Chemistry
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Determination of Triiodide Ion Concentration Using UV-Visible Spectrophotometry†

The concentration of iodine species can be measured using several different analysis methods. One of the most wellknown methods involves iodine titration with a thiosulfate solution. Total iodine can be analyzed using spectrometry, such as inductively coupled plasma-atomic emission spectrometry (ICP-AES), atomic absorption spectrometry (AAS) and inductively coupled plasma-mass spectrometry (ICP-MS). As elemental iodine has various valance states, various stable iodine species are present in aqueous solutions. Stable iodine species can include iodide ions, triiodide ions, iodine and iodate ions. For the quantitative analysis of these species, ion chromatography (IC) has been used in analytical laboratories. In addition, ultraviolet-visible spectrophotometry and Raman spectrometry have been used for the speciation and quantitative analyses of iodine species. Recently, after the severe accident at the Fukushima nuclear power plant in Japan, the chemical behaviour of radioactive iodine received increased attention as a result of interest in evaluating the volatility of iodine species, which depends on the chemical conditions of the exposed environments. The use of γ-ray spectroscopy to measure radioactivity is the best method for determining radioactive iodine concentrations. However, to use radioactive iodine, a licensed Determination of Triiodide Ion Concentration Using UV-Visible Spectrophotometry†


INTRODUCTION
The concentration of iodine species can be measured using several different analysis methods [1][2][3] . One of the most wellknown methods involves iodine titration with a thiosulfate solution 4,5 . Total iodine can be analyzed using spectrometry, such as inductively coupled plasma-atomic emission spectrometry (ICP-AES), atomic absorption spectrometry (AAS) and inductively coupled plasma-mass spectrometry (ICP-MS) 3,6,7 . As elemental iodine has various valance states, various stable iodine species are present in aqueous solutions. Stable iodine species can include iodide ions, triiodide ions, iodine and iodate ions. For the quantitative analysis of these species, ion chromatography (IC) has been used in analytical laboratories 3 . In addition, ultraviolet-visible spectrophotometry and Raman spectrometry have been used for the speciation and quantitative analyses of iodine species 8,9 . Recently, after the severe accident at the Fukushima nuclear power plant in Japan, the chemical behaviour of radioactive iodine received increased attention as a result of interest in evaluating the volatility of iodine species, which depends on the chemical conditions of the exposed environments. The use of γ-ray spectroscopy to measure radioactivity is the best method for determining radioactive iodine concentrations. However, to use radioactive iodine, a licensed The concentration of triiodide ions was determined using UV-visible spectrophotometry. The maximum absorption λmax for the analysis of triiodide ions were observed at 288 and 351 nm, with molar absorptivity values of 2.65 × 10 4 and 1.72 × 10 4 L cm -1 mol -1 , respectively. The absorbance of triiodide ions had little effects on the solution pH in the range of 2.0 to 6.5. However, the absorbance strongly depended on the iodide ion concentration. Consequently, we confirmed that triiodide ion concentrations in the range of 0.005-0.1 mM could be determined using UV-visible spectrophotometry.
Keywords: Triiodide ion, UV-visible spectrophotometry, Iodide ion, Solution pH. †Presented at 5 th International Symposium on Application of Chemical and Analytical Technologies in Nuclear Industries (Nu-ACT 2013), Daejeon, Korea laboratory equipped with numerous radioactivity safety devices is generally required. Therefore, many researchers have performed their experiments using non-radioactive iodine under γ-ray radiation instead of radioactive iodine 10 . In this case, the analysis of iodine species should be performed by chemical methods. Similar to radioactive iodide ions, which are stably soluble in aqueous solution, non-radioactive iodide ions can also be oxidized into volatile iodine under γ-ray radiation.Under the selected experimental conditions, the iodide ion concentration is relatively high, allowing the oxidized species, molecular iodine,to react with iodide ions and form triiodide ions, which are the more stable soluble species. Therefore, it is important to measure the concentration of triiodide ions to evaluate the volatility of iodine species.
In the present work, we established a simple method to analyze the triiodide ion concentration using UV-visible spectrophotometry. First, we identified distinct absorption wavelengths and determined the maximum absorbance for the analysis of triiodide ions. Second, the absorbance of triiodide ions was confirmed to be independent of the solution pH. Third, we evaluated the effect of iodide ions on the formation of triiodide ions by analyzing the UV-visible absorption spectra. Lastly, the detection range of triiodide ions was determined for the UV-visible spectrophotometric method. An iodide solution (100 mM NaI) was obtained by the dissolution of sodium iodide and an iodine (I 2 ) solution was obtained by the dissolution of solid iodine in distilled water. A triiodide solution was prepared by adding the iodine solution to the sodium iodide solution and mixing. Thus, the triiodide ion concentration was mainly determined by the concentration of iodine (I 2 ) in the solutions. Iodide ions were added in excess to stable form the triiodide ion in the solutions. A perchloric acid solution (0.1 M HClO 4 ) was used to control the solution pH. All chemicals used were of analytical grade and all experiments were carried out at ambient temperature. A WPA Lightwave II UV-visible spectrophotometer was used (Biochrom Ltd).

