A polyvalent flow injection system was proposed for spectrophotometric determination of calcium, magnesium and phosphorus in acid plant digests. O-cresolphtalein complexone (CPC) was the chromogenic reagent for calcium and magnesium determination, and ammonium molybdate/vanadate for phosphorus determination. Ammonium-ammonia was elected as buffer system, and EGTA or 8-hydroxyquinoline as the masking agents for calcium or magnesium. Linearity of the calibration equations was observed up to 40.0 mg Ca L-1, 10.0 mg Mg L-1 and 16.0 mg P L-1. Slight variations in the coefficients of the calibration equations (usually < 3 %) were found after 4-h working periods. Precise results (r.s.d. < 0.02), in agreement with alternative method (flame atomic absorption spectrometry) were obtained. When the system was applied to large scale analysis, good repeatability was observed, and a sample throughput, of about 107 h-1 for magnesium and 75 h-1 for calcium and phosphorus was achieved, emphasizing the simplicity, robustness and versatility inherent to flow analysis.
flow injection analysis; calcium; magnesium; phosphorus; plants