NITRATE REDUCTOR EFFICIENCY AS AN ERROR SOURCE IN SEAWATER ANALYSIS

Nitrate analysis in seawater is usually achieved by reduction to nitrite using a cadmium metal reductor, followed by colorimetric determination of the nitrite produced. Reductor efficiency typically declines with use, so that equimolar concentrations of nitrate and nitrite yield different colorimetric signals. Unless this difference is resolved properly in calibration standards, and in samples, errors in estimating nitrate concentration are inevitable, particularly when mixed standards are employed in automated analysis. Errors of more than 1 muM nitrate are possible in deep ocean samples. Quantification of the error and its correction require measurement of reductor efficiency. The problem and correction procedure are described.