- Open Access
A rapid assessment method for determination of iodate in table salt samples
© Kulkarni et al.; licensee Springer. 2013
- Received: 20 June 2013
- Accepted: 18 October 2013
- Published: 15 November 2013
In the present work, a simple and rapid method for determination of iodate is described.
Iodometric reaction between iodate, excess iodide, and acid has been used, and the iodine liberated is allowed to react with variamine blue (VB) dye in the presence of sodium acetate to yield a violet-colored species.
A calibration curve was obtained in the concentration range of 2 to 30 μg of iodate in a final equilibration volume of 10 mL. The effect of different interfering anions on determination of iodate was also studied.
The developed method was applied to iodate determination in various iodized salt samples obtained from local markets in and around Pune city, India. The amount of iodate in various table salt samples was in the range of 10 to 25 ppm.
- Table salt
- Variamine blue dye
- Iodometric reaction
Iodine is an essential trace element for human nutrition. The safe dietary intake of iodine as recommended by the World Health Organization (WHO) is 100 μg day−1 for infants and 150 μg day−1 for adults (Hetzel 1983). Iodine is required by the thyroid gland for the synthesis of T3 and T4 hormones (Visser 2006). The storehouse of iodine in the human body is the thyroid gland. Inadequate intake of iodine leads to iodine deficiency symptoms and disorders like goiter, extreme fatigue, mental retardation, and depression which are collectively called as iodine deficiency disorders (IDDs). In India, about 71 million people suffer from iodine deficiency disorders. Statistics furnished by the Ministry of Health and Family Welfare in its report revealed that Uttar Pradesh, Bihar, Madhya Pradesh, Maharashtra, and Gujarat states contributing to almost 70% population have maximum IDD cases.
The natural dietary sources of iodine include milk, vegetables, fruits, cereals, eggs, meat, spinach, and sea foods (Zimmermann 2009). However, natural sources of iodine may not satisfy its requirement by the body as iodine from these sources may not be in bioavailable form and also the concentration of iodine may be less. Adequate intake of iodine can be achieved by consumption of iodized salt. Iodization of salt is done by addition of iodate to salt samples due to its good stability and bioavailability (Bürgi et al. 2001). Thus, determination of iodate in salt samples is of considerable importance as the amount of iodate in the salt samples may vary with environmental conditions, the nature of transport, packing conditions, and cooking methods (Bruchertseifer et al. 2003).
There are various analytical methods for determination of iodate in seawater and iodized salt samples. Some of the recent methods include kinetic spectrophotometric methods (Ni and Wang 2007), flow injection analysis (Shabani et al. 2011), microspectrophotometry after liquid-phase microextraction (Pereira et al. 2010), using cadmium sulfide quantum dots as fluorescence probes (Tang et al. 2010), liquid-liquid microextraction by high-performance liquid chromatography-diode array detection (Gupta et al. 2011), ion chromatography with integrated amperometric detection (Babulal et al. 2010), transient isotachophoresis-capillary zone electrophoresis (Wang et al. 2009), gas chromatography–mass spectrometry (Das et al. 2004), using polymer membrane selective for molecular iodine (Bhagat et al. 2008), and neutron activation analysis method (Bhagat et al. 2009). A non-suppressed ion chromatography with inductively coupled mass spectrometry (ICP-MS) has been developed for the simultaneous determination of iodate and iodide in seawater (Zul et al. 2007). Most of the techniques are complex and involve sophisticated instruments and complex procedures. It is also observed that application of these analytical methods for iodate determination in table salt is complicated due to the presence of huge excess of chloride, for example, in the case of anion exchange chromatography with conductometric detection which requires the removal of large excess of chloride from the sample matrix (Kumar et al. 2001). Hence, development of a method that is selective for iodate and sensitive and requires simple and inexpensive experimental setup is of considerable scientific interest. Also, accurate determination of the contribution of iodine from table salt to total dietary intake requires novel methods. With this objective in the present work, a simple and rapid method for determination of iodate is described. Iodometric reaction between iodate, excess iodide, and acid has been used, and the liberated iodine is allowed to react with variamine blue (VB) dye to yield a violet-colored species with absorbance maxima at 550 nm. The developed method was applied to determine the iodate concentration in table salt samples obtained from local markets in and around Pune city in India. The kinetics of the method is very fast, and a large number of table salt samples can be screened for their iodate content in a short time. The iodate content thus determined by the developed method was compared with the iodate content determined by conventional iodometric titration. The method developed in the present work has advantages over conventional methods, for example, it is free from losses of iodine and it is interference free.
