4-Acylpyrazolones like 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP), 1-phenyl-3- methyl-4-p-nitrobenzoyl-5-pyrazolone (PMNP) and 1-phenyl-3-methyl-4-(3, 5 dinitrobenzoyl)- 5-pyrazolone (PMDP) have been synthesized and developed as gravimetric reagents for the determination of U (VI) and Th (IV). Uranium (VI) is almost quantitatively precipitated with PMBP, PMNP, and PMDP at pH 2.20, 1.85 and 1.70 respectively. The pH values for the ...