Neptunium

Neptunium has been characterized from the +3 to +7 oxidation states in aqueous solution. The standard potentials for various Np ions have been determined from measured formal potentials of the various redox couples. These data have been thoroughly reviewed by Martinet [94] and Fahey [95]. Recently the standard potentials for the redox couples Np02 /Np02, Np +/Np +, and Np02 /Np in acidic aqueous solution have been reevaluated with more detailed consideration of activity coefficients [49,50]. The standard potential accepted here for the Np02 /Np02 couple is 1.161 0.011 V as determined from 

Kihara et al. employed flow coulometry to study the electrode reactions for Np ions in various acidic media [49]. Flow coulometry has an inherent advantage over the conventional hulk coulometry methods in that the electrolysis can be achieved rapidly to aid in the characterization of unstable electrode products. The resulting coulopo-tentiograms for the Np02 /Np02 and Np /Np couples indicate reversible processes in nitric, perchloric, and sulfuric acids. The differences in potentials between the various acids are attributed to the associated stability constants of the electrode products with the anion of the acid in each case. Table 2 contains the half-wave potentials for each couple in the various acids. 

The electrochemical behavior of Np ions in basic aqueous solutions has been studied by several different groups. In a recent study, cyclic voltammetry experiments were performed in alkali ([OH ] = 0.9 — 6.5 M) and mixed hydroxo-carbonate solutions to determine the redox potentials of Np(V, VI, VII) complexes [97]. As shown in Fig. 2, in 3.1 M LiOH at a Pt electrode Np(VI) displays electrode processes associated with the Np(VI)/Np(V) and Np(VII)/Np(VI) couples, in addition to a single cathodic peak corresponding to the reduction of Np(V) to Np(IV). This latter process at Ep —400 mV (versus Hg/HgO/1 M NaOH) is chemically irreversible in this medium. Analysis of the voltammetric data revealed an electrochemically reversibleNp(VI)/Np(V) 

8 M LiOH, but below this value the Np(VI)/Np(V) couple tends toward a system with reduced electrochemical reversibility. Voltammetric behavior in NaOH solutions is very similar to the voltammograms in LiOH, with a shift in potential for the Np(VI)/Np(V) couple to = 0.106(6) V versus SHE in 3 M NaOH. In the mixed hydroxo-carbonate solutions (0.8 M NaOH/0.4M Na2CO3 and 1.8 M NaOH/0.1 M Na2CO3) the Np(VII)/Np(VI) becomes chemically irreversible and the Np(VI)/Np(V) couple is quasi-reversible. This behavior is 

Neptunium is usually formed in nuclear reactors. However, there are some instances of neptunium occurring naturally such as at Oklo in Gabon, Africa. 

At Oklo, the uranium deposits behaved as natural fission reactors in the Precambrian. 

SYMBOL Np PERIOD 7 SERIES NAME Actinide ATOMIC NO 93 

ATOMIC MASS 237.0482 amu VALENCE 3, 4, 5, and 6 OXIDATION STATES +3, +4, +5, and +6 NATURAL STATE Solid 

ISOTOPES There are a total of 23 isotopes of neptunium. None are stable. All are radioactive with half-lives ranging from two microseconds to 2.144xl0+ years for the isotope Np-237, which spontaneously fissions through alpha decay. 

Energy Levels/Shells/Electrons Orbitals/Electrons 

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. 

These have the same structures as the uranium analogues, except that a second form of NpBrs has the PuBr3 structure. Like UFe and PuFs, NpFe forms a volatile, toxic vapour (bp 55.2 °C). Oxyhalides NpOFs, NPO2F2, NpOp4, NpOCl, and NpOI have been described. 

MOST COMMON IONS Np , Np, Or NpOg, NpOa, NpOs  

Neptunium was discovered by the U.S. physicists Edwin M. McMillan and Philip Abelson, in 1940, via the bombardment of with neutrons. The name of the element is related to the planet Neptune. Neptunium-237 occurs as a product of fission, and appears in uranium fuel elements. 

Neptunium is used to produce plutonium ( Pu), via the irradiation of Np02 with neutrons. The isotope %4Pu is used as a power source for satellites. 