A computer-based spectrophotometer (Systronics, Ahmedabad, India) was used for all the absorbance measurements. A pH meter (Labtronics, Panchkula, India) was used to monitor the pH of the equilibrating solutions. The pH meter was standardized using pH 4, 7, and 10 buffer solutions. A digital balance (Contech, Mumbai, India) was used for weighing all the reagents. Double-distilled water was used throughout all the work which was prepared using Equitron's instrument (Mumbai, India).
Reagents and solutions
All reagents used were of analytical reagent grade (A.R. grade) and used without further purification. Variamine blue (Merck, Mumbai, India), potassium iodate (S.M Chemicals, Mumbai, India), potassium iodide (Loba Chemie, Mumbai, India), sodium chloride (Qualigens, Mumbai, India), potassium bromate (Qualigens), ammonium oxalate (Qualigens), potassium chloride (Qualigens), sodium bicarbonate (Qualigens), potassium nitrate (Qualigens), zinc sulfate (Qualigens), methyl alcohol (Qualigens), and magnesium carbonate (Qualigens) were used. A variamine blue dye solution was prepared by dissolving 20 mg of the dye in methyl alcohol and diluting the solution to 50 mL using distilled water. A potassium iodate solution was prepared by dissolving 0.0122 g of KIO3 in distilled water and diluting it to 100 mL [1 mL = 100 μg IO3 −]. Sulfuric acid (1 M) was prepared by diluting 6.95 mL of stock H2SO4 to the mark in a 250-mL volumetric flask with distilled water. A solution of potassium iodide was prepared by dissolving 25 mg potassium iodide in water and diluting it up to 100 mL [1 mL = 250 μg]. A solution of sodium acetate (2 M) was prepared by dissolving 13.608 g of A.R. grade sodium acetate in distilled water and diluting the solution to 100 mL in a volumetric flask The different interfering ion solutions such as potassium chloride (KCl), sodium bicarbonate (NaHCO3), potassium nitrate (KNO3), zinc sulfate (ZnSO4), potassium bromate (KBrO3), etc. were prepared by dissolving and diluting suitable amounts of the respective salts in distilled water to make a concentration of 1 mL = 100 μg.
Samples for iodate determination
A total of 12 different brands of iodized salt samples were analyzed in the present work. The samples were purchased from local markets in and around Pune city. The samples were stored in cool and dry conditions. The contents of the packets were transferred immediately upon opening into an air tight container.
Optimization of parameters for the iodometric reaction
Various parameters associated with the iodometric reaction were optimized. The amount of potassium iodide and the concentration of acid were optimized in a similar manner as reported in our previous work (Bhagat et al. 2008) The concentration of iodate was fixed as 10 μg during the optimization experiments. Experiments were performed to optimize the dye concentration and pH of the reaction mixture. pH adjustments were done using either 2 M NaOH or 2 M HCl. The time required for the completion of the reaction was measured by studying the changes in absorbance as a function of time. The absorbance values were recorded in the intervals of 30 s till 30 min.
