Page 155 - Analytical Electrochemistry 2d Ed - Jospeh Wang

Analytical Electrochemistry, Second Edition. Joseph Wang
Copyright # 2000 Wiley-VCH
ISBNs: 0-471-28272-3 (Hardback); 0-471-22823-0 (Electronic)

CHAPTER 5

POTENTIOMETRY

5-1 PRINCIPLES OF POTENTIOMETRIC MEASUREMENTS

In potentiometry, information on the composition of a sample is obtained through
the potential appearing between two electrodes. Potentiometry is a classical
analytical technique with roots before the turn of the twentieth century. However,
the rapid development of new selective electrodes and more sensitive and stable
electronic components over the last 25 years of the twentieth century has expanded
tremendously the range of analytical applications of potentiometric measurements.
Selective potentiometric electrodes are currently widely used in many ®elds,
including clinical diagnostics, industrial process control, environmental monitoring,
and physiology. The speed with which this ®eld has developed is a measure of the
degree to which potentiometric measurements meet the need of the analytical
chemist for rapid, low-cost, and accurate analysis. In this chapter, the principles of
direct potentiometric measurements, based on ion-selective electrodes, will be
described. (The second major aspect of potentiometry, the so-called potentiometric
titrations, will not be covered.) General books devoted exclusively to direct
potentiometry may be found in references 1 to 5.
The equipment required for direct potentiometric measurements includes an ion-
selective electrode (ISE), a reference electrode, and a potential-measuring device (a
pH=millivolt meter that can read 0.2 mV or better) (Figure 5-1). Conventional
voltmeters cannot be used because only very small currents are allowed to be
drawn. The ion-selective electrode is an indicator electrode capable of selectively
measuring the activity of a particular ionic species. Such electrodes exhibit a fast
response and a wide linear range, are not affected by color or turbidity, are not

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