Determination of trace iodide in iodised table salt on silver sulfate-modified carbon paste electrode by differential pulse voltammetry with electrochemical solid phase nano-extraction
Introduction
Solid phase micro-extraction (SPME) was developed by Pawliszyn and co-workers as a solvent-free green sample preparation method [1], [2], [3], and have been widely used in many areas [4], [5], [6]. If the adsorbent consists of nanoparticles, the method is called solid phase nano-extraction (SPNE) [7], [8]. The SPNE method retains all the advantages of SPME, and has some new characteristics, such as a larger specific adsorption area, multiple active sites for recognitions of the analyte(s), controllable surface states, and modificative solid nanoparticle surface. The combination of electrochemistry with SPNE (ESPNE) is a new technique for sample preparation. In ESPNE, electrochemistry offers a way to control the redox states of extractants and analytes, and the un-uniformed electric field on nanoparticle-modified electrode surface offers a dielectric force for the separations. Some examples have shown the possibility for integration of analysis with separation [9], [10], [11], [12]. The carbon paste electrode (CPE), composed of a flexable mixture of solid and liquid phase, offers a renewable electrode surface for the solid and liquid extractants, and very suitable for ESPNE [13].
Iodine is one of the biologically important elements, an adult human requires 100–150 μg/day, and it has been suggested that potassium iodate or potassium iodide should be added to table salt called iodised table salt [14]. In practice, however, long-term storage of iodised table salt under adverse conditions results in some loss of iodine due to the inverse disproportional reaction [15]. The trace iodine detection methods, such as UV–visible spectrophotometry [16], catalysis [17], chromatography [18], ion-selective electrode [19], cyclic voltammetry (CV) [20], and ICP-AES [21] with chemical oxidation and liquid–liquid extraction techniques cannot be used directly, and they all suffer from interference by chloride [22] and iodate. Therefore, it is important to develop a method that can determine iodide directly and without interference by the large amounts of chloride and iodate.
We modified the surface of graphite powder in a CPE with Ag2SO4 colloidal nanoparticles (ssn-CPE) and used it to extract iodide from aqueous solution. The amount of iodide extracted was determined directly by anodic stripping of silver with differential pulse voltammetry (DPV). This method was investigated for the detection of iodide in iodised table salt without interference by iodate or chloride, and the results are reported here.
Section snippets
Instrument and reagents
Electrochemical experiments were carried out on a CHI electrochemical system (CHI620B, CHI Co., USA) with three electrodes; a piece of platinum wire as the auxiliary electrode, an in-house built silver sulfate-modified CPE as the working electrode and a KCl-saturated calomel electrode as the reference electrode. All potentials reported here are with respect to this reference electrode.
Graphite powder (200 #, spectrophotometrically pure), epoxy resin, polyacetamide resin, castor oil (chemically
The electrochemical behaviour of iodide at the modified electrode
The different electrodes in 0.5 M NaNO3 electrolyte solution (pH 3.2) including 0.5 mM NaI, gave the CV curves shown in Fig. 1.
The iodide on the bare CPE (with castor oil and ethyl benzoate) shows only one irreversible oxidation peak at 0.559 V (Fig. 1, curve 2) without the reduction peak due to the extraction of iodine into the organic phase. Iodide at the ssn-CPE had a CV with cathodic peaks at 0.400 V and at 0.192 V, and anodic peaks at 0.233 V and at 0.570 V (Fig. 1, curve 1). The ssn-CPE in the
Conclusions
This paper describes the solid phase nano-extraction of iodide from iodised table salt on graphite powder surface modified with silver sulfate nanoparticles in a carbon paste electrode. Some important points are summarized as follows.
- (1)
The extraction mechanism is the exchange reaction between a sulfate anion in a nanoparticle of silver sulfate precipitate and iodide from aqueous solution, depending on the extraction time, solution pH, and the concentration of iodide
- (2)
The extraction isotherm follows
Acknowledgements
The authors acknowledge financial support by the Chinese National Science Foundation (20875063), the Liaoning Education Minister (2004-22) and the national key. Laboratory of Electroanalytical Chemistry (2006-06), Shenyang Sciences and the Technology Bureau Foundation (2007-GX-32).
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