STANDARD POTENTIALS OF THE SILVER/SILVER CHLORIDE
REFERENCE ELECTRODE FILLED WITH SATURATED KCl
WITH RESPECT TO THE HYDROGEN ELECTRODE
Temperature °C Uo (mV)
0 220,5
5 216,1
10 211,5
15 206,8
20 201,9
25 197
30 191,9
35 186,7
40 181,4
45 176,1
50 170,7
55 165,3
60 159,8
65 154,3
1.15.9 The redox sensor
The REDOX sensor measures the oxidation-reduction potential of the REDOX couples present in the
medium; it makes use of the same reference sensor of pH as a reference. The sensor itself consists of a
platinum wire, which ends at the tip of the glass body (in which it is embedded), where the glass body
is rounded. The REDOX state of any natural environment is the result of a combined effect of chemical
and biological processes of reversible and/or irreversible nature and, therefore, difficult to define.
Because of the number of unknown reactions and lack of reversibility, the measured potential is not
representative of a true Eh value but is only the e.m.f. of an electrochemical cell (e.g. Pt + reference
electrode). Such cell difference potentials measured in natural environments are still very often referred
to the hydrogen scale and expressed as REDOX potentials or Eh. The potential of our reference electrode
(Ag/AgCl;KCl sat.) is +202 mV at 20° C which is to be added to the measured value. Therefore, if for
example we measure a value of 100 mV, the true Eh value is +302 mV, whereas, if we measure a value
of -100 mV, the true Eh value is +102 mV, and so on. In other words, the positive potential of the
reference electrode with respect to hydrogen must be always added to the value measured by our CTD.
When using reference electrodes different from the hydrogen ones, it is usual practice to present data
as Eh by obviously taking into account the potential of the reference electrode with respect to the
hydrogen electrode. For this reason, we present REDOX data as Eh.
Type: platinum electrode.
Measurement range: -1000 to +1000 mV.
Accuracy: 1 mV.
Resolution: 0.1 mV.
Max Pressure: 700 bar.
Response time 3s.
Sensor Body: titanium.
Compensation: none.
Life: unlimited.
Calibration frequency: not available.
Maintenance: cleaning of the glass/Pt surface with
special sandpaper.
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