[WilliamYWT]

May I ask why we have to first react sulphur dioxide with sodium hydroxide to from sulphite ion then add acid to "liberate" all sulphite ion to become dissolved sulphur dioxide so that we would be able to react the dissolved sulphur dioxide with iodine?

[Borek]

I don't know exact procedure you refer to (and your post is quite cryptic), but I bet it is about difference between SO₂ and SO₃^2-...

[WilliamYWT]

Sorry for that, I now list the procedures in detail.

First, convert all sulphur dioxide in wine into sulphite ion by sodium hydroxide, let it stands for 15 mins.

Second, acidify the solution with sulphuric acid then liberates all sulphur dioxide (I don't really stand what the word "liberate" means)

Third, titrate the resultant solution immediately with iodine solution using the starch as indicator.

[billnotgatez]

@WilliamYWT
Are you referring to the Ripper method of test for Sulfur Dioxide in wine?

From
http://www.brsquared.org/wine/Articles/SO2/SO2.htm
In section 14.1

Free SO₂ is determined directly while total SO₂ can be ascertained by treating the sample with sodium hydroxide before the titration to release bound SO₂.

[Borek]

If you leave the sample as it is, SO₂ will leave it in gaseous form, thus final result will underestimate SO₂ level.

If you add base to the sample, you will convert SO₂ into SO₃^2-, thus it will stay in the solution:

SO₂ + 2OH^- SO₃^2- + H₂O

Acidifying it later (which is equivalent to removing OH^-) will shift the equilibrium to the left, producing ("liberating") SO₂ which will in turn react with iodine.