Highly selective direct determination of chlorate ions by using a newly developed potentiometric electrode based on modified smectite

Highlights

• A highly selective polyvinylchloride membrane chlorate selective electrode based on modified smectite was proposed.

• The electrode exhibited high selectivity and good stability.

• The proposed electrode was successfully applied for the chlorate determination in synthetic and real water samples.

Abstract

A novel polyvinyl chloride membrane chlorate (ClO₃⁻) selective electrode based on modified smectite was developed for the direct determination of chlorate ions and the potentiometric performance characteristics of its were examined. The best selectivity and sensitivity for chlorate ions were obtained for the electrode membrane containing ionophore/polyvinylchloride/o-nitrophenyloctylether in composition of 12/28/60 (w/w%). The proposed electrode showed a Nernstian response toward chlorate ions at pH=7 in the concentration range of 1×10⁻⁷–1×10⁻¹ M and the limit of detection was calculated as 9×10⁻⁸ M from the constructed response plot. The linear slope of the electrode was −61±1 mV decade⁻¹ for chlorate activity in the mentioned linear working range. The selectivity coefficients were calculated according to both the matched potential method and the separate solution method. The calculated selectivity coefficients showed that the electrode performed excellent selectivity for chlorate ions. The potentiometric response of electrode toward chlorate ions was found to be highly reproducible. The electrode potential was stable between pH=4–10 and it had a dynamic response time of <5 s. The potentiometric behavior of the electrode in partial non-aqueous medium was also investigated and the obtained results (up to 5% (v/v) alcohol) were satisfactory. The proposed electrode was used during 15 weeks without any significant change in its potential response. Additionally, the electrode was very useful in water analysis studies such as dam water, river water, tap water, and swimming pool water where the direct determination of chlorate ions was required.

Introduction

Chlorate is a compound formed as a result of the reaction of oxychlorine compounds such as chlorine dioxide(ClO₂) and dichlorine oxide(Cl₂O) with waters and has high solubility in waters (In 100 mL water:80 g at 0 °C and 96 g at 20 °C) [1], [2].

Chlorates are used in many fields such as the manufacture of fire work, matches, explosives, dyes, leathers, printing fabrics, paper pulp processing, tanning, and as a weak antiseptic. Chlorates of alkali metals are utilized in toothpaste at concentrations of 5% or less in European Union. In addition, sodium chlorate is an active ingredient in a number of commercial herbicides [3]. Recently, alkali metal chlorates have been widely used in soil in order to stimulate flowering of some tree species [4].

Chlorate is not naturally present in waters. The most widely known sources of chlorate is the degradation of hypochlorite solutions, the on-site generation of hypochlorite, and the production and degradation of chlorine dioxide which is often applied as a water disinfectant/oxidant. It can be formed also during the manufacture and storage of hypochlorite solutions. Chlorates are powerful oxidizers in especially highly acidic medium. Thus, they must be kept away from organic compounds and easily oxidized materials. However, Chlorate concentrations are evaluated to be stable in source waters up to 4 weeks without a preservative, concentrated acid and organic compounds in medium at room temperature [5], [6].

Chlorate has some potential harmful effect to human begins. But, there is no detailed toxicological data about its toxicity. The World Health Organization (WHO) advises to be in low levels of chlorate (<700 μg L⁻¹) as much as possible if toxicological data are insufficient [7]. After chlorate is ingested, it is absorbed by the gastrointestinal tract and eliminated by the kidneys and remains unchanged by processes within the body. Chlorate in high concentrations reduces the oxygen transport capability of blood, causes the toxic effects arising from rupturing of the red blood cell membranes. After rupturing of the red blood cell membranes, methemoglobin occurs by the oxidation of free hemoglobin in the bloodstream [8]. Besides its effects on human health, it is reported that chlorate is extremely toxic for marine microalgae which are vital components of the coastal ecosystem [9].

