Why is Cu2 colored

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Copper group

The copper group includes:,,,,.

In contrast to, when heated,,, and dissolve in 20% strength (filtrate I), the dissolution being based on an oxidation of the sulphidic sulfur to or and an associated shift in equilibrium.


At, the oxidation reaction fails because the solubility product of () is so extremely small that the sulfide ion concentration is no longer sufficient to reduce the nitrogen in the nitric acid. The only residue left is. To dissolve aqua regia (1: 3 mixture of concentrated and concentrated), complex formation of too takes place in addition to -oxidation. When this is dissolved, the aqua regia is smoked and the residue is taken up in a diluted form. With tin (II) solution you reduce to what turns white with the chloride as calomel. The addition of ammonia disproportionates the calomel and the change in color to black is observed, caused by the elemental mercury.

Filtrate I is mixed with sulfuric acid and evaporated. The residue is taken up with water and a little dilute sulfuric acid (filtrate II). Lead sulfate remains as a poorly soluble residue. This is dissolved again in ammoniacal tartrate solution and the lead is clearly identified by further evidence.

The sulfuric acid filtrate II is mixed with ammonia, in the course of which bismuth hydroxide precipitates, which is determined in more detail by reduction with stannite solution to form elemental bismuth. Any cadmium and copper hydroxide that may have formed briefly in excess dissolve again with complex formation (filtrate III). The resulting amine complex of the copper is colored intensely blue and thus serves as a positive proof of copper. The alkaline blue solution is added until the solution is colorless. In the meantime, it is possible that the salts or precipitate form, which, however, form the soluble cyano complexes and when further cyanide is added. The copper complex is so stable that when gas is reintroduced into the alkaline solution, it only turns yellow.

Tab. 1
Individual evidence for the copper group
cationPossible detection reactions
Ed2+ • Amalgam sample
Pb2+ • Precipitation as yellow • Precipitation as white
Bi3+ • Reduction to black, metallic bismuth • Detection with diacetyldioxime
Cu2+ • as a blue tetrammine copper (II) complex
CD2+ • precipitation as yellow