The min equilibration in a reducing agent (1 mM

 

The 300nm RuO2
electrode was equilibrated at pH 7 for one week and then pH was measured from
12 to 2. These measurements were then repeated after equilibration for 20 min
in an oxidising agent (1 mM KMnO4); and after 20 min equilibration
in a reducing agent (1 mM ascorbic acid).

Examination of table 9 shows
that sensitivity decreases when the electrode is oxidised and increases when
the electrode is reduced. This would be due to the loss/gain of active sites
when oxidised/reduced. A large increase in E0 value when the
electrode is oxidised; accompanied by a large decrease in E0 when
reduced.

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Redox

Sensitivity
(mV/pH)

E0
(mV)

R2

pH 7

-55.4

598

0.9983

Oxidised

-51.9

844

0.9985

Reduced

-57.0

456

0.9996

Table 9 – Sensitivity and E0 values for
300 nm RuO2-OMC electrode, after exposure to pH 7 buffer, 1 mM KMnO4
(oxidised) and 1 mM ascorbic acid (reduced).

 

 

These differences are
significantly greater than the change in sensitivity can account for. Therefore
a change in E0 value is responsible, which would be due to a change
in the RuIII:RuIV ratio; as shown in equation 2. There is
a possible contribution to E0 value caused by different RuIII:RuIV
ratios. This shows that when RuO2 is highly-oxidised (or
highly-reduced) large changes in potential can occur and that E0 can
shift by approximate ±350 mV when “fully” oxidised/reduced. Interestingly, the
oxidised electrode showed an E0 increase of about 250 mV, suggesting
is was almost completely oxidised. Whilst, an initial reading (not shown) for
the reduced electrode gave an E0 reduction of about -300 mV.

 

However this rapidly drifted up
to -150 mV is likely due to oxidisation of the electrode by dissolved O2
in the buffer solutions 17. It is therefore
possible to explain the observations made as a combination of change in
sensitivity and in E0 value. For example, the base equilibrated
electrode showed more stability and lower hysteresis than the acid equilibrated
electrode; but a lower sensitivity. This is because the base equilibrated
electrode is more oxidised (due to the basic conditions) and therefore less
effected by oxidisation from dissolved O2 in solution; but exhibits
lower sensitivity due to the loss of RuIII sites.

 

 

 

 

 

 

 

 

This can also be used to explain
the negative drift observed at pH 2, which would be due to loss of RuIII
sites (reduction of electrode); which gradually change the RuIII:RuIV
ratio, decreasing the E0 value. Whilst the opposite occurred at pH
12, resulting in a positive drift. Together these effects can be used to
explain hysteresis; which is due to changes in sensitivity caused by loss/gain
of active sites, and shifts in E0 value due changes in the RuIII:RuIV
ratio caused by oxidisation/reduction of the electrode

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