Characterization of aqueous formulations of tetra- and pentavalent forms of vanadium in support of test article selection in toxicology studies.

Tetravalent (VIV) and pentavalent (VV) forms of vanadium were selected for testing by the National Toxicology Program via drinking water exposure due to potential human exposure. To aid in the test article selection, drinking water formulations (125-2000 mg/L) of vanadyl sulfate (VIV), sodium orthovanadate, and sodium metavanadate (VV) were characterized by ultraviolet/visible (UV/VIS) spectroscopy, mass spectrometry (MS), or 51V nuclear magnetic resonance (NMR) spectroscopy. Aqueous formulations of orthovanadate, metavanadate, and vanadyl sulfate in general were basic, neutral, and acidic, respectively. Changes in vanadium speciation were investigated by adjusting formulation pH to acidic, neutral, or basic. There was no visible difference in UV/VIS spectra of pentavalent forms. NMR and MS analyses showed that the predominant oxidovanadate species in both ortho- and metavanadate formulations at basic and acidic pH, respectively, were the monomer and decamer, while, a mixture of oxidovanadates were present at neutral pH. Oxidovanadate species were not observed in vanadyl sulfate formulations at acidic pH but were observed at basic pH suggesting conversion of VIV to VV. These data suggest that formulations of both ortho- and metavanadate form similar oxidovanadate species in acidic, neutral and basic pH and exist mainly in the VV form while vanadyl sulfate exists mainly as VIV in acidic pH. Therefore, the formulation stability overtime was investigated only for sodium metavanadate and vanadyl sulfate. Drinking water formulations (50 and 2000 mg/L) of metavanadate (~pH 7) and vanadyl sulfate (~pH 3.5) were ≥92 % of target concentration up to 42 days at ~5 °C and ambient temperature demonstrating the utility in toxicology studies.