Neptunium III

Neptunium, like uranium, can occur in aqueous solution in four oxidation states trivalent, tetravalent, pentavalent and hexavalent. The most studies have been conducted on the hydrolysis and solubility of neptunium(V) species and phases. There are much fewer studies on the hydrolysis and solubility of neptu-nium(IV). Not many studies are available that have determined the solubility or stability constants of neptuniumfVI). However, it is clear from the studies that have been undertaken that the hydrolytic behaviour of neptunium(VI) is quite similar to that of uraniumfVI). There is only a single study that has determined stability constants for the hydrolytic species of neptunium(III). 

There is only a single reported study that has experimentally determined stability constants for hydrolysis species of neptunium(III), and no data exist for the solid phases, Np20j(s), Np(OH)j(s) or NpfOHljfam). For reaction (2.5) (M = Np , p = l, q=l), Mefod eva et al. (1974), from measurements conducted in 0.3 mol 1 NaClO and at 25 C, obtained a stability constant for NpOH of log = 7.43 0.11. From this value, Lemire et al. (2001) determined a value for the stability constant at zero ionic strength of log = —6.8 0.3. This latter constant can be compared with those estimated by Allard, Kipatsi and Liljenzin (1980) and Brown and Wanner (1987) of log = —7.4 0.5 and -7.71, respectively. The average of these three values is retained in this review  

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Thorium(III) and protactinium(III) complexes are unknown, and relatively few uranium(III), neptunium(III) and plutonium(III) compounds have been described. This is mainly because of the ease of oxidation to the +4 state in all three cases, accentuated for plutonium(III) by the oxidizing nature of the a-radiolysis products formed in solutions. 

Neptunium(III) and plutonium(III) are precipitated from aqueous solution by salicyclic acid the neptunium salicylate, Np(QH4(0H)C02)3T.5H20, can be dehydrated at 50 to 150 °C. 

Neptunium(iii) was reported to be more stable towards oxidation than uranium(iii) when in aqueous solutions of complexing agents in the absence of oxygen. Nephelauxetic shifts for neptunium have been found to be less than for uranium in ter- and quadri-valent states. 

The reaction rate is also sensitive to chloride ion concentration. The overall reaction between neptunium(iii) and uranium(vi)... 

This stability constant is retained and the uncertainty has been estimated in this review. Busey and Cowan (1950) studied plutonium(III) hydrolysis in 0.040 moll NaCl and obtained a stability constant of log = —7.25, in good agreement with the work of Kraus and Dam (1949a). These constants are also consistent with those constants derived for both neptunium(III) and americium(III). 

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