Complexation of Neptunium(V) with Fluoride at Elevated Temperatures Page: 3 of 14
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at 980 nm using the molar absorption coefficient of 395 M cm 1. Gran's potentiometric
method [4] was used to determine the concentration of free acid in the Np(V) stock solution.
Solutions of fluoride were prepared by dissolving solid NaF in water. The ionic strength of all
working solutions in this study was maintained at 1.0 M (25 C) by adding appropriate
amounts of sodium perchlorate as the background electrolyte.
2.2 Spectrophotometry
Spectrophotometric titrations covering the wavelength region of 950 - 1050 nm (0.2 nm
step) were conducted on a Varian Cary-5G spectrophotometer equipped with sample holders
that were maintained at constant temperatures by a 1 x1 Peltier controller. 10 mm quartz cells
were used. Before being inserted into the sample holders, the sealed cells were immerged in
an external constant-temperature water bath to be pre-equilibrated at the required
temperature. This procedure successfully prevented condensation of water on the top of the
cells during the spectrophotometric measurements at high temperatures. The sample holders
are also equipped with magnetic stirrers so that thorough mixing of the titrant was achieved
while the cells were sealed to minimize water evaporation during the titration. Multiple
titrations with different concentrations of Np(V) were performed. Usually 10 - 15 additions
were made, thus generating a set of 10 - 15 spectra in each titration. The stability constants of
the Np(V)/fluoride complex (on the molarity scale) at different temperatures were calculated
by non-linear least-square regression using the Hyperquad program [5].
To allow the comparison of stability constants at different temperatures, the constants in
molar units should be converted to the constants in molal units. According to the method in
the literature [6], the equilibrium constant of a reaction in molality (Km) is related to that in
molarity (KM) by eq.1.
logKm = logKM + rr logS (1)3
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Rao, Linfeng; Tian, Guoxin; Xia, Yuanxian & Friese, Judah I. Complexation of Neptunium(V) with Fluoride at Elevated Temperatures, article, June 16, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc931387/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.