Ion Interaction Models and Measurements of Eu³⁺ Complexation: HEDTA in Aqueous Solutions at 25 °C Containing 1:1 Na⁺ Salts and Citrate pH Buffer

In the TALSPEAK liquid–liquid distribution process, dissolved lanthanides can be separated from actinides using a complexing agent such as N-(2-hydroxyethyl)­ethylenediamine-N,N′,N′-triacetic acid (HEDTA, CAS Reg. No. 150-39-0) in a low pH buffered aqueous phase in contact with an organic phase containing a suitable extractant. This study focuses on the chemical speciation of HEDTA, citrate pH buffer, and Eu³⁺ in aqueous solutions of 1:1 Na⁺ salts (mainly NaNO₃) as a function of ionic strength and pH. New measurements of stoichiometric protonation constants of HEDTA, and the HEDTA complex of Eu³⁺, in aqueous NaNO₃ are reported for ionic strengths from 0.5 to 4.0 M at 25 °C. A Pitzer activity coefficient model of the aqueous mixture has been developed based upon these measurements, available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the HEDTA and buffer speciation, and complexation of Eu³⁺ by both HEDTA and citrate, to be calculated for different solution compositions and pH values. The model of the citrate buffer, which is based on an extensive range of data for NaCl and NaNO₃ media, should also be useful in other practical applications.