Practical investigations

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I'm investigating the sulphur content in wine and read your tutorial on this topic. I was wondering what the addition of sulphuric acid at the start was for, as wine tends to be acidic anyway?

Igloo writes ...
It is important to understand that in water free molecules of SO₂ react reversibly to form hydrogensulphite ions HSO₃^- and sulphite ions, SO₃^2-.
e.g. SO₂(aq) + H₂O(l) <=> HSO₃ ^-(aq) + H⁺(aq)
 
Now, if for example you are using an iodine titration, you need these ions to be reconverted into SO₂ molecules, and the way to do this is to acidify the wine:
HSO₃^-(aq) + H⁺(aq) -> SO₂ (aq) + H₂O(l)
[notice that this is the reverse of the equation above]
 
Next, the iodine can react with the SO₂ formed:
I₂ (aq) + SO₂ (aq) + 2H₂O(l) -> 2I^-(aq) + SO₄^2-(aq) + 4H⁺(aq)
 
You say that the wine is already acidic. This is true, of course, but remember that the equation creating SO₂ from HSO₃^- is reversible. By using a strong acid such as sulphuric acid, the reaction will swing even more to the right-hand side and iodine is therefore likely to “catch” all the SO₂ present, whether as SO₂ or originally bound up as HSO₃^- and SO₃^2-.
 
Incidentally, the sulphuric acid also helps to dislodge SO₂ molecules bound up with compounds such as the sugars and aldehydes present in the wine.

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: 31 January 2006