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338 ELECTROCHEMISTRY

Table 7.11 Advantages and disadvantages of the pH electrode

Advantages
1. If recently calibrated, the GE and pH electrodes give an accurate response
2. The response is rapid (possibly millisecond)
3. The electrodes are relatively cheap
4. Junction potentials are absent or minimal, depending on the choice of reference electrode
5. The electrode draws a minimal current
6. The glass is chemically robust, so the GE can be used in oxidizing or reducing
conditions; and the internal acid solution cannot contaminate the analyte
5
7. The pH electrode has a very high selectivity – perhaps as high as 10 : 1 at room
temperature, so only one foreign ion is detected per 100 000 protons (although see
◦
disadvantage 6 below). The selectivity does decrease a lot above ca 35 C
Disadvantages
Both the glass and pH electrodes alike have many disadvantages
1. To some extent, the constant K is a function of the area of glass in contact with the acid
analyte. For this reason, no two glass electrodes will have the same value of K
2. Also, for the same reason, K contains contributions from the strains and stresses
experienced at the glass.
3. (Following from 2): the electrode should be recalibrated often
4. In fact, the value of K may itself be slightly pH dependent, since the strains and stresses
themselves depend on the amount of charge incorporated into the surfaces of the glass
5. The glass is very fragile and, if possible, should not be rested against the hard walls or
floor of a beaker or container
6. Finally, the measured emf contains a response from ions other than the proton. Of these
other ions, the only one that is commonly present is sodium. This error is magnified at
very high pH (>11) when very few protons are in solution, and is known as the
‘alkaline error’

Justification Box 7.3

At heart, the pH electrode operates as a simple concentration cell. Consider the schematic
+
H (a 2 )||H (a 1 ), then the Nernst equation can be written as Equation (7.48):
+
RT a 2
emf = ln
F a 1
which, if written in terms of logarithms in base 10, becomes

2.303RT a 2
emf = log 10 (7.50)
F a 1

Subsequent splitting of the logarithm terms gives

2.303RT 2.303RT
emf = log a 2 − log 10 a 1 (7.51)
10
F F