Accumet XL 20, User Manual, Page: 182 / 199

171
Appendix:
Determining Isopotenial Points Experimentally
XL15, 20, 25, 50 and 60 meters
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 reported as (pXi
so
,
E
is0
). For an ideal system, this point would be coincident with the system’s Zero Potential Point
(pX
iso
, E
0
).
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 an asymmetric
electrode system are contemplated. However, Isopotential correction is 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 2, the Isopotential Point is well defined as the intersection point
of the three Isotherms (pX
so
, E
iso
). The Isopotential Point is not so well defined in figure 1, but a
value may be interpolated with some accepted error.