Divalent (water hardness) ion-selective electrodes based on poly(vinyl chloride) and poly(methyl acrylate) matrix membranes

Abstract

Poly(methyl acrylate)(PMA) provides an alternative support matrix to poly(vinyl chloride) but the usual 0.17 g of polymer established for functional poly(vinyl chloride) ion-selective electrodes must be increased to 0.3 g to produce master membranes of sufficient mechanical strength. The best membrane compositions were: PMA (0.3 g), calcium bis(didecylphosphate)(0.02 g), didecylphosphoric acid (0.02 g) with either 0.09 or 0.18 g of decanl-ol. Electrodes with such sensor membranes showed near-Nerstian response to calcium and magnesium and functioned for about 10 days. Selectivity coefficients for divalent character with respect to calcium and magnesium measured by separate and mixed solution techniques were close to unity in many instances. The useful pH range lay between approximately 2.7 and approximately 7.2 in solutions of either 10^–1M calcium chloride or 10^–1M magnesium chloride levels and at their 10^–3M levels.

Neither the PMA electrode nor two others based on PVC matrix membranes with divalent sensor systems permitted accurate assays of total calcium and magnesium in standard analates. However, a PVC calcium ion-selective electrode based on calcium bis{di[4-(1,1,3,3-tetramethylbutyl)phenyl]-phosphate} and dioctyl phenylphosphonate facilitated the accurate assay of calcium in the presence of three times its amount of magnesium.