simultaneously measures the temperature so that a rapid reading of pH,
compensated for temperature is obtained. The electrodes are robust and the
response time is a few seconds. An example is given in Topic H1.Gas-sensing electrodes
If gases such as CO 2 or NH 3 are allowed to diffuse into the solution surrounding
a pH electrode, they will alter the pH. The construction is shown in Figure 3(b).
The pH electrode, often incorporating a reference electrode as well, is separated
from the sample solution by a microporous hydrophobic membrane, which
will allow gases but not water to diffuse through rapidly. For CO 2 the overall
equilibrium occurs in 3 stages:(i) carbon dioxide gas diffuses from the outer solution through the membrane
until inner and outer solutions are at the same concentration;
(ii) the solution of CO 2 forms the acid H 2 CO3,which dissociates to form
hydrogen ions:CO 2 +2H 2 O =H 2 CO 3 +H 2 O =H 3 O++HCO 3 -K=a(H 3 O+). a(HCO 3 - )/(p(CO 2 (sample)))(iii) if the internal solution within the membrane has a constant activity of
HCO 3 - , for example sodium hydrogen carbonate, then the pH may be
calculated:pH =-log (H 3 O+) =log (K. p (CO 2 (sample)))so that by measuring the pH we can find the concentration of CO 2.Similar arguments apply with an ammonia-sensing electrode.Selectivity The ideal electrode should respond only to changes in concentration of a single
ion i. This is rarely true and the response of a real electrode system can be
described by the Nikolsky-Eisenmann equation:
Ei=Eni+S log[a(1) +SKpot1,2(a(2))(z1/z2)]where S is the slope of the emf -log(a) plot, which for Nernstian behavior
should be 0.0592/z(1) at 25∞C, a(1) is the activity of the ion selected of charge
z(1), a(2) is the activity of the interfering ion of charge z(2) and Kpot1,2is the selec-
tivity coefficient for ion 1 with respect to ion 2. The smaller the value of this
selectivity coefficient, the smaller the interference by the second ion.By calibration, and the use of appropriate electrode response equations, it is
possible to measure concentrations directly, as indicated in the example above for
fluoride ion in tap water.
A calibration plot should indicate three things:(i) whether the electrode responds to the correct ion;
(ii) whether that response is Nernstian;
(iii) what range of concentrations may be studied.Figure 4 shows the calibration plot for a copper ion selective electrode, where a
total ionic strength adjustment buffer (TISAB), such as 1M NaNO 3 , has been
added to each solution so that the response is effectively at constant ionic
strength and constant activity coefficient.Direct
potentiometry
72 Section C – Analytical reactions in solution