Qualitative Tests on 3d-metal Ions
(Cr, Mn, Fe, Co, Ni, Cu) .
Cr. The most
stable oxidation states of chromium are Cr(III) and
Cr(VI). Green (or blue, or purple) Cr(III) salts give a precipitate of Cr(OH)3
when they react with bases:
Mn. Almost colorless
(actually slightly pinkish) Mn(II) salts can be oxidized in strongly acidic solution to
give purple permanganate ion MnO4-. Several different
oxidants can be employed for that purpose, among them potassium periodate KIO4:
Fe. The most stable oxidation states of iron are Fe(III) and Fe(II). Fe(III) ions
form yellow or brown ( rusty!) or red compounds with various agents. The
reaction with thiocyanate ion is among the best for
colorless Fe(II) salts form deep blue precipitate with Fe(CN)63-:
Co. Most of common cobalt(II) salts
are pink or orange. Some bright blue cobalt (II) compounds form in organic
Attention: the Fe3+ ions need to be masked by NaF while Cu2+ ions require some sodium thiosulfate to mask them.
Ni. A very selective
reaction for Ni(II) determination is known for
almost 100 years. It includes the formation of the complex compound:
The reaction can be performed on the filtration paper. Add some sodium phosphate, then unknown, followed by more sodium phosphate and dimethylglyoxime. Expose the resulting spot on the paper to ammonia vapor.
Cu. Blue color of
copper salts is readily visible in concentrated solutions. To observe it in
diluted solutions, you should convert copper into tetraammine
Large quantities of Ni(II) can
interfere with Cu(II) determination because Ni(II) forms blue-violet hexaammino complex [Ni(NH3)6]2+
Cu2+: To several drops of unknown, add some aqueous ammonia and warm the solution. Filter the precipitate if any. If the resulting color is blue you have Cu. If you have lots of Ni, it also can give you similar effect. In this case, neutralize the filtrate with acetic acid and add some KI. The appearance of brown I2 indicates that you have Cu as well.