Simulations of the O2 concentration and formaldehyde concentration within the capillary channel when using paraformaldehyde as reducing agent after 6.5 s (time needed for depleted for the O2 initially dissolved in the sample), after 10 mins (time used for the coulometric measurements), and after 3.2 days (time needed for O2 to reach the sensing electrode). The grey dash line shows the position of the sensing electrode. The simulations account for the continuous supply of O2 through the capillary opening exposed to ambient air. The parameter values for the simulation are given in Table .