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Chemistry International
Vol. 24, No. 5
September 2002


Highlights from Pure and Applied Chemistry

Sulfate-Sensing Electrodes. The Lead- Amalgam/Lead-Sulfate Electrode (IUPAC Technical Report)

by Patrizia R. Mussini and Torquato Mussini
Pure and Applied Chemistry, Vol. 74, No. 4, pp. 593-600 (2002)

It has long been recognized that sensitive and reproducible sulfate-reversible electrodes (e.g., the Pb|PbSO4 or Hg|Hg2SO4 electrode) are not as readily available as chloride-reversible electrodes (e.g., a widespread Hg|Hg2Cl2 or Ag|AgCl electrode). In this context, two major features are evident: the activity solubility products of PbSO4 and Hg2SO4 are larger than those of Hg2Cl2 and AgCl by several orders of magnitude, and in the case of the Pb|PbSO4 electrode, the preparative and operational procedure had not been assessed satisfactorily until recently, so that the electrode in both Pb|PbSO4 and Pb(Hg)|PbSO4 forms proved difficult to use and/or was unsatisfactorily reproducible.

In this report a new, simplified design and a convenient preparation procedure for the Pb(Hg)|PbSO4|SO42- electrode is proposed. This procedure ensures preparation of stable amalgams and reproducible electrode potentials, which make this electrode useful and attractive for both thermodynamic investigations and electroanalytical applications. For these purposes, the electrode prepared according to the proposed procedure has been exhaustively characterized both thermodynamically and as a sulfate-sensing electrode, in different sulfate solutions, including H2SO4.

The report also proposes a practical standardization procedure. The Pb(Hg)|PbSO4|SO42- electrode can be structured with a built-in concentrated Li2SO4 salt bridge for use as a sulfate-based reference electrode. Li2SO4 also has favorable properties as a salt bridge in some mixed aqueous-organic solvents, e.g., acetonitrile-water mixtures, and its combination with the lead-amalgam|lead-sulfate electrode in such solvents is an interesting perspective, for which further accumulation of data is awaited. This electrode can be operated as a reference electrode alternative to the conventional calomel or Ag|AgCl reference electrodes in electroanalytical practice.



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