Chemistry Reference
In-Depth Information
Table 6.3
Species potentially present or formed during the oxidation of sodium
dithionite at a platinum-disc electrode in alkaline solution
Species in solution and/or adsorbed at
Species adsorbed at the
the electrode surface
electrode surface
S
2
O
4
2
-
SO
3
2
-
S
2
O
4
•
-
SO
2
•
SO
4
2
-
H
2
O
SO
2
-
SO
-
OH
-
HSO
3
-
OH
•
SO
3
-
SO
2
As a starting point, the experimental values of ca. 0.5 and 2 obtained for
the charge-transfer coefficient and the number of electrons transferred in
the first oxidation wave of dithionite, respectively, were linked to the theo-
retical expression for the transfer coefficient:
[
(
)
]
-
a
=-
n
g
v
r
b
[6.7]
where g is the number of electrochemical elementary steps preceding the
rate-determining step (RDS), n is the number of times that the rate-
determining step occurs in the overall reaction, and b is the symmetry factor
presumed to be equal 0.5;
r
= 1 if the rate-determining step is electrochemi-
cal, and
r
= 0 if the rate-determining step is chemical. It is clear that the rate-
determining step of the proposed mechanism should be electrochemical,
otherwise a value of 0.5 cannot be obtained with
n
= 2. Therefore two possi-
bilities remain, namely that the rate-determining step occurs once (n=1),
preceded by another electrochemical step (g=1) or that the rate-determin-
ing step occurs twice (n=2). The latter implies that an intermediate species
should be formed twice in one or two elementary chemical steps preceding
the rate-determining step. Preference was given to the second possibility
because it would be compatible with the experimental reaction order with
respect to dithionite being 0.5. Assuming that the final electrochemical step
is rate-determining and that this step occurs only once can never explain a
dithionite reaction order of 0.5. Therefore it is presumed that the following
dissociation reaction is the initial step of the proposed mechanism:
¨
[6.8]
SO
2
-
SO
-
24
2
Evidence that this reaction occurs in homogeneous acidic solution was
found in the literature
42
. However, the present study was carried out in alka-
line solution, and therefore it is assumed that this dissociation reaction
occurs only at the surface of the electrode. If that is the case, the initial step
of the mechanism, equation 6.8 is replaced by:
¨
(
)
[6.9]
SO
4
2
-
SO
4
2
-
2
2
ads
¨
(
)
(
)
SO
4
2
-
2
SO
-
[6.10]
2
2
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