Question

What is the colour of Nickel Sulphide? How is nickel tested by dimethylglyoxime?

Answer 1

Nickel sulphide is an inorganic compound. It has the chemical formula of $NiS$ . It is produced by treating Nickel (II) salts with hydrogen sulphide ${H_2}S$ . Dimethylglyoxime (DMG) is a reagent used in analytical chemistry and is used for the identification of nickel.

Complete answer:
Nickel sulphide is an inorganic compound with the chemical formula $NiS$ . It is prepared by treating salts of Nickel (II) with hydrogen sulphide ${H_2}S$ . The color of the nickel sulphide is black. Its appearance is like a black solid.
Dimethylglyoxime (DMG) is a chemical compound present in the form of white-colored powder. Its chemical formula is ${C_4}{H_8}{N_2}{O_2}$ . It is soluble in sodium hydroxide solution and methanol but less soluble in water. It is used as a reagent in analytical chemistry. It is used to determine the presence of nickel.
When nickel (II) ion reacts with dimethylglyoxime, it leads to the formation of bright red-rose colored or we can say, cherry-colored nickel dimethylglyoxime complex and they react with an excess of $NaOH$ and bromine water then it forms a black precipitate of Nickelic hydroxide. Here, dimethylglyoxime acts as a chelating agent. The organic functional group that precipitates with the metal ion is facilitated by the chelating agent.
The reaction that occurs is as follows:
$N{i^{2 +}}\, + \,\,2\,{C_4}{H_8}{N_2}{O_2}\,\, \to Ni{({C_4}{H_7}{N_2}{O_2})_2}\,\,\,[red\,ppt.\,]\,\, + \,2{H^ + }$

Note:
Dimethylglyoxime is a common reagent in gravimetric analysis of nickel. It is also used as a precipitant for palladium. In the case of palladium, it forms a precipitate of a yellow colored compound after diluting it with hydrochloric acid $(HCl)$.