| Reagent | Fe2+ | Fe3+ |
| Ammonia or sodium hydroxide (hydroxyl ions) | Green precipitate. Turns brown on exposure to air | Red-brown precipitate |
| Potassium hexacyanoferrate(II), K4[Fe(CN)6] | White precipitate, rapidly turning blue | Prussian blue precipitate |
| Potassium hexacyanoferrate(III), K3[Fe(CN)6] | Dark blue precipitate (Turnbull's blue) | Reddish-brown coloration (no precipitate) |
| Potassium thiocyanate, KCNS | No coloration* | Blood red coloration |
*This test is extremely sensitive and usually sufficient Fe3+ ions are present in an iron(II) salt to give some coloration. The blood red colour appears to be due to a complex.
TESTS FOR COBALT
For a cobalt(II) salt the precipitation of the blue-pink cobalt(II) hydroxide by alkali, or precipitation of black cobalt(II) sulfide by hydrogen sulfide provide useful tests; the hydroxide is soluble in excess alkali and is oxidised by air to the brown CoO(OH). Addition of excess potassium nitrite acidified with acetic acid gives a precipitate of the potassium hexanitrocobaltate(III), K3[Co(NO2)6].
Decomposition of most cobalt(III) complexes by boiling with alkali gives a brown precipitate of the hydrated oxide Co2O3.H2O. This will quantitatively oxidise iodide to iodine.
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| COORDINATION COMPOUNDS OF NICKEL | | | IRON, COBALT, NICKEL |