Voltammetric determination of iodide in iodized table salt using cetyltrimethylammonium bromide as ion-pairing

Abstract

In this work, voltammetric study based on cetyltrimethylammonium bromide (CTAB) as an ion-pairing agent for the determination of iodine level in iodized table salt has been explored. CTAB was used as an intermediate compound between iodide (I⁻) and the electrode due to its ability to dissociate to produce cetyltrimethylammonium ions ([CTA]⁺). The [CTA]⁺ with a long hydrophobic alkyl chain can be directly adsorbed onto the surface of the working electrode, and this in turns coated the electrode with cationic charge and enhance the electrode ability to bind to iodide (I⁻) and other molecular iodine ions. A mixture of iodide and CTAB ([CTA]⁺I⁻) was prepared and potential of 1.0 V for 60.0 s was applied to pre-concentrate the solution on the working electrode causing the [CTA]⁺I⁻ to oxidize to iodine (I₂). The produced I₂ immediately react with chloride ion (Cl⁻) from the electrolyte of hydrochloric acid (HCl) to produce I₂Cl⁻ and form ion-pair with CTA⁺ as [CTA]⁺I₂Cl⁻. The linear calibration curve of the developed method towards iodide was in the concentration range of 0.5–4.0 mg/L with sensitivity of − 1.383 µA mg/L⁻¹ cm⁻² (R² = 0.9950), limit of detection (LOD) of 0.3 mg/L and limit of quantification (LOQ) of 1.0 mg/L, respectively. The proposed method indicates good agreement with the standard method for iodine determination with recovery range from 95.0 to 104.3%. The developed method provided potential application as a portable on-site iodine detector.

Introduction

Iodine is one of the most abundant elements in the world because of its solubility in water, thus making the ocean its main reservoir. Iodide ions (I⁻) from sea water evaporates into volatile iodine, absorbed by rain, returned to soil, and ultimately carried to the sea, completing the iodine cycle (Kocher 1981; Fuge and Johnson 2015). Iodine exists in living and non-living matter, including sources of food (Zimmermann et al. 2008; Rasmussen et al. 2009).

I⁻ is form of bioavailable iodine essential to produce hormones in the thyroid gland that are responsible for human growth and mental development. Lack of iodine may lead to several diseases including goiter and mental development delay in children (Ministry of Health Malaysia 2017; Semba and Delange* 2008; Zimmermann 2009). Iodine deficiency disorder (IDD) was recognized after a randomized controlled study (RCT) in 1983 found that iodine supplementation in iodine-deficient regions could eliminate new cases of cretinism, lower infant mortality and improve cognitive function on the population (Hetzel 1983).

In Malaysia, the use of iodized table salt with iodine (20–40 mg/kg) has been implemented under the Malaysian Food Act of 1983 in the state of Sabah and in the interior area of Sarawak as part of the Universal Salt Iodization (USI) program. Despite the significant improvement of iodine levels in Malaysia has been observed (Lim et al. 2014), the study found that the incidence of IDD still occurs in pregnant woman in the rural areas of Sabah especially in the interior and the west coast area (Lim et al. 2017). Thus, continuous monitoring of iodine level is needed for the successful implementation of the USI program.

According to the World Health Organization (WHO) (2007), the fortification of iodine in iodized salt must be in the form of potassium iodide (KI) or potassium iodate (KIO₃). Since most of the commercial iodized table salts are in the form of iodate (IO₃⁻), and the method presented in this study was suitable for the detection of iodine in the form of I⁻, thus, an additional step of iodine conversion is needed (Mannar and Dunn 1995). In the study reported by Rebary et al. (2010), the Landolt iodine clock reaction (Church and Dreskin 1968; Gaspar and Showalter 1987) was used to reduce IO₃⁻ to I⁻ using bisulfite ion (HSO₃⁻). According to Landolt iodine clock reaction, HSO₃⁻ act as a limiting reactant resulting in the formation of dark blue colored starch-complex after undergoing several continuous reactions. When the concentration of HSO₃⁻ is in excess, iodine will remain in the form of I⁻ and it would not proceed to the next reaction to form a dark blue color to complete the clock reaction. In this conversion, IO₃⁻ stoichiometric was equivalent to the produced I⁻. For conversion, 1 mg/L of IO₃⁻ is equivalent to 0.7255 mg/L of I⁻.

Cetyltrimethylammonium bromide (CTAB) is cationic surfactant with a long alkyl chain and tetraammonium cation salt that has been identified as a promising electrode modifier in electrochemical method for the determination of iodine level in iodized table salt samples (Zou et al. 2007; Brugnera et al. 2010; Yu et al. 2015; Teradale et al. 2017). The nonpolar region of CTAB can adsorbed strongly onto a nonpolar solid electrode such as carbon (Zou et al. 2007), whereas the positively charged polar region is easily absorbs by negatively charge compounds such as I⁻. The adsorbed I⁻, then can be oxidized to molecular of iodine (I₂) on the electrode surface. Several studies have reported application of CTAB for iodine determination using electrochemical techniques which it act as an adsorbent at the surface of carbon paste electrode (CPE) and form ion-pair bonding with iodide species in the samples (Švancara et al. 2002; Huang et al. 2008; Zhu et al. 2010).

In this study, we explore the use of CTAB in combination with the Landolt iodine clock reaction for iodine detection for continuous monitoring of iodine level in iodized table salts in conjunction with the implementation of USI program. The idea of this work is to convert IO₃⁻ in iodized table salt to I⁻ and study the interaction of I⁻ with CTAB for the determination of iodine level (in the form of I⁻) in iodized table salt samples using simple operation, reliable and portable electrochemical technique.

Materials and methods

Reagents

Deionized water with 18.2 MΩ grade (Sartorius, Germany) was used throughout this study. The following reagents were prepared: 0.01 M CTAB (Sigma Aldrich, USA); 2 M hydrochloric acid (HCl) (Merck, Germany); 200 mg/L of I⁻ stock solution from KI (analytical grade, Sigma-Aldrich, USA); 2 M sodium bisulfite (NaHSO₃) (assay 58.5%, Sigma Aldrich, USA); 30% (w/v) sodium hydroxide (NaOH) (Merck, Germany); 10% (w/v) sodium sulfite (Na₂SO₃) (Sigma Aldrich, USA); 5% (w/v) sodium hypochlorite (NaOCl) (Sigma Aldrich, USA); and 0.5 M of ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA-2Na) pH 11 (Sigma Aldrich, USA), respectively.

Electrochemical measurement

A portable instrument of bipotentionstats/galvanostats (µStats 400 model) together with screen printed carbon electrode (SPCE) (DRP-110 model) (Dropsens, Spain) was used in all electrochemical measurements. The SPCE consists of three-electrode system with carbon as a working and counter electrode, while silver chloride as a reference electrode.

Cyclic voltammetric (CV) was used to study electrochemical behavior of iodide species on the electrode surface whereas linear scan voltammetric (LSV) was used for the determination of I⁻ concentration. For measurement, 50 µL of the prepared I⁻ standard solution was placed on the electrode surface and linear scan via cathodic stripping voltammetric was recorded after 60.0 s of pre-concentration of sample at potential of 1.0 V with scan rate of 0.1 V/s. The concentration of I⁻ was investigated in the concentration range of 0.5–4.0 mg/L.

Procedure for determination of iodide in iodized table salt samples

A series of I⁻ solution dilutions (0.5–4.0 mg/L) were prepared daily for the analysis. A qualitative confirmation was firstly done to determine the form of iodine exist in the samples i.e. I⁻ or IO₃⁻, using iodine “IMR Rapid Test Kit” produced by Institute for Medical Research, Ministry of Health Malaysia. Prior to sample analysis, 3 g iodized table salts (6% w/v) were dissolved in a mixture of 1.4 × 10⁻⁴ M CTAB and 0.04 M HCl. If positive result obtained for IO₃⁻ from the test kit, an additional step to add 100 µM of NaHSO₃ was required. A total volume of 50 mL was made up with deionized water.

In order to evaluate the applicability of the developed method towards real sample analysis, eight commercial iodized table salts were purchased from several local supermarkets in Klang Valley, Malaysia. If the iodine level on the packaging was present, it was recorded before the samples were analyzed. The same procedure as mention above was used for the preparation of the commercial iodized table salt sample solutions.

Comparison study for the determination of I⁻ was also evaluated by using polarography method according to the application note provided by Metrohm (VA Application Note V-197). The instrument used is a hanging mercury dropped electrode (HMDE). A salt sample (3 g) was dissolved in 10 mL of deionized water. Then 1 mL of the dissolved salt solution was transferred to another beaker and further diluted to 10 mL. 20 µL of NaOH (pH 11) and 100 µL of 5% NaOCl was added into the beaker, stirred and purged with nitrogen gas for 15 min. Then, 1 mL of Na₂SO₃ was added to remove any excess of NaOCl. Finally, 0.1 mL of EDTA-2Na (pH 11) was added to remove any interference species before differential pulse scan was conducted. All analytical performance figures were assessed based on three replicate measurements and error bars are shown in the figures.

Statistical analysis

Statistical analysis for comparing the result obtained from the developed method and polarography method was calculated based on the t-test formula for equal variance of the samples. The data are presented as mean with standard deviation (SD). At 95% confidence interval, the mean difference between two samples was compared using t-test with the calculated t-value more than 2.78 (t_calc. > 2.78) was considered statistically significant.

Procedure for interference study

For the interference study, each of ions (anions: Br⁻, CH₃COO⁻, ClO₂⁻, ClO₃⁻, ClO₄⁻, F⁻, NO₃⁻, PO₄³⁻ and SO₄²⁻ and cations: Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Ni²⁺, Pb²⁺ and Zn²⁺) were introduced to 3.0 mg/L I⁻ in 6% NaCl solution in the presence of 1.4 × 10⁻⁴ M CTAB and 0.04 M HCl. These ions were added separately at different concentration and the iodide response was observed. In this study, maximum concentration of interfering species that caused error less than ± 5% for determination of iodide is considered as tolerance limit for the proposed method.

Results and discussion

Electrochemical study

Figure 1 illustrates the electrochemical behaviour of I⁻ (3.0 mg/L) in the absence and presence of CTAB studied by CV in the potential range of 0.1–1.0 V. In the absence of CTAB, only small oxidation peak at potential around 0.60 V in a positive potential scanning and no peak was observed at reverse potential scanning (Fig. 1a). As shown in Fig. 1b, a small oxidation peak of I⁻ was observed at potential around 0.55 V at positive potential scanning and at reverse potential scanning, a conspicuous reduction peak of I⁻ at 0.41 V was obtained in the presence of CTAB. This observation could be explored to study the interaction of CTAB-I⁻ on the SPCE surface. The presence of CTAB suggest a quasi-reversible reaction of I⁻ had occurred due to its unsymmetrical shape of redox peak.

Fig. 1.

Cyclic voltammograms of 3.0 mg/L of iodide in the a absence of CTAB, b presence of CTAB (1.4 × 10⁻⁴ M)

Electrochemical behaviour of IO₃⁻ and I⁻ in the presence of CTAB, and HCl as the supporting electrolyte was evaluated by using linear sweep voltammetric technique (LSV). It can be noted that the current signal of IO₃⁻ and I⁻ was not observed in the absence of CTAB (Fig. 2a, b). For CTAB solution only, there was no current signal produced (Fig. 2c). In Fig. 2d, no signal was observed for IO₃⁻ in the presence of CTAB. As shown in Fig. 2e, the current response of I⁻ was observed at the potential of 0.44 V in the presence of CTAB. The LSV scanning confirmed that the interaction of I⁻ on the surface of electrode was not possible without the presence of CTAB. On the other hand, no peak response was observed for IO₃⁻ even with the addition of CTAB, suggesting that the strength of ionic bonding of Br⁻ with CTA⁺ is much stronger compared to IO₃⁻, but weaker than I⁻ (Sangeetha et al. 2016; Ghosh and Manna 2018).

Fig. 2.

Linear sweep voltammograms in 0.04 M HCl medium at 1.0 V deposition potential accumulated for 60.0 s with scan rate of 0.10 V/s. a 3.0 mg/L iodate; b 3.0 mg/L iodide; c 1.4 × 10⁻⁴ M CTAB; d 3.0 mg/L iodate in 1.4 × 10⁻⁴ M CTAB; e 3.0 mg/L iodide in 1.4 × 10⁻⁴ M CTAB

During the LSV scanning, major reduction peak of a compound ([CTA]⁺ I₂Cl⁻) was formed when positive potential was applied. Most of the I⁻ dissociated from [CTA]⁺I⁻ immediately oxidized to I₂ and ion-paired with the Cl⁻ to form a voluminous ion of I₂Cl⁻ (Švancara et al. 1998; He et al. 2003). This phenomenon shows that in the presence of CTAB electrochemical signal of I⁻ species could be obtained, but no signal for iodate species was observed.

Parameter optimization study

In order to improve the performance of the developed method, several parameter were optimized to produce maximum signal including the effect of HCl concentration, CTAB concentration, LSV deposition time and accumulation time, and LSV scan rate, respectively.

The use of supporting electrolyte was crucial as it allow the movement of ions present in the solution to interact on the surface of electrode. The influence of HCl concentration on the signal of I⁻ was investigated in the concentration ranging from 0.002 to 0.180 M. No signal of I⁻ was observed in the absence of HCl. The signal was increased as the concentration of HCl increases and optimum signal was observed at 0.04 M. When the concentration of HCl exceeded 0.04 M, the signal started to decrease. Thus, concentration of 0.04 M of HCl was chosen for further experiment.

The effect of CTAB concentrations was also examined in the concentration ranging from 6.00 × 10⁻⁶ to 3.00 × 10⁻⁴ M. As the CTAB concentration increases the signal response towards I⁻ (3 mg/mL) was also increased. The maximum response was observed at a concentration of 1.4 × 10⁻⁴ M. The signal starts to decrease when the concentration reached above 1.4 × 10⁻⁴ M which might be due to the over saturation of CTAB adsorbed onto the electrode surface where it hinders the adsorption of the ion-pair formed by [CTA]⁺ and I₂Cl⁻. Thus, concentration of CTAB of 1.4 × 10⁻⁴ M was chosen for subsequent study.

The effect of deposition potential was studied in various positive potentials for the pre-concentration of I⁻. The LSV scans were conducted for I⁻ (3.0 mg/L) in the potential range of 0.0–1.10 V. An increase in signal response was observed when applied potential was increased from 0.0 to 0.9 V and slightly decreased at potential of 1.0 V. Above applied potential of 1.0 V, significant decrease in signal was obtained. Therefore, deposition potential of 1.0 V was selected for further study.

The influence of accumulated time towards the iodide-CTAB peak height signal was also studied at fixed concentration of I⁻ of 3.0 mg/L. The peak signal increased as the accumulation time increased and maximum signal was observed at duration of 200.0 s. Above 200.0 s the signal started to decrease. The decreased of the signal may be due to the saturation process occurred over time for the ion-pair on the surface of working electrode. In order to shorten the analysis time, accumulation time of 60.0 s was chosen which sufficient signal was obtained for the analysis.

The effect of scan rate on the peak current was carried out in the range of 0.10–0.50 V/s and concentration of I⁻ was fixed at 3.0 mg/L (Fig. 3). As the scan rate increased, the peak height signal was also increased and the maximum peak height signal was achieved at 0.25 V/s (Fig. 3a). From this study, scan rate of 0.10 V/s was chosen since it gave the highest signal-to-noise ratio of 19.6. A linear correlation between peak height signal over the scan rate with R² = 0.9912 indicates that the mechanism occurs on the surface of the electrode was the adsorption process. Peak splitting was also observed for scan rate of 0.30–0.50 V/s (Fig. 3b). This evidence prove the formation of ion pairs of both I⁻ with CTA⁺ and I₂Cl⁻ with CTA⁺ (Švancara et al. 1998; He et al. 2003). A lower peak from the peak splitting may be related to I⁻ ion pair since it has lower electron affinity compared to I₂Cl⁻ toward CTA⁺.

Fig. 3.

a LSV scans compilation for the effect of various scan rates. b The effect of scan rate parameter for 3.0 mg/L iodide in the presence of 1.4 × 10⁻⁴ M CTAB. Scan parameter: deposition potential, 1.0 V; accumulation time, 60.0 s

Under optimized condition, dynamic response of the developed method towards I⁻ concentration was conducted from 0.2 to 6.0 mg/L (Fig. 4a) and it was observed that a linear signal was achieved from 0.2 to 5.0 mg/L. Beyond concentration of 5.0 mg/L, a plateau curve signal was observed. As can be seen in Fig. 4 (Inset), the linear calibration curve of the developed method towards I⁻ was in the concentration range of 0.5–4.0 mg/L with sensitivity of − 1.383 µA mg/L⁻¹ cm⁻² (R² = 0.9950), limit of detection (LOD) of 0.3 mg/L and limit of quantification (LOQ) of 1.0 mg/L, respectively.

Fig. 4.

Dynamic response of iodide peak height signal with different concentration of iodide in 6% (w/v) NaCl. Inset is the linearity range toward iodide concentration of 0.5–4.0 mg/L

Several possible interfering ions were also evaluated using the developed system to study the effect of response for the determination of I⁻ ions. The ions were anions: Br⁻, CH₃COO⁻, ClO₂⁻, ClO₃⁻, ClO₄⁻, F⁻, NO₃⁻, PO₄³⁻ and SO₄²⁻ and cations: Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, Mg²⁺, Ni²⁺, Pb²⁺ and Zn²⁺, respectively. It can be noted that NO₃⁻ and Zn²⁺ were the two major ions interfering I⁻ signal in which the signal response change more than ± 5%. Other ions show no significant effect in the signal current obtained which is less than ± 5%.

