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(-), NO3-, SCN-, ClO4-) 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 ClO4-. Because of these two latter properties presented by the SA+/Cl-, the derivative SA(+)/ClO4- 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-A,B(pot), were obtained for some interfering anions, and the following interference order was observed: F- > SCN- > NO3- > Br- > Cl- > CH3COO-. The electrode showed fast and stable responses and was useful for approximately 200 measures. (C) 2001 Academic Press.