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.
www.iupac.org/publications/pac/2002/7404/7404x0593.html