Determination of total phosphate in waste waters by on-line microwave digestion incorporating colorimetric detection

A flow-injection system was developed with on-line microwave digestion and subsequent colorimetric detection of phosphate in wastewater samples. Acidified samples were introduced into a water carrier stream and digested under continuous-flow conditions in thin-bore tubing. The samples were then cooled on-line before combining with further reagent streams. The colorimetric determination was based on the reduction of heteromolybdophosphoric acid to Molybdenum Blue with an F1 peak at 690 nm. Under optimal conditions of digestion tube length, tube diameter and reagent concentrations, calibration was linear up to 20 ppm phosphate with a detection limit of 0.1 ppm. Wastewater samples were analysed at a rate of 2 minutes per sample with a relative standard deviation of less than 5 per cent. Results were in good agreement with those obtained by a conventional block digestion autoanalyser method. Samples containing pyrophosphate species were pretreated with pyrophosphate phosphohydrolase prior to total phosphate or orthophosphate determinations.