Chiang Mai Journal of Science

 A hydrodynamic sequential injection system was built for the determination of phosphate and silicate in wastewater and fish pond water samples. It was based on the molybdenum blue method which detected a colored product spectrophotometrically at 710 nm. Stopped-flow procedure was applied in order to increase the reaction time. Two calibration graphs, for the determination of phosphate concentration and phosphate plus silicate concentration were established in the range of 0.1 – 4.0 mg L^-1 and 1.1 – 12.0 mg L^-1, respectively, with a detection limit (3σ) of 80 µg P L^-1and 90 µg Si L^-1. Sample throughput was 20 h^-1 for consecutive determination of both the analytes. Contents of phosphate and silicate found in wastewater samples by the developed method were in good agreement with those obtained by a batch spectrophotometric method.