Finally, measurement of hydrogen peroxide (H2O2) is also conducted ex situ using colorimetric assays. While colorimetric assays allow for a highly sensitive detection of H2O2 and can be performed with common laboratory equipment, it also must be considered that they suffer from possible crosssensitivity. This can result from chemical reactions between components of the photocatalyst dispersion with the assay reagents as well as absorption or scattering of wavelengths relevant for colorimetric quantification by catalyst particles. Therefore, it is crucial to remove photocatalyst particles via filtration as well as to measure a reference sample of the photocatalyst dispersion which was kept in the dark during the experiment. By comparing the irradiated and dark samples, the effect of irradiation can be separated from any interactions between the colorimetric assay and residual components in the filtered reaction solution.