A supersensitive test (SST) for photocatalytic activity is described, based on ISO 22197-1:2016 for the removal of NO, i.e. the standard (NO removal) ISO. The key reaction is the photocatalysed oxidation of nitric oxide (NO) to nitrogen dioxide (NO 2 ) by oxygen, which, for most materials, is the dominant reaction at the end of the 5 h irradiation period employed in the standard ISO. The SST uses the same photoreactor as the standard NO ISO, but with 3 (instead of 1) sample pieces and a flow rate of 0.25 L min -1 (instead of 3 L min -1 ) and, as a consequence, is 36 x's more sensitive than the standard ISO. The SST is used to assess the activities of materials that show little if any activity when tested using the standard ISO, such as commercial self-cleaning samples (such as glass (Activ), paint and plastic), as well as bank notes. An expression is derived, and verified using films of P25 powder, which relates rate, and so photocatalytic activity, directly to the average %NO removed, measured under either standard ISO or SST conditions, in the last 15 min of irradiation, i.e. %NO rem , where the latter value is directly related to the photocatalytic area, A, and the reciprocal of the flow rate, 1/f. This relationship allows all %NO rem values < 25%, measured using either standard ISO or SST conditions, to be combined to create a universal scale of photocatalytic activity. The universal scale of activity is illustrated using the calculated %NO rem values for high, medium and low activity materials which spans over 3 orders of magnitude of activity. The possible uses of this scale are discussed briefly.
