Spectrophotometric analysis of iodide oxidation by chlorine in highly mineralized solutions

A spectrophotometric method for determining the concentrations of various iodine compounds (I⁻: initial compound, I₃⁻: under-oxidized iodine form, I₂ and I₂Cl⁻: target iodine forms and ICl₂⁻: over-oxidized iodine form) in their joint presence has been developed in order to study iodine processing from underground brines in Turkmenistan which are characterized by considerably higher mineralization and lower iodide content compared than those in Japan and USA. It was found that solutions with constant iodine concentrations and variable chloride concentrations had an isosbestic point at 474 nm with a molar absorbtivity of I₂ plus I₂Cl⁻ of 610.2 l mol⁻¹ cm⁻¹, while the absorbance of other iodine forms at this wavelength was negligible. This allowed us to use an absorbance at 474 nm for calculating the iodine concentration in solutions of variable chloride concentration. For calculating concentrations of other iodine compounds, absorbances at other wavelengths were used: 225 nm (I⁻ and ICl₂⁻), 248 nm (I₂Cl⁻) and 350 nm (I₃⁻). Beer’s law was valid for all iodine compounds in solutions with constant salt concentrations at all wavelengths. The authors have also developed a detailed algorithm for calculating the concentrations of the various iodine forms in their joint presence. The method was applied to solutions with various chloride concentrations and additions of microcomponents of natural solutions (bromide and iron ions, naphthenic acid and hydrogen sulfide). The overall precision for calculating the concentrations of various iodine compounds was <5% for solutions with an oxidant excess of <2-fold, and with chloride concentrations of <5 mol l⁻¹.