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

Abstract

Electrochemical solid phase nano-extraction, a novel sample preparation technique, was used for the determination of trace iodide in iodised table salt based on the silver sulfate nanoparticle-modified carbon paste electrode. Electrochemical solid phase nano-extraction was realized in the exchange between the sulfate anion in nanoparticles and an iodide anion from aqueous solution. The released silver cation serves as the electrochemical probe for the determination of iodide. The extraction follows a Freundlich adsorption isotherm, and can be used in the detection of iodide in the concentration range 5.0 × 10⁻¹²–4.0 × 10⁻⁹ M. The amount of iodide in iodised table salt was determined as 0.875 ± 0.002 μg/g, which is about 2.5% of the addition amount of iodate with a relative deviation of 5.92% and a standard addition recovery of 90–110%. The large amounts of chloride and iodate did not interfere with the detection.

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.