SEQUENTIAL DETERMINATION OF CALCIUM AND NITRATE IONS IN WATERS BY POTENTIOMETRIC FLOW-INJECTION

A compact and robust flow injection system for the simultaneous determ ination of calcium and nitrate in waters, with sequential detection by potentiometric sensors, is described. The effects of silver ionic str ength adjuster (ISA) or lead ISA carrier solutions on the response of the tubular electrodes used and on the ability of the system to contro l interferences were studied in detail. Under the selected experimenta l conditions, the response slope and lower limit of linear response we re 53.4 mV per decade and 10(-4.3) mol l(-1) for the nitrate sensor an d 31.3 mV per decade and 10(-4.75) mol l(-1) for the calcium sensor. T he reproducibility (relative standard deviation) was better than 1% an d the sample throughput was approximately 145 h(-1). The results provi ded by the flow system were shown to be accurate, reproducible and fre e from interferences. The results obtained for calcium and nitrate in 23 samples with the flow injection system and with a reference method were in good agreement with each other. No significant interferences f rom other ions, especially chloride and hydrogencarbonate, were observ ed in surface waters, excluding those with a high saline content. The sequential method described was shown to be useful in routine determin ations with a high sampling rate at a low cost.