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Thermodynamic Study of the Complexation of Protactinium(V) with Diethylenetriaminepentaacetic Acid
The
complex formation of protactiniumĀ(V) with DTPA was studied at different
temperatures (25ā50 Ā°C) and ionic strengths (0.1ā1
M) with the element at tracer scale. Irrespective of the temperature
and ionic strength studied, only one neutral complex with (1:1) stoichiometry
was identified from solvent extraction and capillary electrophoresis
coupled to ICP-MS (CE-ICP-MS) experiments. Density Functional Theory
(DFT) calculations revealed that two complexes can be considered:
PaĀ(DTPA) and PaOĀ(H<sub>2</sub>DTPA). The associated formation constants
were determined from solvent extraction data at different ionic strengths
and temperatures and then extrapolated to zero ionic strength by SIT
methodology. These constants are valid, regardless of complex form,
PaĀ(DTPA) or PaOĀ(H<sub>2</sub>DTPA). The standard thermodynamic data
determined with these extrapolated constants revealed a very stable
complex formed energetically by an endothermic contribution which
is counter balanced by a strong entropic contribution. Both, the positive
enthalpy and entropy energy terms suggest the formation of an inner
sphere complex