Sectroscopic study of stability and molecular species of 12-tungstophosphoric acid in aqueous solution

Abstract

The various molecular species of 12-tungstophosporic acid (WPA) in aqueous solutions of different pH values (from 1 to 11.5) were investigated by UV, IR, and NMR spectroscopy. The dependence of the attained equilibrium composition in solution on time, concentration of WPA, and type of buffer used was studied. Obtained results indicate that the buffer type and pH value greatly determine the equilibrium composition in the solution. The Keggin structure of the WPA is sustained only up to pH 1.5. With further increase in pH, the decomposition of Keggin anion does not lead directly to the monovacant lacunary anion. Between 1.5 and 2.0, the structures with 2 phosphorus atoms from the Dawson series are dominant as intermediate species. In the pH range 3.5–7.5, WPA is present in the form of the monovacant lacunary Keggin anion. These results are of special importance for the biomedical and catalytic applications of heteropoly compounds (HPCs) and for an improved understanding of the mechanism of their functioning.Key words: heteropolyacids of the Keggin structure, hydrostability, UV, IR and NMR spectroscopy.

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

“…Assuming that the presence of D 2 O and IPA do not greatly affect the pH, this is a 0.5 m solution of NaHSO 4 , which has a calculated pH of 1.13 using a p K a of 1.9 for hydrogen sulfate ion. At pH 1.5, PW 12 is no longer stable . In addition, at pH 1, it seems as though other POMs are present in solution, specifically the lacunary Wells–Dawson ions .…”

“…At pH 1.5, PW 12 is no longer stable . In addition, at pH 1, it seems as though other POMs are present in solution, specifically the lacunary Wells–Dawson ions . From the EXAFS spectrum, one can crudely estimate that ≈ 25 % of the Tc is present as Tc in a lacunary ion.…”

Large Magnetic Polyoxometalates Containing the Cobalt Cubane ‘[CoIIICo3II(OH)3(H2O)6–m(PW9O34)]3−' (m = 3 or 5) as a Subunit

“…We have also investigated the stability of aqueous solutions of POMs 2 , 3 , and 4 in 0.5 M NaOAc/HOAc buffer solution at pH 4.8 at room temperature (see Figure 5 ). The evolution of the UV-vis spectra of 2 and 3 clearly indicate that, similarly to 1 , a new peak develops in all cases within 24 h at 252 nm, suggesting that these POMs decompose also producing the mono-substituted [PCo(H 2 O)W 11 O 39 ] 5− as the major product (Jabbour et al, 2004 ; Holclajtner-Antunović et al, 2008 ). By comparison with the decomposition of 1 , these mono-substituted species should come from the decomposition of the corresponding subunit in 2 and 3 , which is topologically identical to 1 .…”

Effects of tungstate polymerization on tungsten(VI) adsorption on ferrihydrite

“…The difference in retention with time, therefore, likely reflects slow changes in W(VI) solution speciation which accordingly led to changes in adsorption behavior. The parent 12-W PWO is only stable at low pH (and high concentration), and decomposes to smaller POMs (and produces H + ) over time and eventually to monomeric tungstate and phosphate in basic solutions (Holclajtner-Antunovic et al, 2008;Zhu et al, 2003). PWO added in unbuffered solutions decreased pH over time (data are not shown), with changes between initial and final pH in 48 hour solutions more than changes in 2 hour solutions, which can be explained by the production of H + during depolymerization.…”