Spectrophotometric investigation on the equilibration of monomeric forms of Mo(VI) in aqueous sulfuric acid

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

“…It is also clear from UVevisible spectroscopic comparisons that at equilibrium, 12-MPA in H 2 SO 4 is markedly more dissociated into its precursor molybdates than in HNO 3 [27] or HClO 4 [30], particularly at pH < 1.0, with the effect also induced by addition of sulfate or bisulfate [27]. Spectrophotometric measurements of dilute, monomeric Mo(VI) solutions have shown that an equilibrium system between molybdic acid and hydrogen sulfate does indeed exist, involving a 1:2 reaction between Mo(VI) and HSO 4 À (pK a ¼ 1.9) [133] to form a deprotonated product. Only the 1:2 complex was reported, although ESI-MS data have suggested that a number of other complexes also exist at various Mo(VI) concentrations [20,106].…”

Nature of Monomeric Molybdenum(VI) Cations in Acid Solutions Using Theoretical Calculations and Raman Spectroscopy

“…In strongly acidic solutions, W­(VI) undergoes condensation reactions to form polyoxoanions even when the total W­(VI) concentration, [W­(VI)] t , is <10 –6 M. − In stark contrast, at pH ≲2.5 and [Mo­(VI)] t ≲10 –4 M, uncharged molybdic acid, MoO 2 (OH) 2 (OH 2 ) 2 0 (often written as “H 2 MoO 4 0 ”), undergoes two protonations, forming successively two cations with stoichiometries H 3 MoO 4 + and H 4 MoO 4 2+ (the molybdenyl cation). − Separation of Mo­(VI) and W­(VI) can then be achieved easily using an appropriate ion-exchange resin. − However, optimization of this process requires a detailed and unambiguous understanding of the extraction mechanism. This, in turn, demands precise knowledge of the composition and structure of the dominant Mo­(VI) species in strongly acidic solutions.…”

“…Numerous investigations of the protonation equilibria of molybdic acid have been made over the last 60 years, using a variety of techniques such as UV–Vis spectrophotometry, ,, enthalpimetric titrimetry, and electrophoresis . These studies are in broad agreement and indicate that uncharged molybdic acid undergoes two protonations, with acidity constants p K a2 (H 3 MoO 4 + ) ≈ 0.85 to 1.06 and p K a1 (H 4 MoO 4 2+ ) ≈ −2.13.…”