The Oxidation of Metals with Liebig Acids

In Liebig's definition, an acid is a compound which contains one or more hydrogen atoms which may be substituted by metal atoms. Hence, reactions of Liebig acids in substance, excluding water or any other solvent, with non‐noble metals yield salts and release hydrogen. In this sense, not only the classical mineral acids such as sulfuric or nitric acid, respectively, are Liebig acids. Rather, there is a large variety of organic compounds with, for example, HO‐ or HN‐functions with acid constants that allow for substitution of the hydrogen atoms by a metal atom. Simple covalent hydrides like water and ammonia or even methane may also act as Liebig acids with conditions properly chosen. The ammonium ion, (NH₄)⁺, represents a special case as it is available in a large variety of salts and may react as an acid/oxidant or as a (base)/reductant and is also a pseudo alkali‐metal cation. The versatility of the ammonium ion is reviewed with special emphasis to its ability to function as a Liebig acid, i.e., reactions of, especially, ammonium halides with non‐noble metals.