Here,
we report on systematic investigation of the impact of coextraction
of the aqueous electrolyte and anion interference on the response
of cation-selective bulk optodes. It is evident that to deliberately
manage the properties of chemical sensors and to apply them in routine
analysis, one should have exhaustive insight into their operation
mechanism. Despite the extensive research in the field of ionophore-based
optodes and numerous attempts for their practical application, the
understanding of how coextraction of an aqueous electrolyte influences
its response characteristics has not been developed thus far. Meanwhile,
the electrolyte coextraction determines the detection limits of analogous
ion-selective electrodes. A theoretical model based on phase distribution
equilibrium is proposed to quantitatively describe the effect of Donnan
exclusion failure on the response of polymeric plasticized optodes.
The theoretical conclusions are confirmed by the results obtained
with Na+/pH-selective optodes based on a neutral chromoionophore
as a model system in solutions containing anions of various lipophilicities
(Cl–, NO3
–, I–, SCN–, and ClO4
–).
For the first time, it is shown that coextraction leads to a significant
shift of the response range of the optodes as well as to nonmonotonic
response curves due to the transition from cationic to anionic response.
An approach to estimate the coextraction constants of electrolytes
from the optode response curves is proposed. The limitations in the
applicability of optodes due to co-ion interference are explored.
It is found that neglecting anion interference can cause dramatic
errors in the results of analyses with optical sensors.