Determination of Silicate in Anthropogenically Impacted Natural Waters Using Flow Injection Spectrophotometry

Original Paper

Abstract

An optimised flow injection (FI) method suitable for the determination of silicate in the presence of high concentrations of arsenate (216 µg As L^-1) in anthropogenically impacted natural waters is presented. The proposed method has a practical limit of detection of 10 µg Si L^-1 and typical RSDs of ≈1.5 % (n = 3) and a sample throughput of 40 samples h^-1. Strategies are presented for the removal of matrix interferences. The manifold incorporates a thiosulfate stream to remove arsenate (AsO₄^3-) by reduction to arsenite (AsO₃^3-) and two micro-columns containing a chelating resin (iminodiacetate) to remove metal ions and an anion exchange resin (Dowex 1X8, 200–400 mesh, Cl^- form) to remove phosphate. The chelating column successfully removed matrix cations; Cu(II) at 0.1 mg L^-1, Ni(II), Co(II), Fe(II) and Fe(III) at 1.0 mg L^-1 and Mn(II), Zn(II) and Pb(II) at 10 mg L^-1. The anion exchange column effectively removed phosphate interference by complexing up to 1.5 mg P L^-1 and had no effect on the silicate response. A linear calibration was obtained with the optimised manifold in the range 10 - 1000 µg Si L^-1 (R² = 0.998). This method was applied to the determination of Si in the Tamar Estuary, an area of historical mining activity, and the results compared well with those from a segmented flow analyser standard method with spectrophotometric detection (P = 0.05; t_cal = 1.44 and t_crit = 2.14).