# Evaluating Np(v) disproportionation: effect of Np concentration

## TL;DR

This study explores how neptunium(V) concentration affects its redox behavior in acidic solutions, revealing challenges in stabilizing pure Np(V) and the role of self-complexation.

## Contribution

The study identifies neptunyl self-complexation as a key mechanism affecting Np(V) stability in acidic media.

## Key findings

- Pure Np(V) is difficult to stabilize even in dilute acid, with Np(VI) coexisting in both HNO3 and HCl.
- Electrochemical tuning reveals self-complexation as a significant factor in Np(VI/V) ratios.
- Np(V) stabilization is only brief, and Np(IV) does not coexist with Np(V) in HNO3.

## Abstract

The role of Np(v) concentration as a potential chemical driver of Np(v) disproportionation was investigated by probing Np redox chemistry in two different acid media using visible-near-infrared absorbance spectroscopy. In situ spectroscopy of chemically and electrochemically manipulated Np oxidation states at molar concentrations of Np underscores the difficulty in stabilizing pure Np(v), even in dilute acid. A Np(vi) component coexisting with the majority Np(v) component was observed in both HNO3 and HCl. Electrochemical tuning and subsequent spectroscopic analysis of the Np(VI/V) ratio revealed neptunyl self-complexation as a key mechanism occurring in both acid media. Chemical reduction of the Np(vi) component and stabilization of a minor Np(iv) component in HNO3 were observed; however, spectral data indicated that these two oxidation states were not observed to coexist, and pure Np(v) was stabilized only briefly. These findings demonstrate that high Np concentrations significantly affect the redox chemistry of Np in acidic solutions. Finally, the occurrence of Np(v) disproportionation could not be definitely determined by spectral data, but other mechanisms such as self-complexation and radiolysis, which potentially compete with disproportionation, must be considered.

The role of Np(v) concentration as a potential chemical driver of Np(v) disproportionation was investigated by probing Np redox chemistry in two different acid media using visible-near-infrared absorbance spectroscopy.