ASTM D1681-92(1997)

1.1 Direct titration of an anionic surfactant with a standardized cationic reagent is a simple and convenient method for the quantitative determination of the content of active ingredient. The end point is detected by the transfer of a colored complex from an organic solvent phase to an aqueous phase. The relationship between anionic and cationic agents is not always stoichiometric, and for maximum accuracy the anionic type of interest should first be characterized and then used to standardize the cationic reagent. In most cases, however, the different anionic surfactants likely to be encountered react in the same proportions. That is, a cationic titrating solution standardized against a characterized anionic agent can be used to analyze other anionics of known molecular weights.

1.2 This test method is applicable to alkylaryl sulfonates and fatty alkyl sulfates. Low results are obtained with alkyl- benzene sulfonates having the alkyl chain length less than eight carbon atoms. Low results are also obtained for alkyl sulfates with the alkyl chain length of less than twelve carbon atoms. The anionic surfactants characterized in accordance with Sections 16 to 22 should be the sodium salt and not amine, ammonium, or potassium salts. In case only amine or ammonium salts are available, they should be first converted to the sodium salt before proceeding with this analysis.

1.3 The analytical procedures appear in the following order: