Insights into Tetravalent\nNp Speciation in HNO<sub>3</sub> through Spectroelectrochemistry and\nMultivariate Analysis

In situ optical spectroscopy, spectropotentiometry, and\nmultivariate\nanalysis were applied to the Np(IV) nitrate system to better understand\nspeciation and quantify HNO<sub>3</sub> concentration. Thin-layer\nspectropotentiometry, or spectroelectrochemistry, was leveraged to\nisolate and stabilize Np(IV) without compromising the solution conditions\nand generate representative Vis-NIR absorption spectra from 0.5 to\n10 M HNO<sub>3</sub> and benchmark the corresponding Np(IV) molar\nabsorptivity coefficients. Spectra were described with principal component\nanalysis (PCA) to identify the purest Np(IV) absorbance spectra among\nother oxidation states [e.g., Np(V/VI)] at each acid concentration\nand then to identify the primary sources of variance within each Np(IV)\nspectrum with respect to Np(IV) nitrate complexes. Then, partial least-squares\nregression (PLSR) and support vector regression (SVR) models were\nbuilt to predict HNO<sub>3</sub> concentration from the Np(IV) spectral\ndata. The nonlinear SVR model outperformed the linear PLSR model for\nthe HNO<sub>3</sub> concentration predictions. Finally, the inclusion\nof spectra collected in edge and center point HNO<sub>3</sub> concentrations\nin the calibration set was determined to be crucial for producing\nmodels with strong predictive capabilities. The multivariate approach\nused in this study makes it possible to quantify HNO<sub>3</sub> concentration\nsolely based on Np(IV) absorption spectra, which is essential to quantifying\nprocessing streams in various online monitoring applications.