40
pH/mV Amp
Electrode Response
If two solutions of known pH are measured then these values can be
used to construct a linear plot to determine
E
0
and electrode slope. The
percentage Nernstian response of the electrode is given by:
response
= 100%
×
slope
observed
/
slope
calculated
where:
•
slope
observed
is the observed slope and
•
slope
calculated
=
–2.303(RT/nF)
High quality pH electrodes will have a response in the range
95 – 102%. Cheaper or old electrodes will most likely have a response
outside this range. Ion selective electrodes often exhibit a wider
variation.
The Isopotential Point
At isopotential point, the electrode potential remains constant as
temperature changes. Most pH electrodes are constructed so that the
isopotential is about 0 mV (which for most pH electrodes also
corresponds to pH 7). However, it is not uncommon to find that even in
new pH electrodes this value may be anywhere between ±50 mV and
that the value also changes as the electrode ages. Ion selective
electrodes may have an isopotential completely outside the measuring
range of the electrode.
Most pH meters assume an isopotential point of 0 mV (at pH 7) when
applying manual or automatic temperature compensation. However,
the Chart pH Calibration extension allows you to adjust the isopotential
point if required. In order to calculate the isopotential, two calibration
runs must be made at two different temperatures,
T
1
and
T
2
.
From the Nernst equation:
E
=
E
01
+ (
slope
1
) pH
E
=
E
02
+ (
slope
2
) pH
Summary of Contents for e-corder pH/mV Amp
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