A divalent cation-selective electrode, which utilizes a lipophilic resin as a matrix for the sensing membrane, and which has long-term stability has been developed. The sensing membrane is a lipophilic acrylate resin which is impregnated with a solution of 1-decylalcohol and the calcium salt of bis[4-(1,1,3,3-tetramethylbutyl) phenyl] phosphate at concentrations of 0.08 g ml-1 each. The electrode exhibited nearly equal selectivity to Ca2+ and Mg2+ ions and could be used as a water hardness sensor. The electrode shows a Nernstian response with a slope of 29 mV decade-1 to both Ca2+ and Mg2+ ions in the concentration range from 10-5 M to 10-1 M and could be used in the pH range from 3 to 10 for the determination of 10-3 M Ca2+ and Mg2+ solutions. The initial performance of the electrode could be maintained for 1 year, since the lifetime test of the electrode was conducted in tapwater at a continuous flow rate of 4 ml min-1. The hardnesses of tapwater and upland soil extracts were determined using the developed electrode and the analytical results were in good agreement with those obtained by chelatometric titration using an EDTA solution as the titrant. A coefficient factor of correlation 0.998 was obtained between the electrode method and titrimetry. The long-term stability of the electrode was found to be due to strong affinity of 1-decylalcohol to the lipophilic acrylate resin.
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