Determination of anionic impurities in hexafluorosilicic acid by capillary zone electrophoresis

Abstract

This paper reports an alternative analytical methodology for the determination of chloride and sulphate impurities in hexafluorosilicic acid, FSA, using capillary zone electrophoresis. The influence of different electrophoretic and matrix variables on electrophoretic response was studied. Samples were injected hydrodynamically onto a fused silica capillary of 63.5 cm × 75 μm id, with an effective length of 55 cm. The capillary temperature was kept constant at 25 ± 1 °C throughout the experiments. A voltage of −25 kV was applied (in anionic mode). The electrophoretic zones were detected with a fixed-wavelength UV detector at 254 nm and a diode array detector at 254 and 370 nm. The final background electrolyte consisted of 20 mM sodium chromate at pH 8.0 containing 6% MeOH and 1.5 μM tetradecyltrimethylammonium hydroxide. In these electrophoretic conditions, the separation of chloride and sulphate anions is achieved in less than ten minutes. The introduction of a water gap before the sample injection increases considerably the capillary life. The electrophoretic method developed allows the determination of chloride and sulphate at mg L⁻¹ concentration levels in industrial production FSA samples.