Redox titration between iodine and thiosulfate ions. The concentration of iodine in a solution can be determined by titration with a solution of thiosulphate of known concentration. In this reaction, aqueous iodine is reduced to iodide ions by aqueous sodium thiosulphate which forms the tetrathionate ion S4O62-(aq). Iodine has a blue-black appearance in the presence of starch. During a titration between iodine and thiosulfate ions, this colour will remain as long as iodine is present.