Electrochemical Measurement of Metal Ions in Water

Abstract

Pb2+ between 0 and 50 PPB is measured electrochemically in synthetic and real drinking water by square wave anodic stripping on a gold working electrode. The rate constant of the anodic stripping of Pb0 from gold has a rate constant of approximately k0 = 3 s-1. A set of square wave profiles defined by step height = 1 mV, pulse height = 30 mV, and pulse width = 50, 100, 200 ms yield calibration slopes in synthetic water with relative errors ranging from 1% to 20%. Synthetic water at pH 5, 7 and 9 and conductivity 300, 600, and 900 uS/cm with sodium sulfate electrolyte are measured. In pH 5 solution the magnitude of the Pb0 anodic stripping peak is lower compared to pH 7 and pH 9 for the same lead concentration. Pbx(OH)y(x-y)+ forms at pH 7 and 9 and adsorbs to the gold working electrode resulting in higher anodic stripping current. The presence of phosphate in solution increases the lead signal in the same way. The history of the working electrode potential influences the measurement of Pb2+ by anodic stripping. Potential cycling in the gold oxide region generates a clean gold surface relatively free of adsorbed impurities. The measurement of Pb2+ can be conducted quickly after potential cycling pretreatment to reduce dependence on solution impurities. This strategy is demonstrated on real drinking water samples and qualitative agreement is obtained between the electrochemical signal and the concentration measured by inductively coupled plasma mass spectrometry.