Acid dissociation constants and rare earth stability constants for DTPA

Diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) is an octadentate aminopolycarboxylate complexing agent whose f-element complexes find important practical applications in nuclear medicine and in advanced nuclear fuel reprocessing. This investigation focuses primarily on the latter application, specifically on characterization of lanthanide-DTPA complexes of relevance to the Trivalent Actinide-Lanthanide Separations by Phosphorus reagent Extraction and Aqueous Komplexants (TALSPEAK) process. To function acceptably, the TALSPEAK process requires the presence of moderate concentrations (0.5-2.0 mol·L^-1) of a (Na⁺/H ⁺) lactate (or citrate) buffer. Competition between DTPA, lactate, and the extractant bis(2-ethylhexyl)phosphoric acid (HDEHP) for the lanthanides and trivalent actinides governs the course of the extraction process. To facilitate modeling and to support process improvements, the acid dissociation constants and stability constants for rare earth complexes with DTPA have been determined in 2.0 mol·L^-1 ionic strength (NaClO₄) media. The acid dissociation constants for DTPA and the stability constant for [Eu(DTPA)]^2- also were determined in sodium trifluoromethanesulfonate at 2.0 mol·L^-1 ionic strength to evaluate the potential impact of changing the nature of the electrolyte. The thermodynamic data are compared with earlier reports of similar data at lower ionic strength and used to complete calculations exploring the relative stability of lanthanide-DTPA and lactate complexes under TALSPEAK extraction conditions. Lanthanide-DTPA stability trends are discussed in comparison with literature data on a variety of other metal ions.