eDAQ e-corder pH/mV Amp, User Manual

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28 pH/mV Amp
Isopotential Point
The isopotential point of the electrode is the potential which remains
constant as the temperature changes, see Figure 3–12. Most pH
electrodes are constructed so that the isopotential point is about 0 mV
(which for most pH electrodes also corresponds to pH 7). But it is not
uncommon to find, even in new pH electrodes that the isopotential point
is actually anywhere between ±50 mV and that this value 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
they apply MTC or ATC. However, with the Chart pH extension, you
can enter any isopotential value. To calculate this value you must do
two calibration runs at different temperatures. Refer to Appendix D for
more information on how to determine the isopotential point.
Viewing Electrode Response
The pH extension plots the voltage response of the electrode versus pH
(or other ion) giving a calibration graph of the electrode.
Click the button in the pH dialog box, Figure 3–1 , and
Figure 3–2 . Typical electrode responses are shown in Figure 3–11 . The
isopotential point will be set to zero unless temperature compensation is
turned on and a new value entered.
Adjusting the Scaling
The vertical axis will automatically scale to the full range at which the
data was recorded. For example, if you were recording on the 1 mV
range then the scale would have a range of ±1 mV. You can then drag
the axis tick marks, and tick labels to stretch and offset the axis and
obtain a better scaling.
The horizontal axis represents the ionic concentration. In the special
case of pH, the horizontal scale will be automatically scaled between 0
and 14. The axis tick marks and labels can then be dragged to new
positions to better scale the axis.