INTERFERENCE STUDIES IN THE BATCH DETERMINATION OF NITRATE IN FRESHWATER BY REDUCTION WITH CADMIUM TO NITRITE

A method is described for the batch determination of nitrate-nitrogen in freshwater that uses two different forms of cadmium for reducing nitrate to nitrite. Comprehensive interference tests were made and possible interfering ions in freshwaters were shown to be phosphate, silicate, hydrogen carbonate, sulphide and some organic compounds. Reduction with spongy cadmium is less prone to interference than reduction with cadmium filings. The spongy cadmium method has a criterion of detection of 0.006 6 mg l-1 of nitrate-nitrogen for the 95% confidence level. The relative standard deviation at 0.140 mg l-1 of nitrate-nitrogen is 7.8% and at 2.1 mg l-1 is 2%. The optimum reduction time or the amount of cadmium filings for maximum reduction efficiency for natural samples or solutions containing interferences is shown to be different from that obtained for standard solutions. This difference forms the basis for a suggested quality control procedure, which can be used to monitor the accuracy of nitrate determinations.