Practical investigations

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For my investigation, I have decided to analyse the free SO₂ content in wines and have decided to do this by acidifying the wine with H₂SO₄ and then carrying out a titration with iodine. However, I am stuck as on how to work out the volume of wine that I initially need to start with when reacting it with H₂SO₄ and was wondering if you could help me? Any other hints and tips would be greatly appreciated. Thank you sooo much, hoping for a quick reply.

Igloo writes ..........
This procedure is a relatively straightforward affair in that all you need do is to carry out the titration under acidic conditions in order to ensure that ions in the wine such as HSO₃^- and SO₃^2- are converted to free SO₂. During the iodine titration hydrogen ions are generated, rather than destroyed:
 
SO₂ (aq) + I₂ (aq) + 2H₂O(l) ----> SO₄^2-(aq) + 2I^-(aq) + 4H⁺(aq)
 
Before the titration is carried out you must ensure that the solution has a low pH. You can either use 10.0 cm³, 20.0 cm³ or 25.0 cm³ portions of your wine (depending on which pipettes you have available in your laboratory). Then you should add from a measuring cylinder about 10 – 20 cm³ of ordinary laboratory dilute sulphuric acid, and check, using pH paper or a pH probe, that the pH is low (e.g. about 1) before running in the iodine solution from a burette.
 
Hints:
The concentration of SO₂ in wine will not be very high (maybe round about 0.0005 mol dm^-3), so use iodine solution of approximately the same concentration, so that your titration reading will be “sensible”.
 
Remember that the concentration of the iodine solution will need to be known exactly, so this means that it needs to be standardised before use, since iodine is not a primary standard. This procedure can be carried out with a solution of sodium thiosulphate of known concentration.
 
Always carry out a risk assessment and check with your teacher before starting any practical work.

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: 24 February 2006