To correlate the XANES Ni edge step size with the concentration of Ni ions in the salt, the step sizes in calibration samples with known concentrations were calculated. The step size relates linearly with the concentration of the of Ni in the sample . The results of the fits of the step size correlated to the NiF 2 concentration are shown in Figs. and. Uncertainties in the NiF 2 concentration are determined from the uncertainties calculated from the fitting procedure for the pre-and post-edge regions.