To demonstrate the feasibility of the developed method for iodide (I⁻) detection in iodized table salt, several commercial iodized table salts were evaluated. To perform this analysis, the optimized concentration of HSO₃⁻ has to be established in order to reduce IO₃⁻ to I⁻ that possibly exist in the samples.

Prior to sample analysis, the total iodine content was determined in the form of I⁻. Addition of HSO₃⁻ into the sample solution is needed to reduce the possible IO₃⁻ content in the samples to I⁻ form. Thus, the effect of NaHSO₃ were firstly studied over the IO₃⁻ concentration range of 1.0 mg/L (equivalent to 0.73 mg/L I⁻), 3.0 mg/L (equivalent to 2.18 mg/L I⁻) and 5.0 mg/L (equivalent to 3.63 mg/L I⁻), respectively. It was found that the best concentration of NaHSO₃ produced high yield percentage of I⁻ (between 87 and 97%) converted from the reduced IO₃⁻ was at 100 µM (Fig. 5). Therefore, the addition of 100 µM of NaHSO₃ was used in the sample preparation procedure for real sample analysis.

Fig. 5.

Effect of different concentration of NaHSO₃ towards conversion of iodate (IO₃⁻) (1 mg/L, 3 mg/L and 5 mg/L) to iodide (I⁻) in the presence of 6% (w/v) NaCl

For real sample analysis, the samples were analyzed against external calibration of I⁻ in 6% (w/v) NaCl. As summarized in Table 1, iodide detection using Polarographic method was in the range of 17.13–34.29 mg/kg while the developed method gave the concentration in the range of 11.22–37.84 mg/kg. For recovery study, it was also observed that the developed method recovered approximately 95.0–104.3% of I⁻ when spiked with known concentration of iodide (20.0, 30.0 and 33.3 mg/kg). Statistical for comparing two means of the developed method and polarography method was also evaluated. It can be noted that the calculated t-value of the samples are less the critical t-value (t_calc. < 2.78) indicates the developed method and polarography method are in good agreement and comparable.

Table 1.

Comparison of iodide value in iodized salt sample between the reference method and the developed method

Sample	Polarography (HMDE)		The developed method					Calculated t-value (tcalc.)b
	Iodide concentrationa (mg/kg)	SD	Iodide concentration				Recovery (%)
			Found (mg/kg)	SD	Spiked (mg/kg)	Total (mg/kg)
	(n = 3)		(n = 3)
1	34.05	2.72	37.84	5.75	30.00	68.25	101.1	0.56
2	31.40	4.63	27.17	6.79	30.00	57.99	103.0	0.51
3	20.70	2.01	25.74	2.08	20.00	46.52	103.0	1.89
4	31.49	3.94	33.23	6.94	30.00	64.66	104.3	0.21
5	22.48	1.64	18.89	5.76	33.33	51.77	97.6	0.53
6	25.52	1.91	28.26	0.85	20.00	47.91	98.8	1.86
7	17.13	6.65	11.22	3.06	20.00	31.36	101.3	1.13
8	34.29	2.58	27.38	4.57	33.33	59.33	95.0	1.26

^aValue converted from iodate concentration

^bt_calc. > 2.78 is significantly difference for t-test (the critical value, t₄ = 2.78)

Conclusion

The development of voltammetric method based on CTAB as ion-pairing for the determination of total iodine in iodized table salt using SPCE was successfully established. The interaction of I⁻ and CTA⁺ was achieved by adsorption process of several forms of iodide species (I₃⁻ and I₂Cl⁻) with the adsorbed CTA⁺ on the working electrode surface in acidic medium. Under optimum condition, the developed method produced good iodide signal response through a LSV scanning (deposition potential 1.0 V, scan rate 0.01 V/s and accumulation time 60.0 s). The linear calibration curve of the developed method towards I⁻ was in the concentration range of 0.5–4.0 mg/L with sensitivity of − 1.383 µA mg/L⁻¹ cm⁻² (R² = 0.9950), limit of detection (LOD) of 0.3 mg/L and limit of quantification (LOQ) of 1.0 mg/L, respectively. The developed method demonstrated good agreement with HMDE’s polarographic method for the determination of iodine in iodized table salt. As a proof of concept, the proposed method has shown potential applicability for on-site monitoring of iodine in the commercial iodized table salts.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.