Manganese is sought to fourth analytic group as Mn 2+. It is precipitated as sulphide insoluble at pH 9.8 for ammonium / ammonia buffer. The precipitating reagent is Na 2 S.
Besides manganese precipitate as sulphides, if present, the following cations: Zn 2+, Co 2+, Ni 2+.
Manganese and zinc are separated from the sulfides of cobalt and nickel by treatment with HCl 2N. Although the solubility of the freshly precipitated sulphides is comparable to, those of cobalt and nickel after a few minutes undergo a chemical / physical process known as "aging." Aging is the transformation of the precipitate in a more difficult form of thickened risolubilizzare. For treatment with acid diluted zinc sulfides and manganese are readily risolubilizzati. The nickel and cobalt sulphides aged instead are so insoluble as to be risolubilizzati.
Zn 2+ and Mn 2+ are carried in the solution according to the mechanism:
The sulfide ion S 2- is shifted to hydrogen sulfide. Since [S 2-] is missing from the balance of the solubility of the salt product (Kps = [Mn 2+] [S 2-]) the salt is risolubilizza to maintain constant Kps.
Mn 2+ is therefore in Zn 2+ solution (if any). These two cations are separated by treatment with 6N NaOH. Zinc has amphoteric behavior, and is soluble in strong alkali. Manganese for its part, precipitates as hydroxide.
The manganous hydroxide is colored in pale pink, almost white. We could already be seen, we precipitated in part, to the third analytical group . The manganous hydroxide could stand brown to oxidation by atmospheric oxygen to Mn (OH) 3 or MnO (OH).
Manganese recognition
The manganese is searched from precipitate of manganous hydroxide, Mn (OH) 2 (partially burnished due to the action of atmospheric oxygen (MnO (OH) or Mn (OH) 3). It is with 1 ml of nitric acid concentrated (HNO 3) this precipitate for risolubilizzarlo; is then added a spatula of PbO 2, lead dioxide, commonly known as flea oxide.
This compound is a so strong oxidant that is able to bring Mn 2+ to permanganate, MnO 4 -, where manganese has an oxidation number of +7.
2mn (OH) 2 + 2 + 8H 5PbO 3 O + 2MnO 4 - + 5PB 2+ + 14H 2 O
It 'just the permanganate ion we want to achieve, the manganese compound easier to recognize for the intense violet color that gives the solutions.
The oxidation of manganese (II) to manganese (VII) is justified by the standard reduction potentials of the two semicoppie:
E ° MnO 4 - / Mn 2+ V E ° = 1:49 PbO 2 / Pb 2+ = 1.75 V
For obvious reasons the special test for manganese research , called reaction Crumm, takes place in a similar way.
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