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.