Reaction of iron(ii), Fe2+
Colour : pale green, almost colourless.
| Test | Observation | Notes |
|---|---|---|
|
Add H2O2
Then add dilute H2SO4(aq)
Followed by NaOH(aq) |
Solution turns from light green to yellowish brown Intensity of yellowish-brwon colour increases
A red-brown ppt is formed upon addition of a few drops of NaOH(aq) |
Fe2+(aq) is oxidized by hydrogen peroxide to Fe3+(aq) which is yellowish-brown The action of hydrogen peroxide as an oxidising agent is promoted by an acid
Fe3+(aq) from the oxidation of Fe2+(aq) by hydrogen peroxide is precipitated as Fe(OH)3(s), which is a red-brown solid. |
|
Add NH3(aq)
Leave ppt in the air for a few minutes |
A green ppt is formed
ppt changes from green to red-brown |
Fe(OH)2(s) is produced. Fe(OH)2(s) oxidized by oxygen to red-brown Fe(OH)3(s) |
| Add NaOH(aq) | Pale green ppt is formed insoluble in xs. The ppt slowly turns brown in air | Fe3+ precipitated as Fe(OH)3(s). |
| Add dilute H2SO4 followed by KMnO4(aq), drop by drop | KMnO4 is decolourised. A yellowish-brown solution is formed | Oxidation of Fe3+ by KMnO4 to form a yellowish-green solution of Fe3+. The yellowish-brown colour observed is due to Fe3+. |
Reaction of iron(iii), Fe3+
Colour : yellow brown to orange when unmixed with other colored cations
| Test | Observation | Notes |
|---|---|---|
| Add solid NaCO3 | Effervescence occurs, gas which turns limewater milky is produced. A coffee brown ppt is formed | Brown ppt is formed Fe(OH)3. Fe3+ is acidic, protonate water hence carbonate is dicomposed. |
|
Add KI(sq) followed by either;
Strach solution
Na2S2O3(aq) |
Solution turns deeper brown
a blue black colour appears
Brown colour disappears leaving a pale green solution |
I– oxidized to brown iodine. meanwhile Fe3+ is reduced to pale green Fe2+ whose color is masked by that of iodine. The blue black colour is due to the formation of an iodine/starch complex
Brown iodine is reduced by thiosuphate ions to I– leaving Fe3+ |
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