Calcium‐Selective Electrodes for Measurements in the Presence of Anionic Surfactants

Abstract

Calcium selective electrodes with various membrane formulations were studied in solutions containing CaCl 2 and sodium dodecylsulfate (NaDS). It is shown that electrodes based on neutral ionophores ETH 1001 and ETH 129 cannot be used as Ca 2 + ion sensors in these solutions because of strong anion interference from DS À anion. Among other formulations, that based on calcium bis(tetramethylbutylphenyl)phosphate in tri(2-ethylhexyl)phosphate appear the most promising. The interpretation of the ISE response in solutions under study relied on a novel approach which considers three forms of calcium: Ca 2 + free ions, Ca in Ca(DS) 2 precipitate, and Ca 2 + bound by the DS À micelles. Data needed for the respective calculations were obtained by DS À selective electrode based on tetradecylammonium, and Na + selective glass electrode.