Chemistry Reference
In-Depth Information
3
2
2
3
1
1
0
0
200
400
600
800
1000
Time (min)
6.13
Variation of experimental currents measured at a potential of (1)
0.45 V showing decomposition of sodium dithionite, and (2) 0.8 V
vs. Ag|AgCl showing the decomposition of dithionite and
formation of sulphite as reaction product of dithionite
decomposition. (3) Current contribution of sulphite calculated
according to Equation 6.28. (Reprinted from
Analytica Chimica
Acta
, Vol 486, No 1, Gasana
et al
., 'A wall-jet disc . . .' pp 73-83,
Copyright 2003, with permission from Elsevier.)
From the data shown in Fig. 6.13, the concentrations of sodium dithionite
and sulphite can be calculated by Equations 6.27 and 6.28 after calibration
(determination of B and B¢). This is achieved by measuring the limiting
current in a solution with known dithionite and sulphite concentration
(determined by titration). It was found that B and B¢ were equal to 0.75 ±
0.01 and 0.74 ± 0.04 A l mol
-1
:
c
=
B
I
[6.27]
dit
045
.
V
(
)
c
=¢
I
-
3
I
[6.28]
sul
08
.
V
045
.
V
The results of this calculation for the above-mentioned decomposition
experiment are given in Fig. 6.14 by the continuous lines. The crosses illus-
trate the variation of the concentration of dithionite and sulphite in solu-
tion obtained by titration of samples taken at fixed times. It can be seen
that the concentrations obtained by calculation from the experimental cur-
rents (continuous line) are in good agreement with the ones obtained by
titration (crosses). Note also that the increase of the concentration of sul-
phite is twice as high as the drop of the sodium dithionite concentration.
Indeed, for each dithionite ion that is decomposed, two sulphite ions are
