Figure 6.5
Schematic diagram of a gas-sensing electrode. Selective ion
electrode is shown as a glass electrode. The reference
electrode is an Ag–AgCl electrode. Other electrode
combinations are possible.where k' includes Ke and the response of the electrode is given by
and k" includes k' (cf. equation (6.11)).
Thus, the pH response of the glass/reference pair is a function of the CO 2 content of the sample.
Similar equations for an ammonia-sensing electrode whose internal electrolyte is an ammonium salt
lead to the relation
and the pH response of the glass/electrode pair is a function of the NH 3 concentration in the sample.
Gas-sensing electrodes differ from ion-selective electrodes in that no species in solution can interfere
with the electrode response as only gases can diffuse through the membrane. However, it should be
noted that any gas which causes a pH change in the internal electrolyte solution will affect electrode
response.