Third-Phase Formation in the Extraction of Phosphotungstic Acid by TBP in n-Octane

Abstract

The solvent extraction of 12-phosphotungstic acid, also known as 12-tungstophosphoric acid—H₃PW₁₂O₄₀, the so-called Keggin heteropolyacid—by 0.73 M (20%v/v) tri-n-butyl phosphate (TBP) in n-octane under conditions comparable to those used previously for the extraction of conventional inorganic mineral acids is described. A simplified phase diagram for the pentanary system comprised of H₃PW₁₂O₄₀, HNO₃, H₂O, TBP, and n-octane reveals an extremely low initial concentration of H₃PW₁₂O₄₀ (1.1 mM) at the LOC (limiting organic concentration) condition, far lower than the most effective third-phase-forming inorganic acid, namely HClO₄. The results from small-angle neutron scattering (SANS) indicate that the interparticle attraction energy—U(r) calculated through application of the Baxter sticky sphere model to the SANS data at the LOC condition—does not approach the −2 k _B T value associated with phase splitting in previous studies of TBP third-phase formation. The third-phase formation model based on attractive interactions between polar cores of reverse micelles, successfully developed for TBP and other extraction systems does not apply to the extraction of H₃PW₁₂O₄₀. Rather, the separation of a third-phase from the TBP organic phase stems from the limited solubility of the heavy and highly polar H₃PW₁₂O₄₀–TBP species in the alkane diluent.

Keywords:

• Baxter model
• heteropolyacid
• Keggin ion
• liquid-liquid extraction
• polyoxometalate
• small-angle neutron scattering

ACKNOWLEDGEMENTS

We thank Paul Rickert and Denis Wozniak for assistance with the NMR and SANS measurements. This work was supported by the U.S. Department of Energy, Office of Basic Energy Science, Division of Chemical Sciences, Biosciences and Geosciences, under contract No. DE-AC02-06CH11357.

Notes

^a In 0.984 M HNO₃.

^b Uncertainty intervals: ±20% for [HPT], ±5% for [HNO₃], ±10% for [H₂O], and ±5% for TBP.

^a Incoherent scattering; uncertainty ±1%.

^b Hard sphere diameter; uncertainty ±10%.

^c From S(Q) vs. Q plots like Fig. 5; uncertainty ±10%.

^d Polar core diameter; uncertainty ±10%.

^e Corona thickness; uncertainty ±10%.

^f Weight-average aggregation number; uncertainty ±10%.

^g Stickiness parameter; uncertainty ±5%.

^h Energy of attraction between particles; uncertainty ±5%.

^§Student from INSA, Institut National des Sciences Appliquées de Rouen, France.