chemical tests for redox species. test for so 4 2–, the sulfate ion the sulfate ion is colourless
TRANSCRIPT
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!
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)
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.