Spectroscopic study of Np(V) oxidation to Np(VI) in 3 mol/dm³ nitric acid at elevated temperatures

Abstract

Optical absorption spectra of Np in 3 mol/dm 3 nitric acid at elevated temperatures were measured using an optical glass cell with a water jacket having a light path of 1 cm. Molar extinction coefficients of Np(VI), T M −1 cm −1 , were obtained at various temperatures (297, 313, 334, 352, and 372 K). The values of T were found to decrease with increasing temperature and could be described by the equation T = −0.14 + 85.5, where is the temperature. Oxidation of Np(V) to Np(VI) in 3 M nitric acid at elevated temperatures (336, 342, 354, and 362 K) was observed using the optical glass cell. Oxidation of Np(V) was promoted by increasing the temperature and proceeded as a pseudo-first order reaction with respect to Np(V) concentration. The rate equation in the temperature range of 336-362 K was obtained as follows: − d[Np(V)] / d = 2.2 × 10 7 exp[−65 × 10 3 /( )][Np(V)] , where , , and [Np(V)] indicate the gas constant, temperature, and Np(V) concentration at time , respectively.

Measuring Neptunium Concentration using Optical Spectrometry for the Pu-238 Supply Program

“…Interestingly, Np(VI) concentration did not increase with increasing temperature despite literature reports that suggest otherwise. 13 A possible explanation is the relatively low acidity (1 M HNO 3 ) in these experiments. These fluctuations could arise from alterations of the primary or secondary hydration sphere surrounding the Np(V) ion.…”

“…Studies over the last seven decades have measured Vis-NIR absorption and Raman spectral signatures of various oxidation states of Np in aqueous solutions. [5][6][7][8][9][10][11][12][13] Aqueous absorption spectra of Np(V) and Np(VI) ions have even been characterized to determine electronic structural, optical, and magnetic properties using ab initio calculations. 14,15 The intense 980 nm (Є = 395 M -1 •cm -1 ) band and a less intense band at 616 nm (Є = 22 M -1 •cm -1 ) are commonly used to determine Np concentration in the literature.…”

Quantifying neptunium oxidation states in nitric acid through spectroelectrochemistry and chemometrics

“…Many studies have reported characteristic electronic spectra of Np oxidation states ranging from Np(III) to Np(VI). (Sjoblom and Hindman, 1951;Ryan, 1960;Friedman and Toth, 1980;Ban et al, 2014;Chatterjee et al, 2017;Sadergaski and Morgan, 2022). However, similar redox potentials, differing chemical and redox behavior, the ability to react with certain complexing ligands, and differing stabilities in acid of various concentrations all contribute to the complex redox chemistry of Np and the difficulty in acquiring electronic spectra of single oxidation states.…”

“…A broad system of overlapping peaks occurred near 555 nm, and the most characteristic feature of the Np(VI) spectrum is an asymmetric peak that occurs in the NIR region near 1,223 nm. (Ban et al, 2014).…”

“…Spectrophotometry, or ultraviolet-visible-near-infrared (UV-VIS-NIR) absorption spectroscopy, quantitatively measures the electronic spectra of molecular species and has been used for decades to characterize aqueous Np species in acidic solutions. (Sjoblom and Hindman, 1951;Ryan, 1960;Eisensterin and Pryce, 1966;Varga et al, 1970;Friedman and Toth, 1980;Ban et al, 2014;Edelstein, 2015). Determining the relative proportion of Np oxidation state(s) and total Np concentration by spectrophotometry is challenging because of its complex redox chemistry (adopting oxidation states from +3 to +6), coordination chemistry, interionic associations, and dependence of absorption bands on temperature and solution conditions.…”