THE MEASUREMENT OF NITRITE, NITRATE AND PHOSPHATE WITH TEST KITS AND STANDARD PROCEDURES - A COMPARISON

Portable test kits, which are accepted as analytical techniques for measuring dissolved inorganic nitrite, nitrate, and phosphate were compared with standard methods in terms of analytical performance, colour stability, chemical background of water solutions, salinity interference, filtration and storage procedures. The kit detector, a Hach DR/2000, was also compared to a laboratory spectrophotometer, a Shimadzu UV-2100. Absorbance measurements indicated that the Hach colorimetric detector is a reliable spectrophotometric sensor and compared well to the Shimadzu UV-2I00, but the former has the advantage of being more portable, versatile and cheaper than the latter. Detection limits for nitrite, nitrate and phosphate were 0.300, 0.657 and 0.162, and 0.048, 0.049 and 0.050 mu M for the kits and standard procedures, respectively. Molecular absorption coefficients achieved for phosphate were 21,600 and 22,700 (M.cm)(-1), and for nitrate 29,900 and 47,900 (M.cm)(-1), respectively. However, for nitrate the coefficients differed markedly [12,000 and 44,500 (M.cm)(-1)]. Reagent blanks from the test kits were higher than those from the standard techniques. Differences between filtered and non-filtered water samples from aquaculture facilities and other environments were found. The full chemical reaction (colour) of the kit reagents proved to be stable after 40 min. Information on spectral response, kinetics, multi-point calibration curves and storage of kit reagents is also provided. The studied test kits for determining nitrite and phosphate can be used in monitoring natural waters. But for nitrate, salt interference should be taken into account.