In weakly alkaline medium copper(II) ions react with cuprizone forming a blue colour complex.
Strongly acidic and buffered solutions have to be adjusted to pH 9 with ammonia for the test. Iron(II), chromium(VI), nickel and manganese ions interfere in concentrations above 10 mg/l. Cr(III) concentrations which exceed the copper concentration cause low results, but can be eliminated by oxidation to chromate using ammonium peroxodisulphate / sulphuric acid). Cobalt ions form a red colour complex which, depending on the copper concentration, can already interfere above 1 mg/l. Cyanides and sulphides present in excess of 1 mg/l cause low results.

In water copper(II) can be found in dissolved as well as in undissolved form. Copper(I) compounds and undissolved copper(II) compounds are not determined unless they are decomposed with concentrated nitric acid prior to the test.