Determination of sub-nanomolar concentration of arsenic(III) in natural waters by square wave cathodic stripping voltammetry

A method is presented for the measurement of arsenic(III) in natural waters using square wave cathodic stripping voltammetry at a hanging mercury drop electrode. Pre-concentration was carried out in 2 M HCl in the presence of 0.8 mM CuCl2 at a potential of -0.4 V vs. Ag/AgCl, The deposited intermetallic compound was reduced at a potential of about -0.8 V vs. Ag/AgCl. The square wave frequency was 70 Hz with a staircase step of 2 mV and a purse height of 25 mV. The peak current decreased with scan number, however, this effect can be minimized by the addition of hydrazine. The detection limit is approximately 0.06 nM (5 parts per trillion) when a 10 min deposition time was used. The relative standard deviation was calculated as 8% (n = 11) at 1 x 10(-9) M arsenic for 1 min period of deposition. The method was tested by the measurement of arsenic(III) in both river and sea water.