The redox reaction behind iodometric titration: iodine and sodium thiosulfate. • Sodium thiosulfate, Na₂S₂O₃, is a white crystalline solid that dissolves in water, giving us the thiosulfate anion, S₂O₃²⁻, which is acting as a reducing agent in this demonstration. • Elemental iodine is slightly soluble in water, so in this brown coloured lugol’s iodine we have here it is mixed with potassium iodide to increase solubility via the formation of triiodide ions, I₃⁻ in the following exergonic equilibr