Neptunium VI

Kato etal. (1996) studied the solubihty of Np03-H20(s) at 25 C and in a medium of acidic 0.1 moll NaClO. Lemire et al. (2001) recalculated the data of Kato et al. and obtained a solubility constant of 

Moskvin (1971) studied the hydrolysis of neptunium(VI) and proposed the formation of the following species Np020H, Np02(0H)2(aq), Np02(0H)3,  

The species and their associated stability constants are consistent with those of uranium(VI) and are retained in the present review. Schmidt et al. (1980) used a pulse radiolysis transient conductivity technique to study the hydrolysis of neptu-nium(VI) in a self-medium (very low ionic strength). They could only determine a stability constant for Np020H and obtained a stability constant of between log Pi = -5. 4 to —5.5. This value is considered consistent with that obtained by Lemire et al. (2001) at zero ionic strength from the earlier data of Cassol et al. (1972a). 

Very recently, Gaona, Fellhauer and Altmaier (2013) studied the solubility of Na2Np207(s) in alkaline sodium chloride solutions. From the solubility measurements, the authors were able to obtain stability constants for the neptunium(Vl) species, Np02(0H)3 and Np02(0H)4 . The stability constants they obtained for zero ionic strength were 

Thermodynamic Parameters The thermodynamic values derived for neptu-nium(Vl) phases and hydrolysis species are listed in Table 9.15. Also given in the table are values for the thermodynamic parameters of Np02, as listed by Guillaumont et al. (2003). These are the same values as chosen earlier 

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A single oxo bridge may subtend an angle between 140° and 180°, this angle being determined by steric or electronic factors (Table 3).95 103 Almost all these examples refer to the solid state, but there are also several homo- and hetero-nuclear M—O—M and M—O—M—O—M species known in solution. Often these are intermediates in, or products of, electron transfer reactions with oxide-bridging inner-sphere mechanisms. Examples include V—O—V in V(aq)2+ reduction of VO(aq)2+, and Act—O—Cr in Cr(aq)2+ reduction of UOj+ or PuOj+ a useful and extensive list of such species has been compiled. Tlie most recent examples are another V—O—V unit, this time from VO(aq)2+ and VOJ,105 and an all-actinide species containing neptunium(VI) and uranium-(VI).106 An example of a trinuclear anion of this type, with the metal in two oxidation states, is provided by (31).107... 

Laurinat, J.E. 2006. SASSE modeling of first cycle neptunium(VI) recovery flowsheet. WSRC-TR-2006-00104, Rev.0. 

The donor action of [NOs]" as an anionic ligand towards thorium(iv) and uranium(iv) in the presence of trimethylphosphine and tris(dimethylamino)phosphine oxide in aqueous media has been found to be very similar. The larger nitrate ion was observed to form more stable species with thorium than the chloride ion whereas in the uranium(iv) case both complexes formed equally readily. A study of the sulphate complexes of uranium(iv), neptunium(vi) and plutonium(vi) in HCIO4-H2SO4 solution showed the stability constants to follow the order U[Pg.453]

Purex process, 951 Neptunium(VI) reduction Purex process, 951 Neptunium dioxide ions disproportionation rex process, 950 Nerve growth factor snake venoms zinc, 613... 

The oxidation of plutonium(iii) by neptunium(vi) has been investigated at varying acidities, and the following reactions were identified ... 

In the oxidations of edta and oxalic acid by neptunium(vi) in perchloric acid,carbon dioxide is produced. One mole of edta reacts with six moles of Np, the rate being first-order in both reagents with an inverse hydrogen-ion dependence. On mixing the reactants, a distinct colour change is observed which is ascribed to complex formation, the... 

A kinetic study has been made of the oxidation of ascorbic acid (H A) by neptunium(vi). The rate law is of the form — d[NpVi]/dr=A 2[H2A] [Np i] with no evidence for a hydrogen-ion dependence over the range 0.05 < [H+] < l.OOM. In aqueous media Np i exists as the [O—Np—0] + ion with between four and six equatorial water molecules. A comparison with the reactions with aquo-ions, e.g. or where inner-sphere reactivity is established and with [Fe(phen)3] + indicates that the preferred pathway is outer sphere. The exchange rate for [(5-nitrophen)gFe] +/ + and the redox potential are both greater than for the Np VNp " couple and these features are considered- the reason for the rate of the present reaction being 40 times slower than that of the iron system. 

There is one way in which the actinide series definitely differs from the lanthanide series. This is in the existence of oxidation states higher than IV [protactinium (V), uranium(VI), neptunium (VI), plutonium (VI)] in this series. It must be concluded that the 5f electrons are not so tightly bound as the 4f electrons. This is certainly reasonable. However, the evidence so far is in favor of a maximum oxidation number of VI in this series, so that the removal of three electrons, or four if there are no electrons in the 6d orbitals, from the 5f orbitals is the maximum that occurs in ordinary chemical reactions. 

The system has been used to effect the isolation and final purification of gram quantities of Np. Interestingly, the only difficulty encountered is in maintaining uranium in the uranium(VI) state, since in this system uranium-(IV) closely resembles neptunium(IV) in its extraction behavior. So after two to three aqueous HCl scrubs to remove contaminants from the neptunium product in the organic phase an oxidation cycle must be inserted to oxidize uranium(IV) to uranium(VI). Under the oxidizing conditions used, neptun-ium(IV) is oxidized to neptunium(VI). But through the action of hydro-quinone, following the oxidation step, neptunium(VI) is rapidly reduced to... 

Novak C. F., Nitsche H., Silber H. B., Roberts K., Torretto P. C., Prussin T., Becraft K., Carpenter S. A., Hobart D. E., and AlMahamid I. (1996) Neptunium(V) and neptunium(VI) solubilities in synthetic brines of interest to the Waste Isolation Pilot Plant (WIPP). Radiochim. Acta 74, 31-36. 

The subsequent reaction between hypobromite and bromite has been shown to be rapid, requiring fast reaction techniques for its characterization. The oxidation of neptunium(vi) by hypobromite has been studied in alkaline media, stable neptunium(vn) salts being formed. There is a rate dependence on hydroxide ion concentration, and nickel(n), cobalt(n), or copper(n) ions increase the reaction rate whereas silver(i) or iron(ii) have no effect. 

The redox potential of the Np /Np couple has been measured in alkaline media, the neptunium(vi) species in 0.1—1.5M sodium hydroxide being present as the ion Np02(OH)4 oxidation to Np is considered to involve the loss of two hydroxide groups. Neptunium(vi) is also formed in the reaction of Np with nitrate ion in aqueous nitric acid ... 

Fig. 21.9 Extraction of neptunium(vi) by 0.4 m methyldioctylamine in chloroform from aqueous solutions of hydrochloric, nitric, and sulphuric acids, at 20°C [178]. Initial concentration of neptunium 0.45 mu. The curves give the distribution ratio Das a function of the equilibrium acid concentration in the aqueous phase, Cg.

The oxidation of co-ordinated thiol at carbon rather than at sulphur has been reported in the reaction of cerium(iv) [and neptunium(vi)] with (mercaptoacetato-05 )bisethylenediaminechromium(m). 

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