Iodide-selective carbon paste electrodes based on recently synthesized Schiff base complexes of Fe(III)

Abstract

A new modified carbon paste electrode (CPE) based on a recently synthesized Schiff base complex of Fe(III) as a suitable carrier for I⁻ ion is described. The electrode exhibits a super Nernstian slope of 71.0±0.3 mV per decade for I⁻ ion over a wide concentration range from 1.0×10⁻⁶ to 5.0×10⁻¹ M, with a low detection limit of 6.5×10⁻⁷ M. It has a relatively fast response time, a satisfactory reproducibility and relatively long life time. The proposed sensor shows a fairly good selectivity toward I⁻ ion in comparison to other common anions. The potentiometric response is independent of the pH of the test solution in the pH range 3.5–10.0. Spectrophotometric studies confirmed the redox-type response mechanism of the electrode toward iodide ion. The proposed electrode was used as an indicator electrode in potentiometric titration of iodide ion.

Introduction

Carbon pastes are well known as useful materials for the fabrication of various electrometric sensors for analytical purposes [1]. The operation mechanism of such carbon paste electrodes (CPEs) depends on the properties of the modifier materials used to import selectivity towards the target species [2]. Since the first use of CPEs containing a co-precipitated silver halide–silver sulfide for potentiometric monitoring of halide and silver ions by Mesaric and Dahmen [3] in 1973, a number of CPEs for the determination of different anions [4], [5], [6], [7], cations [8], [9], [10] and biologically important molecules [11], [12] have been reported. In comparison with ion-selective electrodes based on polymeric membranes, the CPEs possess advantages of much lower ohmic resistance, very stable response and easy renewal of surface.

Ion-selective electrodes for iodide ion were first developed based on ionophore free ion exchangers or precipitates long ago [13], [14]. However, carrier-based I⁻ ion-selective electrodes have been reported only recently. These are based on incorporation of suitable ionophores such as di-quaternary ammonium salts [15] and metal complexes of Schiff bases [16], [17], [18], [19], Vitamin B₁₂ derivatives [20], and some aza-substituted macrocyclic ligands [21] into PVC membranes. The observed anti-Hofmeister behavior arises from a strong interactions between I⁻ ion and the ionophores used. It has been shown that the anion selectivity behavior of membrane electrodes based on different transition metal complexes is influenced by both the structure of organic ligands and the properties of the central metal ions [15], [16], [17], [18], [19], [20], [21].

The Schiff bases derived from salicylaldehyde (salens) as polydentate ligands are well known to form very stable complexes with transition metal ions [22], [23]. Schiff base complexes of transition metals have been frequently used as catalyst in such diverse processes as oxygen and atom transfer, enantioselective epoxidation and aziridination, mediating organic redox reactions and as mediator in other oxidation processes [23], [24], [25]. However, very little is known about their complex formation equilibria in solution [26] and also about the use of salens and their complexes as carrier in ion-selective electrodes [19], [27].

We have recently prepared an iodide-selective PVC membrane electrode based on a Schiff base–Mn(II) complex as a suitable ion-carrier which exhibited high selectivity to I⁻ ion over other common inorganic anions [19]. In this paper, we employed two recently synthesized Schiff base–Fe(III) complexes FeL1 and FeL2 as excellent ion carriers to construct CPEs for potentiometric determination of I⁻ which display high selectivity to iodide ion.