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2 • Description
especially true for metal oxide formation and reduction reactions.
Because the reference potential of the combination pH–Ag/AgCl
electrode also shifts by -0.059 V per pH unit, pH-dependent potential
shifts at the working electrode are canceled.
At a mobile phase pH of 7, the reference potential of the entire electrode
is the same as that of the Ag/AgCl half-cell. As the mobile phase pH is
increased, the pH half-cell potential decreases approximately 0.059 V per
pH unit. For example, at a mobile phase pH of 12, the reference potential
of the pH half-cell would be -0.295 V relative to the Ag/AgCl half-cell.
Therefore, at pH 12, the potentials applied to the working electrode must
be raised approximately 0.3 V when switching from the "Ag" reference to
the "pH" reference.
In acidic mobile phases, the reference potential of the pH half-cell is
positive with respect to the Ag/AgCl half-cell, and all applied potentials
must be decreased by 0.059 V per pH unit when switching from the "Ag"
reference to the "pH" reference.
Do not allow the reference electrode to dry out. Make sure that mobile
phase is always pumped through the cell. If the cell will not be used
for a short time (less than 2 days), disconnect the tubing from the inlet
and outlet fittings and install fitting plugs. For longer shutdowns,
remove the electrode from the cell and store it in its storage bottle
filled with saturated KCl solution. See Section 3.9 for detailed storage
instructions.
2.4.2
Monitoring the Amperometry Cell pH Readout
Monitoring the pH readout of a solution with a known composition lets
you detect any shift that may occur over time. A shift in the pH reading
indicates a change in the Ag/AgCl reference potential. Monitor the pH
when the reference electrode is used in the Ag mode, as well as in the pH
mode.
At installation, calibrate the pH electrode by following the instructions on
the
screen (see Section C.2.11). Then, when you run
pH CALIBRATION
your first chromatographic program, note the pH value displayed on the
or
screen (see Section C.1.3). Thereafter, by monitoring the
MAIN
DETAIL
pH value, you can determine when the reference electrode needs to be
regenerated or replaced. If the pH value drifts by 0.5 pH units from the
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