Cobalt is searched to fourth analytic group as Co 2+. Sulfide precipitates as insoluble at pH 8.9 for ammonium / ammonia buffer. The precipitating reagent is Na 2 S (preferred TAA and ammonium sulfide):
In fact, the cobalt is not sought as a sulfide, but directly on the solution that comes from the third analytical group (see diagram of the fourth analytic group ).
The solution coming from the third group is at pH 8-9 for ammonium / ammonia buffer. The cobalt, if present, is in the form of amino complex, tetra or hexacoordinate → Co (NH 3) 6 2+ or Co (NH 3) 4 2+.
Cobalt recognition
- Reaction Vogel
For an aliquot of the solution coming from the third analytical group are added a few drops of 2N HCl to restore the cobalt ion (in the form of amino otherwise complex). They are then added 2-3 ml of a mixture of amyl alcohol / ether, and finally 1-2 spatulate it thiocyanate (or thiocyanate) ammonium, NH 4 SCN. We shake the tube vigorously, and you should get the colored organic phase in more or less intense blue.
The blue coloration is due to the formation of tetratiocianocobaltato ammonium, a compound which has a good coefficient of distribution for the organic phase, but is not particularly stable, hence the need to work in excess of the reagent.
The stoichiometry of the reaction also explains why the complex is observable only in organic phase: H 2 O in fact shifts the balance towards the dissociation of the complex.
Co 2+ + 4SCN - + 2NH 4 + (NH 4) 2 [Co (SCN) 4]
or better: Co (H 2 O) 6 2+ + 4SCN - [Co (SCN) 4] 2- + 6H 2 O
Even the nickel forms a similar complex, but that is less stable and has a less favorable partition coefficient for the organic layer. The complex Ni (SCN) 4 2- is colored in green, and it is therefore possible that as a result of the reaction is achieved in blue colored organic phase and the aqueous colored in green.
A possible interference of the essay is the Fe 3+, which form with thiocyanate a red precipitate (see iron search in the third group).
Since the complex that iron forms with the thiocyanate is more stable than that of cobalt, it is formed preferentially. For this reason it is usually used to add the rate levied by the third party solution that you intend to seek the cobalt with the reaction of Vogel a bit of NaF spatula. These is the function of masking the iron (III), which can be left by the third group of various nature for errors.
-reazione Fisher
To the solution, which is ammoniacal (coming from the second group) is added with CH 3 COOH, to bring to pH 5. It is then added with potassium nitrite KNO 2 excess solid.
The pH must be strictly acetic fact, the compound can degrade both in acid medium and in a basic environment:
-in an acid medium [Co (NO 2) 6] 3- + 10H + 2Co 2+ + 5NO 7no + 2 + 5H 2 O
-in basic environment [Co (NO 2) 6] 3- + 3OH - Co (OH) 3 ↓ + 6NO 2 -
In this reaction the nitrite performs two functions, to oxidize the cobalt (II) cobalt (III) and that of complexing the cobalt (III) to give a hexacoordinated species.
The stoichiometry of the reaction above explains the need for large excess of reagent (7 NO 2 -). The formation of the fisher salt (cobaltinitrito) is quite slow, and the walls of the tube should be rubbed with the glass rod and the reaction must be conducted in hot (water bath), to increase its speed.
A 'trick to increase the yield of the reaction is the addition of a soluble salt of potassium, such as KCl or KI, which provides quantitatively K +, and acts as a common ion in the balance of the cobaltinitrito formation.
* Once the precipitate can be centrifuged and separated from the supernatant. The supernatant can try Ni 2+. It alkalinized by addition of soda, to obtain Ni (OH) 2, precipitated by the evident apple green color. For further confirmation you can spin and wash the precipitate and then treat it with bromine water (Br 2). Ni (OH) 2 is thus oxidized to Ni (OH) 3, of brown color. The precipitate of Ni (OH) 2 has to be washed because in the presence of soda the dismuta bromine.
