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

Abstract

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−, N3−, 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, KpotA, B, 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.