Page 389 - Instrumentation Reference Book 3E

372 Chemical analysis: electrochemical techniques

called “pIon meters.” Scales are provided, analog-
ous to the pH scale, for ions of various valencies
andlor a scale that can be calibrated to read
directly in terms of concentration or valency.
Meters manufactured as pIon meters generally
also have pH and millivolt scales. To date, plon
scales only cover ions with charges of *1 and 52
because no ion-selective electrodes for determin-
ing ions of high charge are yet available commer-
13 cm long 13 cm long cially. Direct activity scales read in relative units
3 mm diameter S mm diameter
only and so must be calibrated before use in the
preferred measurement units.
As with pH meters, pIon meters can be
obtained with analog and digital displays, with
integral microprocessors, with recorder and
printer outputs, and with automatic standardi-
zation. Temperature compensation can be incor-
;kesilver porated but although ion-selective and reference
~
10 mm diameter Platinum rod electrode combinations have iso-potential
d-l mm diameter points, the facility of being able to set the iso-
potential control has so far been restricted to
Silver electrode Platinum electrode pH measurement. On dual pHlpIon meters the
Figure 17.29 Examples of metal redox electrodes. iso-potential control (if it exists) should be
switched out on the pIon and activity scales if
reference electrodes in the sample, read off the one wishes to make a slope correction when work-
generated e.m.f. by means of a suitable measuring ing with an ion-selective electrode at constant tem-
circuit, and obtain the result from a calibration perature.
curve relating e.m.f. and concentration of the For the best accuracy and precision pIon
substance being determined. The difference from meters should be chosen that can discriminate
pH determinations is that most ion-selective elec- 0.1 mV for direct potentiometry; 1 mV discrimin-
trode applications require the addition of a ation is sufficient when using ion-selective elec-
reagent to buffer or adjust the ionic strength of trodes as indicators for titrimetric methods.
the sample before measurement of the potential.
Thus, unlike measurement of pH and redox
potentials, ion-selective electrodes cannot be Practical arrangements For accurate poten-
immersed directly in a plant stream of liquid tiometry, the temperature of the solution being
product, and a sampling arrangement has to be analyzed and the electrode assembly should be
used. However this can usually be done quite controlled and ideally all analyses should be car-
simply. ried out at the same temperature, e.g., by using a
thermostatically controlled water bath. Solutions
must also be stirred; otherwise the e.m.f. devel-
pHandpIon meters High impedance e.m.f. meas- oped by the electrode may not be representative
uring circuits must be used with most ion-selec- of the bulk of the solution. A wide range of
tive electrodes and are basically the same as used stirring speeds is possible but too slow a speed
for measuring pH with a glass electrode. The pH may give long response times and too high a
meters measure e.m.f. in millivolts and are also speed may generate heat in the solution. Precau-
scaled in pH units. Provided the calibration con- tions must also be taken to minimize contamin-
trol on the pH meter (which relates the measured ation.
e.m.f. to a fixed point on the pH scale) has a wide Taking all these items into account, best results
enough range of adjustment, the pH scale can be in the laboratory can be obtained by mounting
used for any univalent positive ion, for example, the electrodes in a flow cell through which the test
measurement with a sodium-selective electrode solution is being pumped; see Figure 17.30. This
can be read on the meter as a pNa scale is a mandatory arrangement for on-stream instru-
(or - log CN~). Measurements with electrodes ments and in the laboratory in cases where the ion
responding to divalent or negative ions cannot concentration being determined is close to the
be related directly to the pH scale. However, limit of detection of the electrode.
manufacturers generally make some modification Flow cells should be constructed of a material
to pH meters to simplify measurements with ion- that will not contaminate a sample with the ion
selective electrodes, and the modified meters are being determined; the flow rates of the solution