The influence of an anionic additive on the properties of cation-exchangerbased calcium-selective electrodes

Potentiometric and electrochemical impedance spectroscopic investigations o f calcium-selective membranes containing poly(vinylchloride), dioctylphenyl phosphonate, calcium (bis[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate) an d different amounts of the lipophilic anionic additive tridodecylmethylammo nium chloride were carried out. The addition of the lipophilic additive cha nges the properties of calcium-selective electrodes, e.g. slope and calcium selectivity. The selectivity for calcium in presence of H+, Na+, K+, NH4+, Mg2+, Ba2+, Sr2+ and (C2H5)(4)N+ was measured by three different methods, namely separate solution method, fixed interference method and matched pote ntial method. Membranes with different concentration ratios between the cal cium-exchanger and tridodecylmethylammonium chloride were investigated with in half a year. The tendency of changing from cationic into anionic respons e for membranes containing nearly equivalent concentrations of cation- and anion-exchanger was shown. This inversion of the electrode response depends not only upon the concentration ratio of both ion-exchangers but also upon the total concentration of calcium-exchanger. Electrochemical impedance sp ectroscopy was used for monitoring the development of membrane resistances during a soaking period of one month. Based on these results dielectric con stants for the calcium-selective membranes depending on the membrane compos ition were determined. Furthermore, the dependence of the membrane resistan ce on the membrane thickness and the concentration of tridodecylmethylammon ium chloride was evaluated.