Redox and redox equilibria
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Please explain why potassium iodate(V) solution, sulphuric acid, and potassium iodide are chosen to be used in standardising sodium thiosulphate (VI) solution. And please tell me what 'liberated' means in the 'liberated iodine' phrase. thank you.
Ulex writes
Neither iodine nor sodium thiosulphate is suitable for direct accurate weighing; iodine because it vaporises so easily, and sodium thiosulphate because it tends to react with even a tiny amount of acid in the water used to make up the solution.
The difficulty of standardising sodium thiosulphate is solved by making up a solution of approximately the right concentration, allowing it to stand for a while so that any reaction with acid is over and done with. The resulting solution is then used to titrate the potassium iodate solution which you describe. Potassium iodate is a stable, anhydrous, crystalline solid which is easy to weigh out and dissolve in water to give a solution of exactly known concentration.
When a pipetted portion of potassium iodate solution is reacted with acid and excess potassium iodide, the reaction produces iodine – the element iodine is, as it were, released from the compounds or ‘liberated’ from them. This is a colourful expression in both the literal and the metaphorical sense!
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updated: 14 December 2006
