he Mechanism of the Electrochemical Tungsten and
Molybdenum Determination
Both the molybdenum and the tungsten responses
show overlapping major and minor peaks (Fig. 1),
which have not been mentioned by Wang and Lu (11).
Similar observations have been reported for several
electroanalytical molybdenum determinations (9, 25,
26). Three possible mechanisms have been proposed in
the literature (10, 25, 27) for electroanalytical molybdenum
determinations using one chelator and catalytic
reduction of chlorate or nitrate. The three mechanisms
involve a reduction of MoVI to MoV at the electrode as a
first step. The proposed catalytic cycle, which follows,
is different in all three cases. The species that have
been proposed to reduce chlorate or nitrate are MoIII,
MoIV, and MoV
. The mechanism of the AdSV method by
Wang and Lu is more complicated than the previously
discussed ones, since it involves two different chelators
(11). The stoichiometry of the different chelators in the
complex is not known. It may be that complexes are
formed with different stoichiometries of the chelators.
The different peaks found for molybdenum and tungsten
can thus be the result of the different oxidation
states of the metals and of different metal–chelate
complexes formed with slightly different reduction potentials.
For a complete understanding of the present
data, these aspects would require a more detailed investigation.