Chemical tests for redox species

Test for SO42–, 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 Ba2+ to form a white precipitate.

We are using HCl because we will add BaCl2. If Ba(NO3)2 is to be used, acidify with HNO3.

Don’t use H2SO4!

Finally, add Ba2+(aq) and look for a white precipitate.

Ba2+(aq) + SO42–(aq)

→ BaSO4(s)

Behaviour of sulfite, SO3

2–

The sulfite ion is colourless in solution.

In sulfites, sulfur exists in the +4 oxidation state.

Other species containing sulfur(IV) include SO2 gas, and HSO3

–, which is found in NaHSO3, and forms when SO2 gas reacts with water.

SO2(g) + 2H2O(l) ↔ HSO3–(aq) + H3O+(aq)

When barium chloride solution is added to sulfite solution a white precipitate of BaSO3 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 Fe3+

A moderately-concentrated solution of Fe3+(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:

Fe3+(aq) + SCN– (aq)

→ [FeSCN]2+(aq)

This test is very sensitive and will detect Fe3+ in the presence of other species such as Fe2+.

Test for iodine, I2

Aqueous solutions of iodine, I2, are often orange/brown because there is usually iodide present.

I2(aq) + I–(aq) → I3– (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, Cl2

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 I2, which then turns the starch blue-black:

Cl2(aq) + 2I–(aq) → I2(aq) + 2Cl–(aq)

If you leave the blue-black paper in the chlorine gas it will be bleached white again.