Chiang Mai Journal of Science

Flow injection amperometric sensor is herein proposed for reliable and expeditious determination of orthophosphate in terms of phosphorus content (P) in the environmentally relevant samples. The analytical method is performed by injecting the standard/sample solution into a stream of 0.1 mol L^-1 potassium chloride and then merged downstream with 0.5% (w/v) molybdate dissolved in 2.5% (v/v) sulfuric acid solution to form the intermediate, which is 12-molybdophosphate complex in a reaction coil. The intermediate was then electrochemically reduced at a glassy carbon electrode (GCE) with the applied potential of 0.20 V vs Ag/AgCl reference electrode. The reduction of intermediate produced electrical current that is directly proportional to concentration of orthophosphate in the injected solution. Linear ranges for determination of orthophosphate are spanned over three ranges from 10-100 μg P L^-1, 0.1-1.0 mg P L^-1, and 1-10 mg P L^-1, with a detection limit (LOD) of 3 μg P L^-1. Relative standard deviations for 11 replicate injections of both 0.5 mg P L^-1 and 5 mg P L^-1 were 0.8%. A sample throughput of 40-60 injections h^-1 was achieved. The proposed method was validated by the analysis of certified reference materials (CRMs) of soil and water, and was successfully applied to reallife environmental samples. Amperometric detection is a straightforward alternative to the conventional spectrophotometric one. Besides, this analytical approach does not deteriorate by interferences such as particulates, colored substances, refractive index effect (Schlieren’s effect). Especially, it can tolerate concentrations of silicate and chloride as high as 1 g L^-1 and 40 g L^-1, respectively.