Studies of calcium ion-selective electrodes in the presence of anionic surfactants

Abstract

Eighteen membrane systems, based on a calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate} ion sensor have been compared in potentiometric studies with a membrane based on the sensor with a dioctyl phenylphosphonate solvent mediator with respect to interferences of calcium ion-selective electrodes by anionic surfactants, especially by sodium dodecylsulphate (SDS) and sodium tetradecylbenzenesulphonates (ABS). Electrodes made from poly(vinyl chloride) matrix membranes of the sensor with trioctyl phosphate solvent mediator are far superior to the other membrane systems in resisting interference by anionic surfactants and are shown to yield calcium ion levels in the presence of a wash liquor to match the expected values.Some improvement over dioctyl phenylphosphonate is also offered by decan-1-ol, dodecan-1-ol and tetradecan-1-ol but the use of such solvent mediators impairs calcium ion selectivity and poly(vinyl chloride) plasticising qualities. The use of alternative polymer matrices based on poly(vinylidene chloride) and VAGH copolymer (hydrolysed vinyl chloride - vinyl acetate) offer no advantages.A nineteenth membrane, obtained from a commercial supplier, exhibited interference by SDS.An interesting effect is the increase (rather than the normal decrease) in e.m.f. observed for those electrodes from membranes in which the amount of free active sensor was reduced to low levels. By using optimum levels of sensor it was possible to fabricate electrodes exhibiting a zero e.m.f. change when changing the background 10–2M calcium chloride to one that is also 10–3M is SDS. Such membranes are based on decyl phosphate grafted to VAGH copolymer.X-ray fluorescence and chromatographic studies on membranes of the sensor with dioctyl phenylphosphonate in poly(vinyl chloride) show SDS to be a significant agent in leaching membrane components, especially of dioctyl phenylphosphonate. Such observations are indicative that the interference of calcium ion-selective electrodes by anionic surfactants may be the result of the different solubilities of calcium - surfactant complexes in the solvent mediator of the membrane.