RESULTS AND DISCUSSION
UV-visible absorption spectrum of triiodide ion: Fig. 1a shows the UV-visible absorption spectra of 0-0.15 mM triiodide ions. The solution pH was maintained at 6.5. The large absorption partially observed between 200 and 250 nm is attributed to the addition of iodide ions to the solutions in excess. The triiodide ions exhibited strong light absorption (ε >10 4 L cm -1 mol -1 ) at 288 and 351 nm. The maximum absorbance values at these two wavelengths increased with the triiodide ion concentration. The molar absorptivity values at 288 and 351 nm were 2.65 × 10 4 and 1.72 × 10 4 L cm -1 mol -1 , respectively.  Using the absorbance values presented in Fig. 1a, we obtained the relationship between the maximum absorbance value at 288 nm and triiodide ion concentration, as shown in Fig. 1b. A linear relationship was observed in the concentration range of 0.005-0.10 mM I3 -, corresponding to the range of triiodide ion concentrations that could be determined using UV-visible spectrophotometry.
Effect of solution pH on the UV-visible absorption spectrum of triiodide ions: Fig. 2 shows the UV-visible absorption spectra of 0.1 mM triiodide ion solutions at three different pH values: 2.0, 4.0 and 6.5. Above a pH of 6.5, it is well-known that the triiodide ion isdisproportionated into iodide and iodate ions according to reactions (1,2). Therefore, in our experiments, we limited the solution pH below 6.5. Interference of iodide ions in the UV-visible absorption spectrum of triiodide ions: Molecular iodine is stably soluble in its triiodide ion form in iodide solution,as shown in reaction (3). Although the equilibrium constant of the reaction is high, the concentration of triiodide ions dissolved in the iodide solution may depend on the concentration of iodide ions.
We obtained triiodide solutions by adding iodine (I 2 ) to the sodium iodide solutions in four different concentrations and mixing. As the substantial concentration of iodine is 0.01 mM, the concentration value of triiodide ion is approaching to 0.01 mM with an increase in the iodide ion concentration.  In Fig. 3b, the maximum absorbance λ max of triiodide ion at 288 nm was plotted against the concentration of iodide ion. As the concentration of iodide ion increased, the absorbance value increased with approaching a maximum value. Thus, the absorbance value increased rapidly when the initial concentration of iodide ion was below 10 mM. However, above the concentration 10 mM, the absorbance value increased slowly and approached a certain value. The result indicates that the concentration of iodide ion in triiodide samples should be constant or relatively high, when the UV-visible spectroscopic analysis is used for a triiodide concentration determination method.
Triiodide ion determination range using UV-visible spectrophotometry: Considering the equilibrium between triiodide and iodide ions, we recorded UV-visible absorption spectra of various concentrations of triiodide ions with a constant iodide concentration of 5 mM. Fig 4 shows the relationship between the maximum absorbance λ max at 288 nm and the initial concentration of molecular iodine added to the iodide solution. It was observed that the absorbance of triiodide ion increased linearly with the initial concentration of iodine without any interference of solution pH. The result indicates that iodine concentrations of up to 0.1 mM are completely converted into triiodide ions in a 5 mM iodide solution. Based  Fig. 4, we also found that the detection limit of triiodide ions was 0.005 mM. Therefore, it was concluded that triiodide ion concentrations of 0.005 to 0.1 mM could be determined using UV-visible spectrophotometry as long as the solution pH was 2.0-6.5.

Conclusion
The triiodide ion was observed to have two distinct absorptions at 288 and 351 nm with molar absorptivity values of 2.65 × 10 4 and 1.72 × 10 4 L cm -1 mol -1 , respectively. The absorbance of triiodide ions was independent of the solution pH from 2.0-6.5. However, the absorbance was strongly affected by the iodide ion up to 5.0 mM. Consequently, it was confirmed that triiodide ion concentrationsof 0.005-0.1 mM could be determined using UV-visible spectrophotometry.