Quaternary Ammonium Salts Immobilized on Silica Gel: Exchange Properties and Application as Potentiometric Sensor for Perchlorate Ions.

Ammonium chlorides immobilized on silica gel, SA(+)/Cl(-) and SE(+)/Cl(-), were obtained from silica previously modified with 3-aminopropyltriethoxysilane and N-[3-(trimethoxysilyl)-propyl]ethylenediamine, respectively. Both materials showed potential use as an anion exchanger: they are thermically stable (up to 413 K), achieve equilibrium rapidly in the presence of suitable exchanger ions, and are easily recovered. The exchange capacities observed for SA(+)/Cl(-) and SE(+)/Cl(-) are 0.70 and 1.19 mmol Cl(-) g(-1), respectively. Through the exchange isotherms and competitive Cl(-)-X(-) exchange (X(-)=F(-), Br(-), I(-), N(3)(-), NO(3)(-), SCN(-), ClO(4)(-)) it was observed that: (i) SE(+)/Cl(-) exchanges the counterion Cl(-) more easily than SA(+)/Cl(-); (ii) SA(+)/Cl(-) presents higher selectivity than SE(+)/Cl(-); and (iii) SA(+)/Cl(-) presents high affinity for ClO(4)(-). Because of these two latter properties presented by the SA(+)/Cl(-), the derivative SA(+)/ClO(4)(-) was used as a potentiometric sensor for this anion, prepared from the supported material on a mixture of graphite powder with epoxy resin. The electrode showed a nernstian behavior and a limit of response of 0.13 mmol L(-1). Potentiometric selectivity coefficients, K(pot)(A, B), were obtained for some interfering anions, and the following interference order was observed: F(-) > SCN(-) > NO(3)(-) > Br(-) > Cl(-) > CH(3)COO(-). The electrode showed fast and stable responses and was useful for approximately 200 measures. Copyright 2001 Academic Press.