Metrohm 2.946.0010, Manual

Page: 72 / 81

7.4 Some voltammetric troubles in detail
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64
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Figure 18 Influence of oxygen on the background current of the
scTRACE Gold electrode. (Measuring solution: c(H
2
SO
4
) = 1
mmol/L, c(KCl) = 50 mmol/L)
with oxygen after 5 min purging with nitrogen
For a determination that means all substances which show a peak more
positive than -0.2 V can be determined without purging the measuring
solution. For substances which show a peak more negative than -0.2 V
oxygen will interfere. The interference can be either removed from the
measuring solution by purging or, in some cases, can be suppressed by
the choice of voltammetric measuring technique (fast square-wave or lin-
ear sweep voltammetry).
Influence of pH
The major influence of the pH can be observed in section
Ⓒ
of the curve
shown in Figure 19 . Section
Ⓑ
is discussed in subchapter Influence of
oxygen
and section
Ⓐ
to a great extent in the subchapter
Influence of
chloride . What affects the background current in section
Ⓐ
in slightly acid
and alkaline solution in addition to what is already described for the chlor-
ide, is the formation of Au(OH)
3
.
The big drop in the current in section
Ⓒ
is related to the reduction of the
electrolyte. The easiest component in the electrolyte to be reduced is H
+
.
The reaction taking place is: 2 H
3
O
+
+ 2 e
-
→
H
2
+ 2 H
2
O. Since the con-
centration of H + is the highest in the measuring solution with pH 2, the
reaction already becomes obvious at potentials more negative than -0.5 V.
In only slightly acid (pH 5) or alkaline (pH 12) solution the concentration of
H
+ is significantly smaller, therefore a significantly more negative potential
is required to have the same rate of H + reduced.