Results of X-ray experiments and comparison with predictions from simulations. (a) Reference K-edge XAS spectra of ferricyanide reactant and ferrocyanide product. The inset shows a zoom into the K pre-edge region. (b) Reference Fe Kβ XES spectra for the same species. (c) Predicted (line) and measured (symbols) depth-averaged, unreacted sample fraction MR versus the sample residence time t. The shaded area near the line shows the range of possible MR given variations in sample and sheath species diffusivities of 10%. Vertical uncertainty bars denote the standard error of the X-ray measurement and horizontal uncertainty bars denote the sample residence time uncertainty. In this diffusion-limited scenario, mixing and reactions occur simultaneously. We therefore report the average sample residence time since sample species were exposed to sheath species. Data are for Qsa = 30 µl min−1, Qsh = 1 ml min−1, = 40 mM, and [H2Asc]0 = 200 mM, and pH = 10.