Answer:
Dr. Toshihiko Sakai, who is studying solar cells using iodine solution, kindly answered the question above:
 
 
My major is material chemistry, and one of my studies is solar cells using a titanium oxide photocatalyst. Iodine solution is used as the electrolyte in this cell, and oxidation-reduction reaction in the iodine solution is used to maintain generation of electricity. Since I am not studying the reaction of iodine but just using iodine as a tool, I cannot say that I am specialist in the reaction of iodine. However, I will try to answer the question as much as I can.
 
I guess that the problems which this questioner mentions would be:

Write the reaction formula

1. when sulfur dioxide is bubbled in hydrogen sulfide solution 

2. when sulfur dioxide is bubbled in iodine solution

The high school chemistry textbook says “sulfur dioxide works as both an oxidant and a reducing agent”, and the problems above would be typical examples to both of these.

“Iodine solution” in the high school textbook is actually “iodine - potassium iodide solution” . (I am using iodine - lithium iodide solution in my solar cell research.) Actually iodine (I₂) is minimally dissolved in water. On the other hand, potassium iodide (KI) is easily dissolved in water. I₂ can be dissolved in this KI solution, probably because I₂ combines with I^- and becomes I₃^- ion, which is stable in solution. When sulfur dioxide (SO₂) is bubbled in iodine solution, the I₃^- ion in the solution becomes 3I^- because SO₂ works as a reducing agent. In the high school textbook, the reaction above is expressed as the following equation:

I₂ + 2H₂O + SO₂ ↔ 2HI + H₂SO₄

• The high school textbook does not mention I₃^- but instead refers to I₂.

I₃^- ion is brown, but I^- ion is colourless. So the colour of the solution changes from brown to colourless in this reaction.
 
When SO₂ is bubbled in hydrogen sulfide solution, SO₂ works as an oxide. Thus, 
 

2H₂S + SO₂ ↔ 2H₂O + 3S

 
Since sulfur (S) is buff yellow, the colour of solution changes from colourless to buff yellow in this reaction.
 
 
Acknowledgement
We would like to thank Dr. Toshihiko Sakai for his kind answer.
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.