Chemical tests for redox species

Test for SO 4 2–, the sulfate ion The sulfate ion is colourless.

The solution is acidified to remove other anions such as carbonate, hydroxide or sulfite which react with Ba 2+ to form a white precipitate. We are using HCl because we will add BaCl 2. If Ba(NO 3 ) 2 is to be used, acidify with HNO 3. Don’t use H 2 SO 4 !

Finally, add Ba 2+ (aq) and look for a white precipitate. Ba 2+ (aq) + SO 4 2– (aq) → BaSO 4 (s)

Behaviour of sulfite, SO 3 2– The sulfite ion is colourless in solution.

In sulfites, sulfur exists in the +4 oxidation state. Other species containing sulfur(IV) include SO 2 gas, and HSO 3 –, which is found in NaHSO 3, and forms when SO 2 gas reacts with water. SO 2 (g) + 2H 2 O(l) ↔ HSO 3 – (aq) + H 3 O + (aq)

When barium chloride solution is added to sulfite solution a white precipitate of BaSO 3 forms.

This precipitate dissolves when acid is added.

The final solution is normally slightly cloudy because even a freshly-made solution of sodium sulfite, prepared from a newly- opened container of powder, usually contains trace amounts of sodium sulfate.

Test for Fe 3+ A moderately- concentrated solution of Fe 3+ (aq) is yellow- orange in colour. Dilute solutions appear colourless.

Add a drop or two of SCN – (thiocyanate) solution. A blood red complex will form: Fe 3+ (aq) + SCN – (aq) → [FeSCN] 2+ (aq) This test is very sensitive and will detect Fe 3+ in the presence of other species such as Fe 2+.

Test for iodine, I 2 Aqueous solutions of iodine, I 2, are often orange/brown because there is usually iodide present. I 2 (aq) + I – (aq) → I 3 – (aq)

Add a drop or two of starch solution. The iodine forms a strongly coloured blue- black complex. This test is very sensitive and will detect iodine in solutions which are so dilute they appear to be colourless.

Test for chlorine gas, Cl 2 Chlorine gas is a pale green-yellow gas with a characteristic ‘swimming pool’ odour. Very small amounts of chlorine gas appear colourless.

Chlorine will turn blue litmus paper red and then bleach it of colour.

Chlorine will turn damp starch-iodide paper blue-black.

Starch-iodide paper is filter paper that has been soaked in a solution of potassium iodide and starch. Chlorine oxidises the I – to I 2, which then turns the starch blue-black: Cl 2 (aq) + 2I – (aq) → I 2 (aq) + 2Cl – (aq) If you leave the blue-black paper in the chlorine gas it will be bleached white again.