Spectrophotometric determination of phosphite in fertilizers in a flow injection system with online sample preparation

A flow injection system with online sample preparation is proposed for the determination of phosphite in liquid fertilizers by spectrophotometry. Afte r loop-based injection, phosphite is oxidized by an acidic permanganate sol ution (1.0 10(-2) mol L-1 KMnO4 + 1.0 mol L-1 H2SO4) in a heated reactor (5 0 degreesC). The phosphate generated is then determined by the molybdenum b lue method. Influence of flow rates, temperature, and concentration and ord er of addition of reagents, sample volume, and reactor configuration for th e blue complex formation on recorded signals were investigated. The pow sys tem was applied to phosphite determination in commercial samples of liquid fertilizers. The proposed system handles about 80 samples per hour [0.05-0. 40% (w/v) H3PO3; R = 0,9998], consuming about 80 muL sample, 1 mg KMnO4, 25 mg (NH)(6)Mo7O24, and Ia mg ascorbic acid per determination. Results are p recise [relative standard deviation less than or equal to 3.5% for 0.1% (w/ v) H3PO3, n = 12] and in agreement with those obtained by gravimetry at 95% confidence level. (C) 2000 John Wiley & Sons, Inc.