SPECTROPHOTOMETRIC DETERMINATION OF URANIUM AND NITRIC-ACID BY APPLYING PARTIAL LEAST-SQUARES REGRESSION TO URANIUM(VI) ABSORPTION-SPECTRA

An application of the multivariate calibration technique of partial least-squares (PLS) regression to U(VI) absorption spectra in the spectral range 380-500 nm is presented. This method uses the information contained in the absorption spectra of U(VI)-nitric acid solutions to determine both U(VI) and nitric acid. For the determination of U(VI) its visible spectrum is used. Nitric acid is determined indirectly from its distinct effects on the U(VI) spectrum. The investigations reported show that PLS regression can be used efficiently to determine both constituents. A calibration set spanning the concentration ranges 1-30 g l-1 U(VI) and 0.02-3.0 mol l-1 nitric acid was modelled with standard deviations of 0.19 g l-1 and 0.067 mol l-1, respectively. Two PLS calibration models based on the original and on normalized U(VI) absorption spectra are compared. The calibration models were evaluated by predicting the concentrations of independent test set samples in the same U(VI)-HNO3 concentration range. The standard error for the test set samples was of the order of the standard error of the calibration models. The calibration model in combination with a fibre-optic spectrophotometer could be valuable for the in-line monitoring of both species in PUREX (plutonium-uranium recovery by extraction) process operations, e.g., U(VI) extraction procedures.