A series of heterocyclic dyestuffs having the same basic ligand structure but varying in their substituents on the 5-positions of both the heterocyclic and benzene rings has been prepared. The dissociation constants of three of these compounds have been determined and the pK 0 H(ortho) found to be of the same order as those observed for PAR and its derivatives, while pKNH values for the diethylamino and pyridyl nitrogens are somewhat lower than in related dyes. Complex formation between uranium

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... (VI) and 5-Bromo-PADAP has been thoroughly studied. Two different complexes have been shown to form; one, from pH 2 to pH 5 which is a 1:1 neutral complex exhibiting a double peak in its spectrum and thought to be a hydroxy bridged complex, the other a terniary fluoro complex 1:1:1 forming between pH 5.5 and pH 8 also neutral and exhibiting the same double peak character. The constancy