The first reaction of iodine with SO  and water is as follows:
SO+I+2HO→HSO+2HI
As the reaction proceeds, all available SO will be consumed and the starch indicator added to the solution will bind with the unconsumed iodine, turning the solution black.
The second step of the reaction requires pretreating with solution with NaOH to release bound SO. The reaction with iodine can then be done.
HSO⇌HSO⇌SO