applied to normal quality sodium soaps, as the presence of certain additives presents sources of error. Not applicable to potassium soaps due to the solubility of potassium carbonate in ethanol.

Barium Chloride Method:


It should be applied to all mild potassium soaps or soaps with sodium and potassium mixtures. It is not recommended to apply this method to normal quality sodium soaps, which generally do not contain enough free caustic alkali to be determined by this method.

Because soaps often contain small amounts of unsaponified neutral oil, there is no perfect procedure for determining free caustic alkali, because when the soap sample is dissolved, a neutral oil is more or less saponified by any free caustic alkali present.

Therefore, both methods described in the International Standard are of a conventional nature.
Conventionally, free caustic alkali is expressed as sodium hydroxide (NaOH) for sodium soaps and potassium hydroxide (KOH) for potassium soaps.

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