SELECTIVITY OF ION-SENSITIVE BULK OPTODES

The selectivity behavior of solvent Polymeric optode membranes with two different ionophores incorporated is discussed. One of these is competing reversibly for the analyte ion; the other (chromoionophore), for a hydrogen ion. The two ions are extracted by mass transfer into the bulk of the organic phase. A novel selectivity formalism is developed on the basis of different equilibria in the absence and presence of interfering ions. The chromoionophore was assumed to bo ideally hydrogen ion selective. For a constant pH, the selectivity coefficient is given by the ratio of the two activities of the primary and the interfering ion, which load, if separately calibrated, to the same degree of protonation of the chromoionophore. The novel theory is especially helpful for ions of different valencies, where no appropriate description has been available so far. In these cases, the selectivity coefficient is differently defined as compared to the Nicolsky-Elsenman formalism and is dependent on the sample pH value. The calcium selectivity of a sodium sensor based on the ionophore ETH 4120 Is measured and compared to the now theoretical approach. It is shown that the selectivity coefficient of log K(NaCa)opt = 0.27 for a sample at pH 4.54 fulfills the requirement of less-than-or-equal-to 0.4 for measurements in diluted serum.