Homogeneous precipitation of thorium oxalate: Structural, kinetic, and morphological aspects

Thorium oxalate hexahydrate, Th(C₂O₄)₂·6H₂O was produced by homogeneous precipitation from a thorium nitrate solution through the decomposition of oxamic acid (NH₂COCOOH). Typically, lanthanide oxalates and actinide oxalates are prepared by heterogeneous precipitation using oxalic acid (COOH)₂. However, in the present homogeneous precipitation reaction, oxalic acid was slowly generated by the acid-catalyzed hydrolysis of oxamic acid. While heterogeneous precipitation is rapid and typically yields small microcrystals, the slow generation of the precipitation agent (oxalic acid) during homogeneous precipitation resulted in the formation of large thorium oxalate crystals with atypical morphology. The reaction exhibited first-order kinetics and was assessed at 443, 453 and 463 K (70, 80, 90 °C). The morphology of crystals obtained at these different temperatures were investigated. Additionally, the sensitivity of thorium oxalate hexahydrate to drying conditions and its decomposition during calcination to ThO₂ were examined. Thorium oxalate hexahydrate tends to lose crystalline water, resulting in transition phases toward the dihydrate when dried under vacuum at 313 K (40 °C). This loss of crystalline water was not observed when drying was performed under ambient conditions. The further decomposition of the oxalate dihydrate to ThO₂ followed the well-known decomposition path. The developed reaction is affordable, convenient, and does not require demanding apparatus, making it a versatile preparation route for various thorium oxalate crystals of variable morphology suitable for crystallographic studies or applications demanding powders with large particle sizes.