Formation and conversion of molybdophosphate and -arsenate complexes in aqueous solution

The difference between the formation and conversion processes of molybdophosphate and -arsenate complexes was elucidated in a 100 mM Mo(VI)-0.5 M HCl system as a function of the P(V) or As(V) concentration. In the Mo(VI)-P(V) system, the Keggin-type [PMo12O40](3-) complex was stable only at P(V) concentrations <7 mM; further addition of P(V) caused the [PMo12O40](3-) complex to decompose mainly into the A-[PMo9O34H6](3-) complex at an ambient temperature. The latter was subsequently converted into the Dawson-type [P2Mo18O62](6-) complex at an elevated temperature; the Dawson complex was kinetically stable even in the presence of excess of P(V). At As(V) concentrations <10 mM, in contrast, the corresponding Mo(VI)-As(V) system did not produce the Keggin-type [AsMo12O40](3-) complex but the previously unknown complex (tentatively formulated as [AsMo10O37H5](4-)) at an ambient temperature; with further addition of As(V), the [AsMo10O37H5](4-) complex was transformed directly into the [As2Mo6O26H2](4-) complex. The Dawson-type [As2Mo18O62](6-) complex, which was formed upon heating the system at 80 degrees C for 5 h, also underwent subsequent transformation into the [As2Mo6O26H2](4-) complex at As(V) concentrations >20 mM. (C) 1999 Elsevier Science S.A. All rights reserved.