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The new elements neptunium and plutonium have been produced in quantity by neutron bombardment of uranium. Subsequently many isotopes have been obtained by transmutation and synthetic isotopes of elements such as Ac and Pa are more easily obtained than the naturally occurring species. Synthetic species of lighter elements, e.g. Tc and Pm are also prepared. 

Initially, the only means of obtaining elements higher than uranium was by a-particle bombardment of uranium in the cyclotron, and it was by this means that the first, exceedingly minute amounts of neptunium and plutonium were obtained. The separation of these elements from other products and from uranium was difficult methods were devised involving co-precipitation of the minute amounts of their salts on a larger amount of a precipitate with a similar crystal structure (the carrier ). The properties were studied, using quantities of the order of 10 g in volumes of... 

Planet pluto) Plutonium was the second transuranium element of the actinide series to be discovered. The isotope 238pu was produced in 1940 by Seaborg, McMillan, Kennedy, and Wahl by deuteron bombardment of uranium in the 60-inch cyclotron at Berkeley, California. Plutonium also exists in trace quantities in naturally occurring uranium ores. It is formed in much the same manner as neptunium, by irradiation of natural uranium with the neutrons which are present. 

Its importance depends on the nuclear property of being readily fissionable with neutrons and its availability in quantity. The world s nuclear-power reactors are now producing about 20,000 kg of plutonium/yr. By 1982 it was estimated that about 300,000 kg had accumulated. The various nuclear applications of plutonium are well known. 238Pu has been used in the Apollo lunar missions to power seismic and other equipment on the lunar surface. As with neptunium and uranium, plutonium metal can be prepared by reduction of the trifluoride with alkaline-earth metals. 

Because of the high rate of emission of alpha particles and the element being specifically absorbed on bone the surface and collected in the liver, plutonium, as well as all of the other transuranium elements except neptunium, are radiological poisons and must be handled with very special equipment and precautions. Plutonium is a very dangerous radiological hazard. Precautions must also be taken to prevent the unintentional formulation of a critical mass. Plutonium in liquid solution is more likely to become critical than solid plutonium. The shape of the mass must also be considered where criticality is concerned. 

A rather more specific mechanism of microbial immobilization of metal ions is represented by the accumulation of uranium as an extracellular precipitate of hydrogen uranyl phosphate by a Citrobacter species (83). Staggering amounts of uranium can be precipitated more than 900% of the bacterial dry weight Recent work has shown that even elements that do not readily form insoluble phosphates, such as nickel and neptunium, may be incorporated into the uranyl phosphate crystallites (84). The precipitation is driven by the production of phosphate ions at the cell surface by an external phosphatase. 

Each of the elements has a number of isotopes (2,4), all radioactive and some of which can be obtained in isotopicaHy pure form. More than 200 in number and mosdy synthetic in origin, they are produced by neutron or charged-particle induced transmutations (2,4). The known radioactive isotopes are distributed among the 15 elements approximately as follows actinium and thorium, 25 each protactinium, 20 uranium, neptunium, plutonium, americium, curium, californium, einsteinium, and fermium, 15 each herkelium, mendelevium, nobehum, and lawrencium, 10 each. There is frequently a need for values to be assigned for the atomic weights of the actinide elements. Any precise experimental work would require a value for the isotope or isotopic mixture being used, but where there is a purely formal demand for atomic weights, mass numbers that are chosen on the basis of half-life and availabiUty have customarily been used. A Hst of these is provided in Table 1. 

Kilogram amounts of neptunium ( Np) have been isolated as a by-product of the large-scale synthesis of plutonium in nuclear reactors that utilise 235u and 238u as fuel. The following transmutations occur ... 

The wastes from uranium and plutonium processing of the reactor fuel usually contain the neptunium. Precipitation, solvent extraction, ion exchange, and volatihty procedures (see Diffusion separation methods) can be used to isolate and purify the neptunium. 

In general, the absorption bands of the actinide ions are some ten times more intense than those of the lanthanide ions. Fluorescence, for example, is observed in the trichlorides of uranium, neptunium, americium, and curium, diluted with lanthanum chloride (15). 

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