Sequential automatic on-line determination of aquiculture nutrients: phosphate and nitrate

A continuous-flow method for the determination of nitrate and phosphate in water is proposed. The method is based on insertion of reagents and sample solutions into a single channel through a programmable switching valve. The method depends upon heteropolyacid with phosphate and on the modified Griess reaction for nitrate, and permits the determination of PO(-3)(4) in the range 1-20 μg/ml and NO(-)(3) between 5 and 100 μg/ml, the achievable sampling frequency being 45/hour. The two analytes can be determined in N/P ratios between 0.25 and 100. The method has been applied to the determination of nitrate and phosphate in the sea-water used in fish farms.