The reactions involved are as follows: Na2S2O5 H2O → 2 NaHSO3 IO3− 3 HSO3− → I− 3 SO42− 3 H This reaction reduces iodate ions to iodide ions. Hg2 2 I− → HgI2 Orange mercury iodide solid is precipitated until the mercury is used up. IO3− 5 I− 6 H → 3 I2 3 H2O The excess I− and IO3− undergo the iodide-iodate reaction I2 starch → a blue/black complex A blue/black starch-iodine complex is formed. | Chemistry