Optimization of a bipotentiometric flow injection analysis system for simultaneous determination of calcium and chloride ions in natural waters

The modified simplex method was used to optimize a bipotentiometric flow injection analysis system for the simultaneous determination of calcium and chloride in water. The system employed 2 different ion selective electrodes connected in series. The response function adopted was a linear combination of 3 weighted variables selected to maximize the calcium electrode sensitivity, minimize the interference of the calcium signal on the chloride response and maximize the sampling rate. The optimized system was linear for the concentration ranges 0.05 to 5.0 mmol per litre for calcium and 0.2 to 20 mmol per litre for chloride and provided a sensitivity of 27 and 55 mV per decade, respectively. An effective sample throughput of 40 samples per h was achieved. The effect of ionic strength on the results is discussed. The accuracy of the bipotentiometric method was demonstrated by comparison of results for various drinking water samples with values for calcium and chloride concentrations determined by ASTM certified methods (ASTM D 511-88 and ASTM D 512-88, respectively).