IGCSE Chemistry 0620: Oxidising and Reducing Agent Tests on Vitamin C

A food scientist investigates the redox behaviour of vitamin C (ascorbic acid) using the two standard test reagents for oxidising and reducing agents.

Test 1: a few drops of acidified aqueous potassium manganate(VII) are added to a vitamin C solution. The purple colour disappears.

Test 2: a few drops of aqueous potassium iodide are added to a fresh sample of the vitamin C solution. There is no colour change.

(a) State what test 1 shows about the redox behaviour of vitamin C, and identify which species, vitamin C or the manganate(VII) ions, is reduced in test 1. [2]

(b) Use the result of test 2 to explain why vitamin C is not acting as an oxidising agent. [2]

(c) State the term used to describe a substance, like vitamin C, that reduces another species and is itself oxidised. [1]

(d) The oxidation number of manganese in the manganate(VII) ion, $\text{MnO}_4^-$ , is $+7$ . In test 1 the manganese ends up as $\text{Mn}^{2+}$ ions. State the change in the oxidation number of manganese during test 1, and use it to confirm that the manganese has been reduced. [2]

Part (a): Interpreting test 1

Acidified potassium manganate(VII) turns from purple to colourless in the presence of a reducing agent. Since the purple colour disappears when vitamin C is added, test 1 shows that vitamin C is a reducing agent.

A reducing agent works by being oxidised itself while reducing another species. So it is the manganate(VII) ions that are reduced in test 1, not vitamin C.

Part (b): Interpreting test 2

Aqueous potassium iodide turns from colourless to brown in the presence of an oxidising agent, as iodine forms. Test 2 shows no colour change, so no iodine has formed. The iodide ions have not been oxidised.

Since vitamin C has not oxidised the iodide ions, vitamin C cannot be acting as an oxidising agent here.

Part (c): Naming the role of vitamin C

A substance that reduces another species, and is itself oxidised in the process, is called a reducing agent. This matches vitamin C’s behaviour in test 1.

Part (d): Oxidation number change of manganese

The oxidation number of manganese starts at $+7$ (in $\text{MnO}_4^-$ ) and ends at $+2$ (in $\text{Mn}^{2+}$ ):

$+7 \longrightarrow +2$

This is a change of:

$7 - 2 = 5$

so the oxidation number of manganese decreases by 5. A decrease in oxidation number corresponds to a species gaining electrons, which is the definition of reduction. Confirming that the manganese has indeed been reduced.

Final answers

(a) Vitamin C is a reducing agent; the manganate(VII) ions are reduced.
(b) No iodine forms with potassium iodide, so vitamin C is not an oxidising agent.
(c) A reducing agent.
(d) Manganese’s oxidation number decreases from $+7$ to $+2$ (a decrease of 5), confirming reduction.