Chemistry of Neptunium

Np + is in many ways the most important oxidation state. It is formed by reduction of the higher oxidation states, and by aerial oxidation of Np +. Strong oxidizing agents like Ce + oxidize it back to [Np02], whilst electrolytic reduction of Np + affords Np +, which is stable in the absence of air (unlike U). 

As neptunium has one more outer-shell electron than uranium, it has the possibility of a (4-7) oxidation state, a possibility realized in alkaline solution, when ozone will oxidize Np to Np , an oxidation also achieved by Xe03 or 104 at higher temperatures. The potential for this is estimated as -1.24V (IM alkali). The relevant standard potentials for neptunium (IM acid) are  

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Aqueous coordination chemistry of neptunium. S. K. Patil, V, V. Ramakrishnan andM. V. Rama-niah, Coord. Chem. Rev., 1978, 25, 133-171 (216). 

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

Cohen, D. Fried, S., "Some Observations on the Chemistry of Neptunium in Basic Solution. Inorg. Nucl. Chem. Letters 1969,5,653-663. 

LaChapette, T. J., L. B. Magnusson, and J. C. Hindman The Chemistry of Neptunium. First Preparation and Solubilities of some Neptunium Compounds in Aqueous Solution. In G. T. Seaborg, J. J. Katz, and W. M. Manning (Eds.), The Transuranium Elements, National Nuclear Energy Series, Div. IV, Vol. 14B, p. 1097. New York McGraw-Hill 1949. 

Very recent results are included, for instance, the chemistry of neptunium and plutonium, cyclooctate-traene, tropolone, the theory of colour, etc, and the coordination of data from many chemical fields often throws new light on well-known facts. This is a hook which will well repay the reader s effort be he student, teacher or practising chemist—by a new insight into many aspects of chemistry. 

V. A. Mikhailov, Analytical Chemistry of Neptunium, Halsted Pres, New York, 1973... 

A.A. Lychev, L.G. Mashirov, Yu.I. Smolin, Yu.F. Shepelev, Abstracts on Third All-Union Conf. on Chemistry of Neptunium and Plutonium, Nov.24-26 1987, Leningrad, USSR, p.98. 

Initial studies of the chemistry of neptunium and plutonium actually preceded the official establishment of the Manhattan Project. But as soon as the project got underway, they became the subject of intensive investigation at several of the Manhattan Project laboratories (Seaborg and Katz 1954). Both elements turned out to have four major oxidation states -F3, -F4, -F5, -F6, similar to uranium, hut plutonium is unique in that these four states can all exist simultaneously in aqueous solution. Microchemical techniques were applied to prepare and study microgram quantities, such as the first weighahle sample of a man-made element, 2.77 ig Pu02, in September 1942 (Cunningham and Wemer 1949). At the Los Alamos Laboratory, chemists and metallurgists learned to produce metallic plutonium and studied its complex properties, which eventually turned out to involve no less than six allotropic phases, more than any other element. 

A new research field in the chemistry of neptunium, plutonium, and other light actinides developed in the late 1970s in the context of environmental contaminations and nuclear waste management. The aim was to unravel their behavior in natural environments (see, e.g., Watters et al. 1983 Kim 1986). This is a difficult task because of the complex chemistry many oxidation states, polymerization, interaction with less defined natural partners such as humic acids, clays, and various rocks play a role. Included is also the development of techniques for the determination of actinide elements and their speciation in extremely low concentrations. 

MO2 ions are formed by the four elements from uranium to ameridum. For uranium, the hexavalent oxidation state is the most stable one. Though easily reduced, it is also prominent in the chemistries of neptunium and plutonium. Ameridum(vi) is a very strong oxidizing agent. As the ameridum isotopes available are quite radioactive, a steady radiation-induced reduction of AmO occurs in aqueous solution, as soon as a strongly oxidizing system is not present [19]. 

Neptunium. Np is in a class with Pa no efforts have been made to use it as a fuel solute, but consideration has been given to its formation in and removal from blanket solutions of [30a]. The chemistry of neptunium has been reviewed by Hindman et al. [30b], and the hydrolytic behavior has been reviewed by Kraus [30c]. Continuous separation of Np239 would provide a Pu product of high purity by radioactive decay, whereas plutonium recovered from long-term irradiation of usually contains appreciable amounts of Pu °. Spectrophotometric cells for use at elevated temperatures and pre.ssures in the study of the chemistry of neptunium (and other materials) have recently been developed by Wag-gener [30d] and have been used to measure the absorption spectra of dilute neptunium perchlorate in its six-, five-, four-, and three-valence states, using heavy w ater as the solvent. Dilute solutions of neptunyl nitrate in nitric acid have been so studied at temperatures up to 250°C the pentavalent state was found to be stable under the test conditions [30e]. 

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