Optimization of a flow injection differential potentiometric system with two identical electrodes for the determination of calcium ion in water with improved sensitivity

An experimental optimization by the modified simplex method of a flow injection analysis (FIA) system for the differential potentiometric determination of calcium ion in mater is reported, The performances of the optimized system and of an equivalent conventional potentiometric FIA system are compared, The most important features of the proposed system are a substantial increase in sensitivity (from about 28 to shout 50 mV per decade) in the linear response range (10(-4) - 10(-2) mol l(-1) in both cases) and better repeatability (0.5 and 0.8% versus 1.7 and 1.9%, respectively, at 1 x 10(-3) and 6.2 x 10(-4) mol l(-1) calcium ion), The influence of the ionic strength is discussed, showing that it significantly affects the accuracy of the results. The relative accuracy of the differential system was demonstrated try the determination of the calcium content of drinking Waters in parallel with an ASTM certified method.