Novel Anion Exchangers for Electrodes with Improved Selectivity to Divalent Anions

Abstract

It has been found that replacing of several long-chain alkyl substituents at the nitrogen atom of lipophilic quaternary ammonium salts (QAS) by methyls results in a dramatic increase of the potentiometric selectivity of ion-selective electrodes (ISE) with QAS-based plasticized PVC membranes to some divalent anions against the monovalent ones. The discussed effect of QAS cation nature on the potentiometric selectivity is also partly retained for ISE with neutral carrier-based membranes doped with QAS to provide anion permselectivity. This opens up new possibilities to control the potentiometric selectivity of ISE for divalent anions by the appropriate selection of the anion exchanger.Keywords: Liquid anion exchangers, Anion-selective electrodes, Quaternary ammonium salts, Selectivity, Sulfate Lipophilic quaternary ammonium salts used in the membranes of anion-selective electrodes usually contain four long-chain hydrocarbon substituents or three long-chain and one methyl substituent at the nitrogen atom [1 ± 3]. Such electrodes possess extremely low selectivity towards hydrophilic divalent anions as compared to monovalent ones. One of the reasons for this behaviour seemed to be steric hindrance preventing simultaneous approach of two QAS cations to the divalent anion. So the possibility of improvement of the ISE selectivity to divalent anions against monovalent ones by means of introduction of methyl substituents into QAS cation has been studied in a series of works [4 ± 6]. However, the development of this idea and its practical realization was hindered by relatively high hydrophilicity of QAS containing more than one methyl substituent. Because of this, we have synthesized several anion exchangers with the general formula [(C 43 H 79 , and investigated the selectivity of toluene extraction systems as well as PVC plasticized membranes containing these compounds. The obtained results demonstrate that increasing the number of methyl substituents in QAS cation leads to a dramatic improvement both of the exchange and potentiometric selectivity to double-charged anions against the single-charged ones. It has been established that the above-mentioned effect is also partly retained for the membranes based on neutral anion carrier ± hexyl 4-trifluoroacetylbenzoate (HTFAB) ± doped with QAS to provide anion permselectivity. Therefore the appropriate selection of the anion exchanger can provide an additional means for the selectivity control of the ISE for determination of divalent anions. It is well-known that the potentiometric selectivity coefficients (K ij Pot) of liquid ion-exchange membranes are the function of the relative efficiency of interactions of the ion-exchange sites with the principal and interfering ions. However, experimental values of K ijPot depend upon the parameters of interphase and intramembrane equilibria and upon the diffusion processes in the membrane phase in a rather complex way [7 ± 10]. Besides that, ionic impurities of the membrane can affect both the boundary and diffusion potential...

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