In the literature, in first studies, the determination of chlorate ions covered the reduction of chlorate to chloride ions by using different reductants and determination of chloride ions with silver(I) nitrate titration (Mohr method) [10]. Later, various analytical methods such as titrimetic methods based on potentiometry [11], [12], voltammetric methods [13], [14], chromatographic methods [15], [16], [17], and spectrophotometric methods [18], [19], [20] have been used for the determination of chlorate. The iodometric methods requires the amplification reaction of chlorate ions with iodide ions (ClO₃⁻+6I⁻+6H⁺→3I₂+Cl⁻+3H₂O) [21], [22], [23]. In these reactions, concentrated acid solutions are required for the rapid reduction of chlorate to chloride by iodide ions [24]. Such methods require a very different reagents that these reagents may enable unexpectedly greater toxicity for the environment [3]. Methods based on spectrophotometry and titration involve the acid-catalyzed reduction of chlorate with the subsequent oxidation of an indicator chromophore. In recent years, chromatographic methods with mass spectrophotometry such as ion chromatography-mass spectrometry (IC-MS) and liquid chromatography-mass spectrometry (LC-MS) has been described for simultaneous determination of chlorate, bromate, iodate, and chlorine dioxide. But these methods have been usually used for relatively clean matrices as water samples and require sample pre-treatment [25], [26]. Chlorates can be measured also by ion chromatography with conductivity detection in water [27], [28], [29], [30], [31]. However, in the case of a highly chloride matrix such as swimming pool water sample, it is almost impossible the direct determination of chlorate ions by ion chromatography with conductivity detection due to highly interference of chloride ions [32].

Potentiometric ion selective electrodes (ISEs) has been recently used widely for the direct determination of many anionic and cationic species due to a number of its advantages, such as high selectivity, low limit of detection, high accuracy, shorter analysis time, simple measurement process, mostly no sample pretreatment and applicability to colored and turbid samples [33]. Direct determination of chlorate ion is also possible with the use of a chlorate selective electrode. A review of the literature indicated that there are a few study related to the direct determination of chlorate ions by using potentiometric chlorate selective electrode. But their potentiometric performance characteristics is not satisfactory with respect to linear range, detection limit and selectivity [34], [35].

Ionophores used as an active component in ion selective electrodes are of major importance with regard to prepare a selective electrode against certain species. Recently, modified zeolites and modified clays have been used as an active component in potentiometric ion selective electrodes [36], [37], [38], [39], [40]. The most important advantage of these compounds is that they have permanent positive or negative charges in their crystal structures making them appropriate cation or anion exchangers in polymeric membrane [41], [42], [43]. Clay plate face have permanent negative charge sites on the basal planes owning to the isomorphous substitution of the central Si- and Al-ions in tetrahedral and/or octahedral sheets by the cations with lower positive charge whereas octahedral Al-OH and tetrahedral Si-OH groups situated at the broken edges are positively or negatively charged depending on the pH [44]. Generally, organo-modified clays are prepared by intercalation of organic surfactants. The intercalation of cationic surfactants may both change the charge of the raw smectite layer from negative to positive when the excess of organic cations available and increase anionic selectivity in the potentiometric determination of anionic species [45]. However, the surface chemistry and structural properties of organo-modified clays limit their further potentiometric sensor applications in various electrochemical systems depending on the loaded surfactant amounts and weak interactions between clay surface and surfactant molecules [46]. Therefore, we have synthesized a covalently-grafted silane smectite (APTES-S) by reacting 3-aminopropyltriethoxysilane (APTES) with smectite. The synthesized N-pyridin-2-ylmethylsuccinamic acid (PMSA) was attached to APTES molecules with amide bond formed by interaction between the amine groups of APTES-S and the carboxyl group of the PMSA molecule (PMSA-S) [47].

In this study, we have reported a highly sensitive and selective PVC membrane chlorate selective electrode based on modified smectite (PMSA-S) for the direct potentiometric determination of chlorate ions. The potentiometric performance characteristics of the electrode were investigated and described in detail. Additionally, the developed electrode was used for the determination of chlorate ions in spiked river and dam water samples. Because the determination of chlorate species in swimming pools is highly important for the health of athletes considering the above mentioned negative health effects, chlorate determination in swimming pool water samples were also performed. The obtained results for river and dam water samples were compared with the spiked values while the results for swimming pool samples were compared with daily official analysis results of the pool waters. The potentiometric analysis results were in good harmony with the spiked values and ion chromatographic determination value at 95% confidence level.