Thermo Fisher Scientific CyberScan PC 5000 - Manual (Page 156)

Instruction Manual CyberScan PC5500/ 5000
151
Figure 1
Figure 2
17 APPENDIX
17.1 Determining Isopotential Points Experimentally
The isopotential point of an electrode system is the point at which
electrode potential is unaffected by a change in temperature. The
coordinates of this point would be coincident with the system’s Zero
Potential Point (pX
iso,
E
o
).
In practical systems, however, this coincidence rarely occurs, and for
some systems, there is no true Isopotential Point but a general
Isopotential area. If a system exhibits an apparent Isopotential Point,
or at least an Isopotential area with relatively small spread.
Isopotential coordinates may be established and possibly used to
some advantage.
Isopotential correction may be used only in conjunction with a One or
Two Point Standardization ; it is required only when both accuracy
over a significant temperature range as well as operation with
asymmetric never necessary if all measurements will be performed
on samples which are at similar temperatures. Most conventional pH
electrode systems are designed and manufactured to be highly
symmetrical cells. Consequently, for most pH work, unless ultimate
accuracy over broad temperature ranges is required, Isopotential
may be ignored. Specifically, this is accomplished by setting the
Isopotential at its reset value of zero millivolts.
The following is an example for determining an Isopotential Point:
Data points are first established for three different standards at three
different temperatures. The data points are then plotted and will
produce three isotherms which should resemble the plot in either
figure 1 or figure 2. in figure 1, the Isopotential point is well defined as
the millivolt difference between the coincidence point of the three
isotherms (pX
iso,
E
iso
) and the Zero Potential Point (E
o
). The
Isopotential Point is not so well defined in figure 2, but a value may
be interpolated with some accepted error.
(pX iso , E iso )