The experimental procedure for potassium ferrioxalate actinometry was based on literature. A solution of 6.0 mM of potassium ferrioxalate (Alfa Aesar/Thermo Scientific) in 50 mM sulfuric acid (Sigma Aldrich) solution, using demineralized water as solvent, was prepared. The solution was pumped through the Uflow and pRS-SDR at selected liquid flow rates. Gases were supplied with a mass flow controller (MFC, Bronkhorst) and the gas and liquid phase were mixed using a T-mixer. Gas equivalents are based on the volume under normal conditions (0 °C and 1 atm). The mixture was irradiated with a UV-light source (Uflow: Kessil PR160L 370 nm, input power 43 W, operated at 25% power intensity, pRS-SDR: UV-A curing 365nm floodlight, input power 175 W). Sampling at the outlet was done at steady-state conditions (i.e., after waiting at least two residence times). The microcapillary reactor (PFA, 1.6 mm outer diameter, 0.8 mm inner diameter, 4.1 mL) was not operated in stop-flow to increase the mixing and prevent unwanted dilution. Blank samples were taken after each experiment to account for any conversion of the actinometer caused by background irradiation.