Practical investigations
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I have investigated the sulphur dioxide content in the white wine. I use an iodine titration to determine the SO2 content, but I don't know why it is necessary to add sodium hydroxide solution to the wine before adding sulphuric acid. Can you tell me the purpose?
Igloo writes ...
Sulphur dioxide is found “free” in the wine, but much of it is bound to organic components present, such as carbonyl compounds and sugars. If you simply titrate the wine in acid conditions with iodine, having not used sodium hydroxide beforehand, only the “free” sulphur dioxide reacts.
However, warming the wine up with alkali first causes the “bound” sulphur dioxide molecules to become detached from the various chemicals in the wine, so that they too can be taken into account when you carry out your iodine titration.
To sum up:
Titration without adding NaOH analyses for the “free” SO2 within the wine.
Titration using NaOH analyses for the total SO2 present: “free” and “bound”.
Since the SO2 binds to a wide range of different and complex components within the wine, it isn’t possible to give an equation, even a general one. However, if you wish to carry out more research, you will find further information under “bisulphite addition compounds”.
Risk assessment
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updated: 25 January 2008
