The electrode responds to Ca2+ down to 10–^6 M (0.04 ppm) and is independent of pH in the range 6 to
- Other electrodes of this type include (Ca2+ + Mg2+), K+,. Some selectivity ratios
are given in Table 6.2.
Electrodes based on solutions of cyclic polyethers in hydrocarbons show a selective response to alkali
metal cations. The cyclic structure and physical dimensions of these compounds enable them to
surround and replace the hydration shell of the cations and carry them into the membrane phase.
Conduction occurs by diffusion of these charged complexes, which constitute a 'space charge' within
the membrane. Electrodes with a high selectivity for potassium over sodium (> 1000:1) have been
produced.
(d)—
Gas-sensing Electrodes
The concentration of gases such as CO 2 , NH 3 , SO 2 and NO 2 in aqueous solutions can be measured with
an electrode consisting of a glass electrode/reference electrode pair inside a plastic tube which is sealed
with a thin gas-permeable membrane and containing an appropriate electrolyte solution (Figure 6.5).
The membrane consists of a microporous hydrophobic plastic film through which only gases can
diffuse. On immersion of the electrode in a solution containing a dissolved gas (CO 2 , NH 3 , SO 2 or NO 2 ),
rapid diffusion through the membrane occurs until an equilibrium is established in which the gas
concentrations are the same in the internal electrolyte and the sample solution. Any change of the gas
concentration in the internal electrolyte solution results in a change in its pH which in turn is sensed by
the glass/reference pair. Such changes are directly proportional to the concentration of the gas in the
sample solution.
The following equations summarize the equilibria involved for a CO 2 - sensing electrode which utilizes a
sodium hydrogen carbonate solution as the internal electrolyte:
The overall reaction may be represented as:
By using a high concentration of in the internal electrolyte, can be considered constant,