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THE STUDY OF COMPLEX FORMATION BETWEEN URANIUM(VI) AND 5-BROMO-PADAP 3.1 Introduction 3.2 pH Dependence of Complex Formation 3.3 The Composition of the Uranyl Complex at pH 4.1 3.4 The Effect of Fluoride Upon Complex Formation 3.5 pH Dependence of Complex Formation in the Presence of Fluoride 3.6 The Composition of the Complex at pH 7. 6 3.7 The Nature of the Complexes formed at pH 4.1 and pH 7.6 3.8 The Determination of Stability Constants 3.9 The Ratio of Peak Heights 3.10 Rates of Complex Formation 3.11 Summary THE NATURE OF THE SPECTRA OF PADAP DERIVATIVES AND THEIR COMPLEXES AND THE ANALYTICAL POTENTIAL OF THESE DYES 4.1 Introduction 4.2 Qualitative Observations 4.3 The Analytical Significance of Metal Complex Spectra 4.4 The Influence of Substituents on the Spectra of the Uncomplexed Dyes

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fidence level for a sample to be significantly different from a blank, it must contain urea equal to twice the standard deviation. The lower limit of detection of the analysis in the estimated relative standard deviation is 10% at a urea nitrogen concentration of 1 . O pg-atom per liter.Similarly, from the munonia w~a t e r data, for a sample to be significantly different from the blank at the 95% confidence level, it must contain at least 0.2 pg-atom ammonia nitrogen per liter. The relative standard deviation is 20% at the ammonia nitrogen concentration of 0.5 pg-atom per liter.Ship Systems Command ocean science program, Work was taken in part from a thesis by the author submitted in partial fulfillment of the M.A. degree in oceanography at The Johns Hopkins University. The opinions and assertions of the writer are his alone and do not necessarily reflect those of the Navy or the Naval establishment at large. seawater is thus 0.2 m a t o m urea nitrogen per liter and 1969. The principal research was sponsored by the Naval A RECENT REVIEW (1) showed the wide application and rapid growth in popularity of heterocyclic azo dyestuffs in analytical chemistry. Some dyes of this class have proved particularly useful for the spectrophotometric determination of uranium. The first such dye, 1-(2-pyridylazo)-2-naphthol (PAN) was superseded by the water-soluble, and more sensitive reagent, 4-(2-pyridylazo) resorcinol (PAR) (2, 3). At pH 8 uranium (VI) forms an intense red color with PAR, the complex having a molar absorptivity of 3.87 X lo4 at 530 nm (4). Florence and Farrar ( 4 ) developed a mixed complexing solution containing (1,2-cyclohexylene-dinitrilo) tetra-acetic acid (CyDTA) sulfosalicylate, and fluoride which effectively masks most metals, including thorium, and prevents their interference in the PAR method for uranium. This method has been in use in our laboratories for over six years, and has proved reliable, accurate, and versatile.We have now synthesized the diethylaminophenol analog of PAR, 2-(2-pyridylazo)-5-diethylaminophenol (PADAP). This new reagent is almost twice as sensitive towards uranium (VI) as PAR, with a molar absorptivity of 7.61 X lo4 at pH 8.2 and 564 nm. In the presence of the mixed complexing solution it is also even more selective than PAR, and because the color is developed in an acetone-water mixture, uranium can be determined directly in methyl iso-butyl ketone (MIBK) extracts, By a simple preliminary extraction of uranium into MIBK from a calcium nitrate-EDTA medium, the PADAP method can be made specific for uranium. EXPERIMENTALReagents. Synthesis of PADAP (zinc complex). Sodium amide (2.1 grams, as a 50% suspension in toluene) was introduced into a dry 250-ml flask fitted with a reflux condenser.Dry ethyl ether (20 ml) and 2-aminopyridine (5 grams in 5 ml ether) were added while nitrogen was passed through the flask. The mixture was gently refluxed under nitrogen for 30 min, and iso-amyl nitrite (5 ml) added. Refluxing was continued for another hour. The precipitated sodium 2-pyridyldi...

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