This dark blue to purple solid is a sulfuric acid salt of the metal complex [Cu(NH3)4(H2O)]2+ (tetraammineaquacopper(II) cation).

[2] The correct concentrations of ammonia and copper(II) sulfate solution needed to synthesize the complex can be determined by colorimetry.

The combination of the correct concentrations will produce the highest absorbance read out on the colorimeter and as a result the formula of the complex can be verified.

The characteristic deep blue colour of the tetraammine complex is found in brass and copper alloys where attack from ammonia has occurred, leading to cracking.

The problem was first found in ammunition cartridge cases when they were stored near animal waste, which produced trace amounts of ammonia.