Equivalent ionic conductances selected ions

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

According to the above definition, direct detection is feasible when using carefully selected eluents such as phthalate [5] or benzoate [6], which exhibit a low equivalent ionic conductance (see Table 6-1). This results in a conductivity increase when a solute ion passes the conductivity cell. 

Table 2 Equivalent ionic conductivity of selected ions at infinite dilution at 25°C (Scm mol ) ...

Conductometric titrations. Van Meurs and Dahmen25-30,31 showed that these titrations are theoretically of great value in understanding the ionics in non-aqueous solutions (see pp. 250-251) in practice they are of limited application compared with the more selective potentiometric titrations, as a consequence of the low mobilities and the mutually less different equivalent conductivities of the ions in the media concerned. The latter statement is illustrated by Table 4.7108, giving the equivalent conductivities at infinite dilution at 25° C of the H ion and of the other ions (see also Table 2.2 for aqueous solutions). However, in practice conductometric titrations can still be useful, e.g., (i) when a Lewis acid-base titration does not foresee a well defined potential jump at an indicator electrode, or (ii) when precipitations on the indicator electrode hamper its potentiometric functioning. 

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