The Th-Acetate Chemical Equilibria: Is It Really That Simple?

Area of Science:

• Coordination Chemistry
• Analytical Chemistry
• Solution Chemistry

Background:

• The thorium-acetate chemical system was previously understood to involve complexes with one to five acetate ligands.
• Existing models did not account for hydrolyzed thorium acetate species.

Purpose of the Study:

• To redefine and revisit the thorium-acetate chemical system by investigating the formation of hydrolyzed species.
• To re-evaluate the formation constants of thorium acetate complexes.
• To determine the speciation of thorium in acetate solutions.

Main Methods:

• Coupling of capillary electrophoresis (CE) with inductively coupled plasma mass spectrometry (ICP-MS) for simultaneous analysis of kinetically stable and labile species.
• Evaluation of peak areas in CE-ICP-MS to study kinetically stable species.
• Analysis of variations in electrophoretic mobility to study labile species.

Main Results:

• The formation of hydrolyzed thorium acetate species, specifically [Th(OH)(AcO)_i]^3-i (i = 3,4), was discovered.
• [Th(AcO)₅]^- was not observed under the experimental conditions.
• The first Th⁴⁺ hydrolysis constant was determined (log*K⁰ = -2.7 ± 0.2), consistent with NEA recommendations.
• Formation constants (β_i, i = 1-4) for [Th(AcO)_i]^4-i complexes were determined at various ionic strengths and extrapolated to zero ionic strength using SIT.
• Formation constants for hydrolyzed thorium acetate complexes, [Th(OH)(AcO)_i]^3-i (i = 1-4), were calculated.

Conclusions:

• The Th/acetate system is more complex than previously thought, involving hydrolyzed species.
• CE-ICP-MS is a powerful technique for studying both kinetically stable and labile metal complexes simultaneously.
• This study provides a revised understanding of thorium speciation in acetate solutions and offers new thermodynamic data.