Page 164 - Analytical Electrochemistry 2d Ed - Jospeh Wang

5-2 ION-SELECTIVE ELECTRODES 149

across the glass membrane, with respect to the hydrogen ions in the inner and outer
solutions, produces a potential:

RT ‰H Š inner
‡
E ˆ K ‡ ln 5-10†
‡
F ‰H Š outer
The potential of the electrode is registered with respect to the external reference
electrode. Hence, the cell potential (at 25 C and after introducing the de®nition of

pH) follows the relation

E cell ˆ K ‡ 0:059pH 5-11†
0
The measured potential is thus a linear function of pH; an extremely wide (10±14
decades) linear range is obtained, with calibration plots yielding a slope of 59 mV
per pH unit. The overall mechanism of the response is complex. The selective
response is attributed to the ion-exchange properties of the glass surface, and in
particular the replacement of sodium ions associated with the silicate groups in the
glass by protons:

Na ‡ ‡ H O ‡ „ Na ‡ ‡ H O ‡ 5-12†
glass
glass
3
soln
soln
3
The theory of the response mechanism has been thoroughly discussed (15).
The user must be alert to some shortcomings of the glass pH electrode. For
example, in solutions of pH 11 or more, the electrode shows a so-called alkaline
error in which it responds also to changes in the level of alkali metal ions
(particularly sodium):
‡
‡
E cell ˆ K ‡ 0:059 log ‰H O Š‡ k H;Na ‰Na Š† 5-13†
3
As a result, the pH reading is lower than the true value (Figure 5-7). This error is
greatly reduced if the sodium oxide in the glass is replaced by lithium oxide. Still,
even with new glass formulations (with k < 10 10 ), errors can be appreciable
H;Na
when measurements are carried out in highly basic solutions (e.g., NaOH). Many
glass electrodes also exhibit erroneous results in highly acidic solutions pH < 0:5†;
the so-called acid error yields higher pH readings than the true value (Figure 5-7).
Before the pH electrode is used, it should be calibrated using two (or more)
buffers of known pH. Many standard buffers are commercially available, with an
accuracy of 0.01 pH unit. Calibration must be performed at the same temperature
at which the measurement will be made; care must be taken to match the temperature
of samples and standards. The exact procedure depends on the model of pH meter
used. Modern pH meters, such as the one shown in Figure 5-8, are microcomputer
controlled, and allow double-point calibration, slope calculation, temperature adjust-
ment, and accuracy to 0.001 pH unit, all with few basic steps. The electrode must