# Standardization of sodium thiosulfate using potassium dichromate

In this process first we need to liberate iodine to react with sodium thiosulfate.

First, sodium bicarbonate is added to a iodate-free solution of potassium iodide. The mixture is shaken until the salts dissolve, then conc. hydrochloric acid is added. The solution is shaken again, then covered and keept in a dark place. I have two questions here:

1. What is the role of sodium bicarbonate here?
2. Can conc. sulfuric acid be used to provide the acidic media instead of conc. hydrochloric acid?

1. I think your procedure is incorrect. Typically you would mix $\ce{KIO3}$ (limiting reagent) with access of $\ce{KI}$. Then you add $\ce{HCl}$ to have a reaction: $$\ce{KIO3 + 5KI + 6HCl -> 6KCl + 3I2 + 3H2O}$$ This is a way to generate a known amount of $\ce{I2}$.
You would then dissolve $\ce{Na2S2O3}$ in a buffer (sodium bicarbonate). This is done because $\ce{Na2S2O3}$ is not stable under acidic conditions. Then you can titrate $\ce{I2}$ in presence of starch with $\ce{Na2S2O3/NaHCO3}$. $\ce{NaI}$ and $\ce{Na2S4O6}$ are formed in this reaction.
2. In any case you cannot replace $\ce{HCl}$ with other acids. $\ce{H2SO4}$ and $\ce{HNO3}$ are oxidizers strong enough to turn some more $\ce{KI}$ into $\ce{I2}$ and mess up your measurement.