Measurement of iodate in the aqueous solution
An aliquot of iodate solution containing 2 to 30 μg of iodate was taken in 10-mL volumetric flasks. Excess of KI (250 μg) was added to each flask followed by 1 mL of H2SO4 (1 M). The solution turned yellow due to liberation of iodine. At this stage, 1 mL of dye solution was added followed by addition of 2 mL sodium acetate (2 M). The solutions were diluted to 10 mL with distilled water and kept for 5 min to allow the reaction to complete. After 5 min, the absorbances of all the solutions were recorded at 550 nm against water as a reagent blank. A calibration plot of absorbance values of VB dye was plotted against the amount of iodate in solution.
The effect of common interfering anions like Cl−, SO4 2−, NO3 −, Br−, PO4 3− HCO3 −, C2O4 2−, and BrO3 − on determination of iodate by the VB method was studied by the following procedure. The concentration of iodate in the reaction mixture was kept fixed as 5.72 × 10−8 M, and the concentration of interfering anions in the equilibrating solution was varied.
Application to table salt samples
Before application of the method to table salt samples, it was applied to A.R. grade laboratory reagent NaCl to study the effect of sodium chloride on the absorbance values. In the case of iodized table salt samples, a homogenous portion of 2 g of sample was weighed accurately on a balance and dissolved in distilled water. The final volume was made up to 25 mL, and the solution was used for further analysis. The concentration of iodate in the samples was calculated using a calibration curve. Each sample was analyzed five times, and the standard deviation was calculated. The iodate content in these salt samples was also analyzed by conventional iodometric titration using Na2S2O3 with starch as an indicator.
Optimization of parameters for iodometric reaction
Optimized parameters used for analysis
2 to 30 μg
1 mL [2 M]
2 mL of 2 M
Calibration curve and detection limits for iodate determination
Effect of interfering anions on determination of iodate
Effect of some interfering anions on iodate determination
Interfering anion [X−]
Tolerated ratio [X−]/[IO 3 −]
Determination of iodate in iodized salt samples
Iodate values obtained in local brands of table salt
Iodate (ppm) ± SD (n = 3)
by variamine blue method
Iodate (ppm) ± SD (n = 3)
15.31 ± 2.645
14.3 ± 0.2
14.67 ± 2.336
13.6 ± 0.1
11.83 ± 1.98
10.8 ± 0.2
15.76 ± 3.80
16.3 ± 0.1
10.78 ± 0.46
9.8 ± 0.1
15.22 ± 1.90
15.5 ± 0.2
16.01 ± 1.46
14.3 ± 0.1
21.09 ± 2.50
23.2 ± 0.1
16.02 ± 0.48
15.6 ± 0.2
16.20 ± 0.56
15.9 ± 0.1
18.20 ± 0.66
19.3 ± 0.2
17.60 ± 2.34
18.5 ± 0.2
According to WHO, the daily dietary intake of iodine is 150 μg for adults. Considering the spicy food habits in India, the average intake of salt through food may be in the range of 2 to 3 g per day. Using this approximation, the contribution of iodine from table salt can be calculated to be around 30 to 40 μg per day. The bioavailability of iodine may be considered to be 100% due to its solubility in digestive fluids. The contribution of iodine from table salt is estimated to be 20% to 40% of the total iodine requirement. This contribution may not be enough in regions where the soil is deficient in iodine content. Consequently, iodine deficiency disorders will be prevalent in these regions.
A simple and rapid method has been developed for determination of iodate in aqueous samples. The method is applicable to iodate determination in the concentration range of 2 to 30 μg in a final equilibration volume of 10 mL. Optimized parameters for the method are IO3 − (2 to 30 μg), KI (250 μg), 2 M H2SO4 (1 mL), pH (5.0), time of equilibration (20 min), 2.44 μM dye (20 μg, 1 mL), and 2 mL sodium acetate (2 M). The concentration of IO3 − obtained in the salt samples was in the range of 10 to 22 ppm. The results obtained by the present method are in good agreement with those obtained by conventional iodometry, thus validating the method.
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