Some salts although containing two different metallic elements give test for one of them in solution. Such salts are:

Complex compounds or complex salts containing two different metallic elements give test for only one element. For example, potassium hexacyanoferrate (II), K4[Fe(CN)6] gives test only for K+ ions and not for Fe2+ ions.K4FeCN6⇌4K++FeCN64−