Evaluation of spectrophotometric methods for determination of orthophosphates by sequential injection analysis

Three methods for the determination of orthophosphate have been studied by using the same sequential injection analysis manifold, namely the vanadomolybdate, the Malachite Green and the molybdenum blue methods. The calibration graphs were linear up to 18.0, 0.4 and 4.0 mg P l−1, respectively. Relative standard deviations and detection limits for vanadomolybdate, Malachite Green and molybdenum blue were 2.1%, 18% and 1.7% and 0.15, 0.01 and 0.01 mg P l−1, respectively. The three methods were applied to the determination of phosphate in several water samples, both from natural and waste origin at a frequency of about 30 determinations per hour.