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Abstract
Titanium chemistry in aqueous acidic media has been extensively investigated over the last decades. Hydrolyzed species such as Ti(OH)3+, TiO2+, Ti(OH)22+ or Ti(OH)3+ have been identified and their equilibria have been studied in nitric and perchloric acid. A predominance of the divalent cations was found for low pH (i.e., pH <2). Nonetheless, recent literature reports the existence of small titanium oxo-clusters in aqueous acidic media for large titanium(iv) concentration (typically., >0.1 mol L−1), as stable precursors for the formation of condensed titanium dioxide. The present paper reconsiders firstly previous knowledge about the speciation of titanium(iv) in non-complexing acidic media by giving evidence for the presence of polynuclear hydrolyzed species, even at very low Ti(iv) concentration (i.e., typically <0.1 mmol L−1). UV-visible absorbance spectra recorded for diluted nitric acid solutions (a model of non-complexing acidic medium) containing titanium(iv) were compared to time-dependent density functional theory (TD-DFT) predicted excitation energies. Experimental and predicted maximal absorbance wavelengths showed significantly improved matches when polynuclear species were considered in TD-DFT calculation. Then, 0.1–12.7 mol L−1 phosphoric acid solutions containing titanium(iv) were studied by means of spectroscopic techniques (UV-visible, NMR) in order to identify qualitatively the presence of titanium(iv) complexes and to link this speciation to the acid concentration. Two different titanium(iv) orthophosphate complexes, potentially polynuclear, were detected, and the presence of free titanium(iv) is also expected for low phosphoric acid concentration (i.e., <0.1 mol L−1). A general complexation scheme for a large range of H3PO4 concentration was thus formulated.
Highlights
The wet phosphoric process produces different grades of phosphoric acid and phosphate salts that can be used in the agriculture sector, in food, or in the pharmaceutical or electronic industries
The speciation of Ti(IV) was investigated at its turn in a large range of concentration of phosphoric acid by 31P NMR spectroscopy and UV-visible spectroscopy coupled with chemometric analyses
The use of UV-visible spectroscopy and molecular modeling techniques was found to be relevant to reinvestigate the speciation of titanium in non-complexing acidic solutions
Summary
The wet phosphoric process produces different grades of phosphoric acid (known as “wet” phosphoric acid by opposition to H3PO4 produced by pyrometallurgy) and phosphate salts that can be used in the agriculture sector (fertilizers), in food, or in the pharmaceutical or electronic industries. This process is based on the leaching of the phosphate rock with concentrated sulfuric acid and a series of puri cation steps to reach the purity speci cation requested by the target application.[1,2,3] The puri cation steps aim at removing inorganic anions such as uorides, sulfates, chlorides and metal impurities including. The divalent titanium(IV) aquocation was generally found to be predominant in perchloric or nitric acid solutions in the absence of any complexing agents.[13,16,18] The exact structure of this divalent cation is still debated as it was initially identi ed as a titanium oxocation (titanyl TiO2+)[20] whereas more recent studies suggested an intramolecular rearrangement leading to
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