[Zir0_Cool]

Hello everybody!
For a university project I'm trying to determine the amount of SO2 in lemon juice.
The first step is a simple Iodine titration with starch as indicator.
The second step (according to the text book) is adding a base (NaOH) and sulfuric acid (H2SO4), then doing another titration.
Why is that?? what's the purpose?
also, can anyone write the reactions?
The final step is adding to another sample fornaldehyde and sulforic acid and doing another titration.
Again, why?? what are the reactions?
Many thanks to everybody in advance for helping.

[hmx9123]

You need to show what work you've done thus far to receive any help.