Determination of mercury(II) at trace levels by gas-diffusion flow injection analysis with amperometric detection

A gas-diffusion flow injection (GD-FI) system for the determination of Hg(II) in aqueous samples at trace levels is described. The analytical procedure involved injection of Hg(II) samples and standards into a 1.5 M H2SO4 stream which was subsequently merged with a reagent stream of 0.6% SnCl2 in 1.5 M H2SO4 to reduce Hg(II) to metallic mercury. The gas-diffusion cell was thermostated at 85 °C to enhance the vaporization of mercury. The mercury vapour diffused across a Teflon membrane into an acceptor stream containing 0.050 M KNO3 where it was detected amperometrically at a gold electrode set at +600 mV versus Ag/AgCl reference electrode. Under optimal conditions, the flow injection method is characterized by a linear calibration range between 0 and 80 μg l−1, a detection limit of 900 ng l−1 and a sampling rate of 12 samples per hour. The flow injection system was successfully applied to the determination of mercury in freshwater samples containing colored compounds and suspended particulates and to digested sediment and soil samples. Satisfactory agreement with results obtained by cold vapour atomic absorption spectrometry was observed.