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ABSTRACT: Through a combination of Raman spectroscopy, multi-element NMR spectroscopy and chemical analysis, the differences between the action of carbonate and carbamate as agents for dissolving Cs3PMo12O40⋅xH2O(s) (CPM) and ZrMO2O7(OH)2(H2O)2(s) (ZM) have been elucidated. Alkaline H2NCO−2/HCO3−/CO32− solutions, derived from the dissolution of ammonium carbamate (NH4H2NCO2; AC), dissolve CPM by base hydrolysis of the PMo12O403− Keggin anion, ultimately forming [MoO4]2− and PO43− when excess base is present. If the initial concentration of H2NCO2−/HCO3−/ CO32− is lowered, base hydrolysis is incomplete and the dissolved species include [Mo7O24]6− and [P2Mo5O23]6−, and undissolved solid Cs3PMo12O40, CsxNH7−xPMo11O39, and CsxNH6−xMo7O24 remain. Na2CO3 solutions dissolve Cs3PMo12O40 through a similar mechanism, but the dissolution rate is much lower. We attribute this difference to the different buffering effects of H2NCO2−/HCO3−/CO32− and CO32−/HCO3− solutions, and the instability of carbamic acid, the protonated form of H2NCO2− (which rapidly decomposes into NH3 and CO2). The ability of NH3 to produce NH4+ and OH−, together with the evolution of CO2 gas, drive the reaction forward. Low temperature measurements under conditions where pure H2NCO2− is kinetically stable, allowed the rates of dissolution of CPM by H2NCO2− and CO32− to be compared directly, confirming the faster dissolution by H2NCO2−. Compared to CPM, the dissolution of ZM by H2NCO2−/HCO3−/CO32− is a much slower process and is driven by the formation of soluble ZrIV-carbonate complexes and MoO42−. The driving force for the dissolution of ZM is the superior complexing ability of carbonate over carbamate; consequently solutions containing a higher carbonate concentration dissolve ZM faster.
Journal of Solution Chemistry 04/2005; 34(4-34):443-468. · 1.41 Impact Factor
