20
pH/mV Amp
Electrode Calibration
Choosing the pH... command (available after installing the pH
Calibration extension) from a channel pop-up menu accesses the
Electrode Calibration dialog box,
Figure 3–1
and
Figure 3–2
.
If a region of the electrode signal has first been selected in the Chart
window then that region will also appear in the display area of the
Electrode Calibration dialog box.
The raw signal values (with units of V or
mV) and the corresponding pH values are
entered in the boxes (at right) to calibrate the electrode. For single
point calibration only the top set of boxes need to have values
entered. The software will calculate the electrode response based on
an assumed isopotential point of zero mV and an ideal 100%
Nernstian response. For more accurate calibration two calibration
points should be used and the actual electrode response will be
calculated.
While the raw signal values can be entered directly by typing them
in, it is more usual for the electrode to be immersed in a standard
buffer solution of known pH (or ion concentration) and the signal
recorded for a minute or so until it has stabilized. If two point
calibration is required, then the electrode is immersed in a second
standard buffer solution of a different pH (or ion concentration).
After recording the raw signal from the two buffer solutions, a signal
similar to that shown in
Figure 3–3
should be obtained. Select a
region that includes the signal response from both buffer solutions,
and choose pH... from the Channel Function pop-up menu
.
This
opens the electrode calibration dialog box with the selected
waveform displayed,
Figure 3–1
and
Figure 3–2
.
Select a suitable part of the signal in the data display by using the
mouse to drag the pointer across it. You can also shift and stretch
the vertical Amplitude axis to make the best use of the available
display area.
If you click in the data display area, a vertical line indicates the
active point, and an indicator arrow, , shows the point of
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