We were doing a titration experiment to find out how much ascorbic acid a vitamin pill actually contains. In a flask, there is a solution of water, sulfuric acid, potassium iodide and ascorbic acid. Also starch as an indicator. When adding potassium iodate $(\text{KIO}_3)$, the following reactions will appear:

$$\text{IO}_3^- + 5 \ \text{I}^- + 6 \ \text{H}^+ \longrightarrow 3 \ \text{I}_2 + 3 \ \text{H}_2\text{O}$$

After the iodine is formed, the following reaction will appear:

$$\text{C}_6\text{H}_8\text{O}_6 + \ \text{I}_2 \longrightarrow \text{C}_6\text{H}_6\text{O}_6 + 2\ \text{H}^+ + 2 \ \text{I}^-$$

After using the right amount of potassium iodate, there will be no ascorbic acid anymore, causing to eliminate the second reaction. When there is no ascorbic acid, iodine will not react back to iodine-ions, which causes the starch to give a brown/blue color.

My question is, why isn't this possible without the sulfuric acid?

  • 4
    $\begingroup$ Look at the first reaction: it requires $\ce{H+}$ ions. $\endgroup$
    – bon
    Jun 22 '15 at 20:38
  • $\begingroup$ It is possible, sulforic acid is added just to increase oxidation power. $\endgroup$ Apr 23 '18 at 21:06

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