Cationic or anionic sites? Selectivity optimization of ion-selective electrodes based on charged ionophores

The influence of ionic sites on the selectivities of ionophore-based ion-se lective electrodes (ISEs) is described on the basis of a phase boundary pot ential model. The discussion presented here is significantly more general t han previous ones. It is formulated for primary and interfering ions of any charges and it is valid for ISEs based on electrically charged or neutral ionophores, Furthermore, it also applies to membranes that contain more tha n one type of complex of the primary or interfering ion. It has been believ ed thus far that only ionic sites of the same charge sign as the primary io n improve the selectivities of ISEs based on charged ionophores, How ever, it is shown here that the charge sign of the ionic sites that give the high est potentiometric selectivities depends on the charge number of the primar y and interfering ions and on the stoichiometry of their complexes with the ionophore. The validity of our model was confirmed experimentally with thr ee ISEs based on different charged ionophores. ISEs based on lasalocid or 1 -(N,N-diclo-hexylcarbamoyl)-2-(N,N-dioctadecylcarbamoyl)ethylphosphonic aci d monomethyl ester (ETH 5639) as the ionophore responded selectively to Sr2 + or Mg2+, respectively, and discriminated well against other alkaline eart h cations when their membranes contained anionic sites. These two electrode s are the first examples of ISEs based on charged ionophores for which maxi mum selectivities are obtained with membranes containing ionic sites with a charge sign opposite to that of the primary ion. On the other hand, the ex perimental F- selectivities of membranes based on oxo(5,10,15,20-tetrapheny lporphyrinato)molybdenum-(V) improved gradually when the concentration of a nionic sites was increased from 0 to 75 mol %. The selectivity-modifying in fluence of ionic sites for these three types of ISEs can be explained by co nsidering the different stabilities of the 1:2 ion-ionophore complexes of t he primary and of the interfering ions.