Page 384 - Instrumentation Reference Book 3E

Potentiometry and specific ion measurement 367

g Gate
d Drain
s Source

Figure 17.23 pH measuring circuit using field effect transistors.

If the iso-potential point is arranged to be the log meters have a facility of expanding the scale
locus of the slope of' the measuring instrument, so that the precision of the reading can be
the pH measuring circuit can be modified to increased up to 10 times. Digital outputs are also
include a temperature sensor arranged to change available, with the most sensitive ones reading to
the negative feedback so that the circuit compen- 0.001 pH unit (unlikely to be meaningful in prac-
sates for the change in slope of the e.m.f./pH tice) or 0.1 mV. Instruments incorporating micro-
relationship. It is important to realize that the processors are also now available-these can
temperature compensation only corrects for the calculate the concentration of substances from
change in the electrode response due to tempera- pH measurements and give readout in concentra-
ture change and the iso-potential control setting tion units. Blank and volume corrections can be
therefore enables pH electrodes calibrated at one applied automatically.
temperature to be used at another. The iso-poten-
tial control does not compensate for the actual Precision and accuracy Measurements reprodu-
change in pH of a solution with temperature. cible to 0.05 pH units are possible in well buffered
Thus if pH is being measured to establish the solutions in the pH range 3 to 10. For routine
composition of a solution one must carry out measurements it is rarely possible to obtain a
the measurements at constant temperature. reproducibility of better than kO.01 pH units.
A few commercial pII meters have a variable In poorly buffered solutions reproducibility may
iso-potential control so that they can be used with be no better than &O. 1 pH unit and accuracy may
several different combinations of electrodes but it be lost by the absorption of carbon dioxide or by
is more generally the case that pH meters have the presence of suspensions, sols, and gels. How-
fixed iso-potential control settings and can only ever, measured pH values can often be used as
be used with certain combinations of pH and control parameters even when their absolute
reference electrodes. It is strongly recommended accuracies are in doubt.
that, with fixed iso-potential control settings,
both the glass and reference electrodes be Sodium ion error Glass electrodes for pH meas-
obtained from the manufacturer of the pH meter. urement are selective for hydrogen ions, not
Temperature compensation circuits generally uniquely responsive to them. and so will also
work only on the pH and direct activity ranges respond to sodium and other ions especially at
of a pH meter and not on the millivolt, expanded alkaline pH values (more than about 11). This
millivolt, and relative millivolt ranges. effect causes the pH value to be underestimated.
Modern pH meters with analog displays are Sodium ions produce the greatest error, lithium
scaled 0 to 14pH units with the sniallest division ions about a half, potassium ions about a fifth.
on the scale equivalent to 0.1 unit giving the and other ions less than a tenth of the error due to
possibility of estimating 0.02pH units by inter- sodium ions. One can either standardize the elec-
polation. The millivolt scale is generally 0 to trode in an alkaline buffer solution containing a
1400mV with a polarity switch, or -700 to suitable concentration of the appropriate salt, or
+700mV without one. The smallest division is better, use the special lithium and cesium glass
10mV, allowing estimation to 2mV. Many ana- electrodes developed for use in solutions of high