Sequential determination of calcium and nitrate ions in waters by potentiometric flow injection

A compact and robust flow injection system for the simultaneous determination of calcium and nitrate in waters, with sequential detection by potentiometric sensors, is described. The effects of silver ionic strength adjuster (ISA) or lead ISA carrier solutions on the response of the tubular electrodes used and on the ability of the system to control interferences were studied in detail. Under the selected experimental conditions, the response slope and lower limit of linear response were 53.4 mV per decade and 10–4.3 mol l–1 for the nitrate sensor and 31.3 mV per decade and 10–4.75 mol l–1 for the calcium sensor. The reproducibility (relative standard deviation) was better than 1% and the sample throughput was approximately 145 h–1. The results provided by the flow system were shown to be accurate, reproducible and free 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 from other ions, especially chloride and hydrogencarbonate, were observed in surface waters, excluding those with a high saline content. The sequential method described was shown to be useful in routine determinations with a high sampling rate at a low cost.