49
To make correct redox measurements the
following conditions must prevail:
- The surface of the electrode must be
cleaned and smooth.
- The surface of the electrode must un-
dergo a preventive treatment depending
on the solution to be measured has
oxidizing or reductive characteristics.
Because the Pt/PtO system depends on the
pH, the pretreatment of the electrode may be
determined by the pH and the redox potential
of the solution to be measured.
As a general rule, if the ORP mV reading
corresponding to the pH solution value is
higher than the value in the Table below, an
oxidizing pre-treatment is necessary; other-
wise a reducing pre-treatment is necessary:
pH mV pH mV pH mV pH mV pH mV
0 990 1 920 2 860 3 800 4 740
5 680 6 640 7 580 8 520 9 460
10 400 11 340 12 280 13 220 14 160
Reducing pre-treatment: immerse the elec-
trode for some minutes in HI 7091.
Oxidizing pre-treatment: immerse the elec-
trode for some minutes in HI 7092.
If the pre-treatment is not performed, the
electrode will take significantly longer to re-
spond.
When working with electrodes of the refilling
type, the electrolyte used for filling must be
constantly kept at an adequate level (no less
than 2½ centimeters from the filling hole) and
topped up if necessary with HI 7071 refilling
solution.
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