Nickel confirmation also is carried out on an quote of supernatant solution from the analysis of group 3.
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).