Effect of Temperature on the Protonation of the TALSPEAK Ligands: Lactic and Diethylenetrinitropentaacetic Acids

Abstract

The protonation reactions of two ligands that play important roles in the TALSPEAK process for the separation of trivalent actinides from lanthanides, lactic acid, and diethylenetrinitropentaacetic acid (DTPA), have been studied at variable temperatures. The protonation constants at 10–70°C were determined by titration potentiometry and the protonation enthalpies were determined at 25°C by titration microcalorimetry. The protonation constants remain essentially unchanged (25–70°C) within the experimental uncertainties, indicating that the effect of temperature on the protonation of lactate is insignificant. In contrast, the protonation constants of DTPA (logβ_H's) generally decrease as the temperature is increased. Results from this study indicate that the effect of temperature on the protonation of DTPA could alter the speciation of metal ions (actinides and lanthanides) in the TALSPEAK system, since lower values of logβ_H at higher temperatures suggest that the hydrogen ions would compete less strongly with the metal ions for the complexation of DTPA at higher temperatures.

Keywords:

• Diethylenetrinitropentaacetic acid
• lactic acid
• protonation
• temperature effect

ACKNOWLEDGEMENTS

This work was supported by the U.S. Department of Energy, Office of Nuclear Energy, the Fuel Research and Development Program, under Contract No. DE-AC02–05CH11231 at the Lawrence Berkeley National Laboratory.