Practical investigations

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I was reading your help sheet online about the wine analysis investigation, and saw that you mentioned using an iodine titration to work out the concentration of sulphur dioxide in the wines. I understand the technique involved and the necessary calculations that I have to perform, but despite all of my searching in books and on the internet, I have been unable to find the reaction equation between sulphur dioxide and iodine. I have guessed that the products would be pure sulphur and iodate ions. Is this correct and if not, what is the balanced equation for the reaction? Thanks.

Igloo writes
Solid sulphur and aqueous iodate ions might have been formed if sulphur dioxide acted as an oxidising agent in this reaction, but it doesn’t! It behaves as a reducing agent here, so the equation is:
 
SO₂(aq) + I₂(aq) + 2H₂O(aq) -> H₂SO₄(aq) + 2HI(aq)
 
Or in an ionic form:
SO₂(aq) + I₂(aq) + 2H₂O(aq) -> 4H⁺(aq) + SO₄^2-(aq) + 2I^-(aq)
 
In other words, one mole of iodine molecules is reduced by one mole of sulphur dioxide molecules.

Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website. For further guidance see our tutorial on Risk Assessment.

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updated: 28 January 2006