[Anne]

I have worked in a laboratory of a winery where I analyzed the free and total sulfur dioxide in wine with a manual titrator by the Ripper method.

This method is usually a simple iodine titration where iodine reacts with sulfur dioxide according to the following equation:

SO₂ + I₂ + H₂O  H₂SO₄ + 2HI

The reagents used in the method I followed are slightly modified. Instead of titrating the wine sample acidified with sulfuric acid against an iodine solution, I titrated it against a solution of potassium iodide (a starch indicator is used to see the color change and distilled water is added to the sample).

I have thought of the following equations to represent this reaction:

1. SO₂ + 2H₂O + H₂SO₄ + 4KI  2K₂SO₄ + 4HI

2. SO₂ + 2H₂O + 2KI  H₂SO₄ + 2HI + 2K⁺

3. HSO₃^- + KI  KSO₃^- + HI

However, in equation 1 the hydrogen is not balanced, and in equation 2 the charges are not balanced. Equation 3 looks fine at first, but the bisulfite ion would imply that this reaction only occurs when analyzing the free fraction of sulfur dioxide, which is not true.

How does potassium iodide in solution react differently to iodine, supporting this explanation with a chemical equation preferably?

If you are interested, you can find the user's manual of the apparatus I used in the following link: http://shop.gabsystem.com/data/descargas/Inst%20TONING%20A5_En.pdf

I contacted the company but their response was not very helpful. They only said that, in their method, the common iodine solution is replaced by potassium iodide solution because as a product it is cheaper and more stable.

[Borek]

Their procedure calls for iodate, not iodide.

1. SO₂ + 2H₂O + H₂SO₄ + 4KI  2K₂SO₄ + 4HI

It can't be balanced. Note that most of the things you listed are just spectators.