Question

1) What does it mean to be a reducing agent? What ion does vitamin C reduce in this experiment? Explain your answer using the oxidation numbers.

b) If you look at the chemical reaction for this experiment, sodium bicarbonate is not a part of the reaction. If so, why do we need to add sodium bicarbonate during the analysis of vitamin C? Explain your answer using balanced chemical equation

c) Identify the oxidizing agent in chemical equation (i) from the discussion. Explain your answer using oxidation numbers before and after the reaction.

Info below. Thanks!

The analysis of vitamin C is performed under acidic condition. Standard potassium

iodate, KIO3 solution is used as a titrant.

The analysis undergoes following three step process.

(i) Oxidation of iodide to triiodide complex
IO3-(aq) + 8 I-(aq) + 6 H+(aq) ? 3 I3-(aq) + 3 H2O(l)

(ii) Ascorbic acid reduces triiodide to iodide (ascorbing acid is oxidized) C6H8O6(aq) + I3-(aq) + H2O(l) ? C6H8O7(aq) + 3 I-(aq) + 2 H+(aq)

(iii) Once the ascorbic acid is consumed, the excess of I3- is generated which produces deep-blue/purple color with starch, indicating end-point of the titration.

Answer 1

1)Reducing agent is one which under go oxidation itself & reduces other substance

2)Vitamin-C reduces IO^-3 ion into I^- ion

b) NaHCO₃ is added during the tthe estimation of vitamin-C to maintain _PH of the solution.By adding NaHCO3 forms CO2 which blankets over the sample which prevents the oxygen intereference during the titration.

3)Oxidation of IO^-3 to I^-3

i) IO3^-(aq.) + I^-3 (aq.) + 6H+ (aq.) ------>3I₃^- (aq.) + 3H₂O

Oxidation number of Iodine in IO₃ is +5 changes to -1

ii)C₆H₈O₆(aq) + I₃-(aq) + H₂O(l) ---> C₆H₈O₇(aq) + 3 I-(aq) + 2H+(aq)

Vitamin C is reducing while I₃- _{oxidizing species.}