Abstract: | What is “Molybdic Acid” or “Polymolybdic Acid”? According to a comparative study of the literature, supplemented by well-aimed experimental investigations and equilibrium calculations, the terms “molybdic acid” or “polymolybdic acid”, used for many substances, species, or solutions in the literature, are applicable to a species, a solution, and two solids: - a) The monomeric molybdic acid, most probably having the formula MoO2(OH)2(H2O)2(? H2MoO4, aq), exists in (aqueous) solution only and never exceeds a concentration of ≈ 10?3 M since at higher concentrations it reacts with other monomemeric molybdenum (VI) species to give anionic or cationic polymers.
- b) A concentrated (>0.1 M MoVI) aqueous molybdate solution of degree of acidification P = 2 (realized, e. g., by a solution of one of the MoVI oxides; by any molybdate solutions whose cations have been exchanged by H3O+ on a cation exchanger; by suitable acidification of a molybdate solution) contains 8 H3O+ and the well-known polyanion Mo36O112(H2O)168? exactly in the stoichiometric proportions.
- c) A glassy substance, obtained from an alkali metal salt-free solution prepared according to (b), refers to the compound (H3O)8Mo36O112(H2O)16]·xH2O, x = 25—29.
- d) A solid having the ideal composition (H3O)Mo5O15(OH)H2O·H2O]∞ consists of a polymolybdate skeleton (the well-known ?decamolybdate”? structure), in the tunnels of which H3O+ and H2O are intercalate. The structure is very unstable if only H3O+ cations are present, but it is enormously stabilized by a partial exchange of H3O+ by certain alkali or alkaline earth metal cations.
For the compounds MoO3, MoO3·H2O, and MoO3·2H2O the term ?molybdic acid”? is unjustified. The commercial product ?molybdic acid, ≈85% MoO3”? is the well-known polymolybdate (NH4)2O·4 MoO3 with a layer structure of the polyanion. |