Anal. Chem.,1999, 71, 1176-82
Copyright © 1999 American Chemical Society

Effects of Chloride Ion Concentration on Mercury(I) Chloride Formation during ex Situ and in Situ Mercury Deposition with Selected Electrode Substrates and Electrolytes

Melissa A. Nolan andSamuel P. Kounaves*

Department of Chemistry, Tufts University, Medford, Massachusetts 02155 

Received for review July 27, 1998. Accepted January 6, 1999.

Abstract:

Ex situ and in situ mercury film formation was investigated in solutions containing NO₃^-, SCN^-, Cl^-, and ClO₄^-, and at glassy carbon macro- (3 mm) and micro- (10 mm) electrodes, a platinum macro- (1.6 mm) electrode, and a microfabricated Ir-ultramicroelectrode array (Ir-UMEA). The formation of mercury(I) chloride (calomel) during Hg deposition was investigated by varying the Cl^- concentration. The performance of the Hg films was evaluated using Cd²⁺ and Pb²⁺ with square wave anodic stripping voltammetry. For ex situ Hg film formation on Ir and GC substrates, the highest efficiency was obtained when no Cl^- or 1 M Cl^- was present. The Pt surface was affected by the Cl^- ions; thus for ex situ Hg film formation, the best results were obtained with no Cl^- present. Calomel formation during in situ Hg deposition occurred for 0.001 > [Cl^- ] < 0.5 M and was independent of the substrate. Calomel interference was identified by the presence of an anomalous cathodic peak between -0.6 and -0.3 V during the anodic scan. For in situ Hg film formation, the Pb²⁺ stripping peak current increased with increasing ionic strength, while the Cd²⁺ peak current increased dramatically in the presence of SCN^- or Cl^-. Although conditioning of the Ir-UMEA and Pt macroelectrode eliminated the cathodic peak, calomel still remained on the surface after the Hg film was removed. When the cathodic peak was present, no calomel remained after removal of the Hg film; thus for in situ film formation, the calomel interference can be avoided. When [Cl^-] > 0.01 M, the best results can be obtained by adjusting [Cl^-] to 1 M. For [Cl^-] < 0.01 M, addition of SCN^- is recommended. Although analyses can be preformed without any Cl^- present, precautions should be taken to avoid unintentional contamination.