where R is defined by R 2 = 4Dt, as a measure of the interfacial width. Using this equation, the concentration profiles were fitted and parameterized for each time point. Fits are indicating by red lines in Fig. , and demonstrate that this is a good model for the observed process. A total of 9 data sets at varying times were analysed, which were at the beginning (at 45 min, 78 min and 111min), in the middle (at 624 min, 678 min and 708 min) and at the end of the experiment (at 1366 min, 1397 min and 1427 min), with the fitted concentration profiles becoming broader flatter over time, in accordance with the data. The experiment was repeated twice, giving a total of four samples/tubes measured.