Nickel is sought to fourth analytic group as Ni 2+. It is precipitated as sulphide insoluble at pH 9.8 for ammonium / ammonia buffer. The precipitating reagent is Na 2 S (preferable to TAA and ammonium sulfide).
In reality, the nickel and cobalt are not searched as sulphides. They are searched directly in the solution coming from the third analytical group . This solution is at pH 8-9 for ammonium / ammonia buffer.
The nickel if present is in the form of complex or tetra amino hexacoordinate → Ni (NH 3) 6 2+ or Ni (NH 3) 4 2+.
Remember that the solution from the third analytical group is divided in two. In a portion are searched cobalt and nickel, zinc and manganese in the other.
-Reazione Of Tschugaeff
We must make sure that the pH of the solution from the third group is actually post 8-9. A pH more alkaline, given by an excess of ammonia, mean low concentration of Ni 2+ "free" (because complexed by ammonia).
At the rate in which the search nickel are added 2-3 drops of dimethylglyoxime (alcoholic solution). If this Ni 2+ forms a flocculent precipitate cherry red.
The precipitate is formed because the nickel-dimethyl gliossima whole is more stabila nickel-ammonia complex. The complex is however soluble in acid environment, as you can see from the stoichiometry of the reaction.
In fact, even the cobalt forms a complex with dimethylglyoxime. The complex is rust colored and soluble. This is usually used an excess of reagent (dimethylglyoxime), so as to delineate a cherry red precipitate (insoluble complex of nickel) in the field rust (soluble complex of cobalt).
* (See → search of cobalt )
The search of the nickel is also carried out from the solution in which it is precipitated as a salt of the cobalt Fischer (cobaltinitrito or potassium). This solution (after centrifuging and removed the yellow precipitate cobaltinitrito) is made alkaline with soda. If this Ni 2+ precipitates Ni (OH) 2, nickel hydroxide, a precipitate by the evident apple green color. For further confirmation, and thoroughly washed after separating the precipitate, it can be treated with bromine water. Br 2 oxidized Ni (OH) 2 Ni (OH) 3, of brown color.
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