Complex Copper Compounds

In this the new ion Cu(NH₃)₄^•• is formed, the salts of which are obtained by adding excess of ammonia to the solutions of the respective copper salts.

Of these salts the best known is the sulphate, Cu(NH₃)₄SO₄, which is easily obtained by adding ammonia to a concentrated solution of copper sulphate until the solution has again become quite clear, and then pouring a layer of alcohol on the top. The salt is only very slightly soluble in alcohol, and is deposited in well-formed, dark blue crystals as the alcohol slowly diffuses into the underlying liquid. In the same way various other salts of the same ion can be obtained. Solutions of these salts are used in combating certain parasites (mildew) on vines.

Copper forms other complex compounds of a similar colour on adding excess of alkali to cupric salts in presence of certain organic compounds, e.g. sugar or tartaric acid. In this case also a dark blue liquid is produced from which a salt of the same colour can be obtained. In these salts acids containing copper are present, the composition of which depends on that of the substance employed, and cannot be discussed here. In general the property of forming such compounds is found in the same organic hydroxyl compounds as hinder the precipitation of ferric oxide by bases.

Of such compounds the best known is Fehling's solution, which is obtained by adding tartaric acid and excess of caustic potash to a solution of copper sulphate. It is a dark blue liquid, which is changed by various reducing agents in such a manner that it deposits a precipitate of red cuprous oxide. It can serve, therefore, for the detection of such substances, and it is used for this purpose in analysis, e.g. for the detection of grape-sugar in urine.

In the cuprous series some complex copper compounds have been already mentioned. It has still to be remarked that cuprous cyanide dissolves in potassium cyanide to form a colourless liquid, from which the complex salt KCu(CN)₂, the potassium salt of cuprocyanidion, is obtained. This solution is very stable, and contains exceedingly little cuprion, so that all solid copper compounds, even copper sulphide, dissolve in potassium cyanide with formation of this complex salt. Advantage is also taken of this behaviour in analysis.

Copper also forms a number of complex compounds in which sulphur plays a part, and which are derived from sulphurous and thiosulphuric acids.