The elements may for example be used in the form of easily obtainable salts of the corresponding acids, for example NH4 VO3 or Na2 Cr2 O7 .sup.. 2 H2 O. Tetravalent and/or pentavalent vanadium or hexavalent chrome are active also in the presence of impurities of dilute sulfuric acid which have a reducing effect, such as organic components.