Understanding the chemistry behind: Why O2 and not I2?

[Tyto alba]

We were doing O₂ estimation by Winkler's method

The procedure and reactions involved in this experiment are:

1. Carefully fill a 300-mL glass stoppered bottle brim-full with sample water.
2. Immediately add 2mL of manganese sulfate to the collection bottle.
3. Add 2 mL of alkali-iodide reagent (NaOH + KI) in the same manner.
4. Stopper the bottle with care to be sure no air is introduced. Mix the sample by inverting several times.
5. Add 2 mL of concentrated sulfuric acid.Then mix well.
6. In a glass flask, titrate 100 mL of the sample with sodium thiosulfate to a pale straw color. Titrate by slowly dropping titrant solution from a calibrated pipette into the flask and continually stirring or swirling the sample water.
7. Add 2 mL of starch solution so a blue color forms.
8. Continue slowly titrating until the sample turns clear.
   
   
   
   Now the formula to calculate the amount of O₂ is
   
   
   
   
   
   Well don't we use the formula V1 N1 = V2 N2 only when two substances, compound-1 and compound-2 are reacting?
   If we look at the reactions 1 Na₂S₂O₃ is not reacting with 1O₂ but 1/2 molecule of I₂. Well then again may be had the number of I₂ produced by one O₂ was the same as the number of I₂ reacting with Na₂S₂O₃, we could still use O₂ in place of I₂. But here it is not so.
   This doesn't make sense at all.
   
   Summing up: Why/How do we use this formula with O₂ and not I₂ when it is not reacting with Na₂S₂O₃?

 

[Borek]

A lot depends on how you define the normality of thiosulfate.

In general you should derive the formula following the stoichiometry. Can you write the overall reaction? Intermediates should cancel out and in the end you should get the correct stoichiometric ratio between oxygen and thiosulfate.