The
selectivities of ionophore-doped ion-selective electrode (ISE)
membranes are controlled by the stability and stoichiometry of the
complexes between the ionophore, L, and the target and interfering
ions (I
zi and J
zj, respectively). Well-accepted models predict how these selectivities
can be optimized by selection of ideal ionophore-to-ionic site ratios,
considering complex stoichiometries and ion charges. These models
were developed for systems in which the target and interfering ions
each form complexes of only one stoichiometry. However, for a few
ISEs, the concurrent presence of two primary ion complexes of different
stoichiometries, such as IL
zi and IL2
zi, was reported. Indeed, similar
systems were probably often overlooked and are, in fact, more common
than the exclusive formation of complexes of higher stoichiometry
unless the ionophore is used in excess. Importantly, misinterpreted
stoichiometries misguide the design of new ionophores and are likely
to result in the formulation of ISE membranes with inferior selectivities.
We show here that the presence of two or more complexes of different
stoichiometries for a given ion may be inferred experimentally from
careful interpretation of the potentiometric selectivities as a function
of the ionophore-to-ionic site ratio or from calculations of complex
concentrations using experimentally determined complex stabilities.
Concurrent formation of JL
zj and JL2
zj complexes of an interfering ion is shown here to shift the ionophore-to-ionic site ratio that
provides the highest selectivities. Formation of IL
n–1
zi and IL
n
zi complexes of a primary ion is less of a concern because an optimized membrane
typically contains an excess of ionophore, but lower than expected
selectivities may be observed if the stepwise complex formation constant, K
ILn, is not sufficiently large and the ionophore-to-ionic
site ratio does not markedly exceed n.