Determination of polyoxyethylene alkyl ether non-ionic surfactants in waters at trace levels as potassium picrate active substances

Non-ionic surfactants of the type R(OCH2CH2)nOH, where R represents an n-alkyl group and n is the degree of polymerisation, are widely used in house-hold detergent formulations and are found at trace levels (0.01–1 mg l–1) in waters receiving untreated urban liquid sewage. A method based on the extraction of potassium picrate active substances (PPAS) has been investigated in the determination of non-ionics with R = dodecyl by considering both mono- and polydisperse surfactants.The reactivity of monodisperse surfactants (4 ⩽n⩽ 14) as PPAS is explained qualitatively by considering the equilibria involved in the extraction. Commercial polydisperse surfactants with 3.8 ⩽text-decoration:overlinen⩽ 28 (where text-decoration:overlinen is the number-average degree of polymerisation) are also considered and compared with other polydisperse surfactants in which R =p-tert-nonylphenyl. In both surfactant series the proposed method allows the absolute determination of the polydisperse surfactants in the range of text-decoration:overlinen from 13 to at least 28.