ASTM D1681-05(2014)

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 detffected 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.