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Determination of the sulfur sioxide content of wine

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danielf:
Hi,
Just wondering if someone could transcribe the following paragraph into equation form:
'Sodium hydroxide is added to the sample of wine to convert the hydrogen sulfite compounds to sulfate ions. When sulfuric acid is added, sulfite and hydrogen sulfite ions are converted to sulphur dioxide molecules in aqueous solution’
This is only a part of the background theory for the titration however help in turning the above into equations would be greatly appreciated
cheers

Collin Li:
I am having trouble with this because (hydrogen sulfite) has an oxidation number of +4, which changes to +6 in (sulfate). This is an oxidation, and that is okay, but NaOH is not an oxidant - in fact, it is a weak reductant - it will oxidise. I can't think of the reduction half-equation.

The only reaction that should take place with sodium hydroxide and the hydrogen sulfite would be an acid-base reaction, which protonates (donates a proton) the hydrogen sulfite ion to form the sulfite ion. Perhaps it was a typo because "sulfate" ions don't seem to have any further role in this experiment, so I think it meant sulfite. In this case, it is just an acid-base reaction, where a proton from hydrogen sulfide is donated to the hydroxide ion to from water in a typical neutralisation.

This equation would be:

In the second reaction, the sulfuric acid will act as an acid and a dehydrating agent (not on the course any more). The reaction will protonate the hydrogen sulfite and sulfite ions into sulfonic acid: . This sulfonic acid will then have the water stripped from its formula, so it will lose 2 hydrogen atoms and 1 oxygen atom, becoming . It cannot be a redox reaction because the species you have at the beginning are +6 and +4 (sulfuric acid and the sulfite ions respectively), while your product is +4. The sulfite ions are converted into sulfur dioxide in a non-redox reaction (+4 to +4), while the sulfuric acid is just a facilitator of the reaction - to protonate and dehydrate.

First step (protonation):

Second step (dehydration): in the presence of sulfuric acid.

Overall equation:



Note that this equation is based on the assumption that the original question is a typo, and that it should read "sulfite" rather than "sulfate."

Also note that sometimes you don't need to know the equation. For example, you might only need to know how much sulfur you have, and you don't need an equation for that if you have enough data about the sulfur containing compounds.

danielf:
thanks for the help mate. and yeah, you're right, that was typo

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