Nickel identification
Nickel confirmation also is carried out on an quote of supernatant solution from the analysis of group 3.
-Colorimetric essay
In first place, make sure that the pH of the solution of the solution (from group 3) is about 8. A more alkaline pH, and then an excess of ammonia means a low concentration of "free" Ni2+ because this ion is favorably complexed by ammonia (tetra or hexacoordinate complex).
At this quote we add 2-3 drops of dimethylglyoxime (alcoholic solution). If is there Ni2+ in solution it will form a flocculent precipitate cherry-red colored.
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. For this reason it is usually used an excess of the reagent. It outlines a cherry red precipitate (insoluble nickel-dimethylglyoxime) complex field rust (complex soluble cobalt-dimethylglyoxime).
