The thermal stabilities of thorium(IV) salts of ortho-, meta-and para-hydroxy-and aminobenzoic acids were studied. The salts were prepared as hydrated compounds with general formula Th(OH)2(R-C6H4COO)2.nHzO, were R = OH or NH2, and n = 2, 3 or 4, while the salt of 3-aminobenzoic acid was anhydrous. On heating, the salts undergo dehydration in two or three steps and di(R-benzoato)dihydroxothorium(IV) or di(2-hyroxybenzoato)oxothori,um(IV) is then transformed directly to ThO2.The temperatures of beginning of decomposition and ThO2 formation decrease with decreasing values of the Hammett constant 8 and yon Bakkum constant 8 t~.
The conditions of thermal decomposition of Th(IV) benzenedicarboxylates have been studied; On heating, thorium(IV) phthalate and isophthalate are dehydrated and the anhydrous complexes decompose to ThO2 with intermediate formation of a mixture of ThOCO 3 and carbon.Th(IV) terephthalate obtained at room temperature loses crystal water and then decomposes directly to ThO2, while the complex isolated from a hot solution on heating is first dehydrated, and the anhydrous complex is decarboxylated and then decomposed to ThO2 with the intermediate formation of ThOCO3. The activation energies of dehydration have been calculated for the Th(IV) benzenedicarboxylates.
The thermal decompositions of thorium(IV) trimesinate and trimellitate, prepared under various conditions, have been studied. On heating benzenetricarboxylates of Th(IV) decompose in many stages. First, the hydrated complexes lose crystallization water in one, two or four stages, forming anhydrous comtlexes which decompose to ThO 2 with the intermediate formation of ThOCO 3. The dehydrations of the complexes are associated with strong endothermic effects, and the decompositions of the anhydrous complexes are accompanied by exothermic effects.
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