Qualitative Tests on 3d-metal Ions
(Cr, Mn, Fe, Co, Ni, Cu) .

Introductory remarks
Most of compounds of the above mentioned ions are colored. These colors could be enhanced in order to give a characteristic effect which would undoubtedly show the presence of the ion in an unknown mixture.

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:
Cr³⁺ + 3 OH^- ® Cr(OH)₃ or CrCl₃ + 3 KOH ® Cr(OH)₃ + 3 KCl
Chromium hydroxide Cr(OH)₃ dissolves in strongly alkaline solutions forming tetrahydroxochromate(III) ion:
Cr(OH)₃ + OH^- ® [Cr(OH)₄]^-
tetrahydroxochromate(III) can in turn be readily oxidized to chromate(VI) using hydrogen peroxide:
2 OH^- +2[Cr(OH)₄]^- + 3H₂O₂® 2CrO₄^2- + 8 H₂O

All other 3d transition metal ions for precipitates of appropriate hydroxides at these conditions. Only Zn(II) forms colorless tetrahydroxozincate [Zn(OH)₄]^2- that does not interfere with chromium determination. The overall reaction can be written as
2Cr³⁺ + 10 OH^- + 3 H₂O₂ ®2CrO₄^2- + 8 H₂O
or
2 CrCl₃ + 10 NaOH + 3 H₂O₂ ®2 Na₂CrO₄ + 8 H₂O + 6 NaCl
Cr³⁺: Add an excess of NaOH solution and some H₂O₂. Heat the resulting mixture. Filter the solution and observe the color of the filtrate. If it is yellow, you have CrO₄^2- ion in filtrate.

CrO₄^2-

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:
2Mn²⁺ + 5IO₄^- + 3 H₂O®2MnO₄^- + 5IO₃^- + 6 H⁺
Mn²⁺: Put 1 droplet of unknown (NO more than 1!!!) solution into a test tube and dilute it with some H₂SO₄. Add KIO₄ (white solid) and warm gently. The appearance of characteristic purple color shows the presence of Mn. Very sensitive reaction.

MnO₄^-

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 Fe(III) determination:
Fe³⁺ + 3 SCN^- ® Fe(SCN)₃
Fe³⁺: To a drop of unknown solution, add several drops of NH₄SCN. Bloody red color appears if Fe(III) is present.

Fe(SCN)₃

Almost colorless Fe(II) salts form deep blue precipitate with Fe(CN)₆^3-:
3 Fe²⁺ + 2 Fe(CN)₆^3-®Fe₃[Fe(CN)₆]₂
Fe²⁺: To a drop of unknown , add several drops of K₃Fe(CN)₆ . The deep blue color is the indication of Fe²⁺.

Fe₃[Fe(CN)₆]₂

Co. Most of common cobalt(II) salts are pink or orange. Some bright blue cobalt (II) compounds form in organic solvents:
Co²⁺ + 4 SCN^- ® Co(NCS)₄^2-
Co²⁺: To a drop of unknown solution, add several drops of NH₄SCN in acetone or butanol. The bright blue color is the indication of Co²⁺.

Co(NCS)₄^2-

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:

Ni²⁺: To several drops of unknown, add some tartric acid , dimethylglyoxime solution, and then slowly add aqueous ammonia. Drops of ammonia solution must go by the wall of the test tube. A pink color of nickel glyoximate appears.

nickel glyoximate

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 complex:
Cu²⁺ + 4 NH₃  ® [Cu(NH₃)₄]²⁺

[Cu(NH₃)₄]²⁺

Large quantities of Ni(II) can interfere with Cu(II) determination because Ni(II) forms blue-violet hexaammino complex [Ni(NH₃)₆]²⁺ .
Contrary to Ni(II), Cu(II) reacts with iodide forming molecular iodine I₂ and copper(I) iodide:
Cu²⁺ + 4 I^- ® 2 CuI + I₂

I₂

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.