Wall-Jet Electrode and a DISP 1 Process

The reduction of 1 mM fluorescein in aqueous solution has been studied at a wall-jet electrode. At pH 13 a simple one-electron reduction is known to occur  

Use the pH 13 data to calculate the diffusion coefficient of fluorescein in aqueous solution. Also shown in Fig. 8.7 are data obtained at pH 9.65 using a carbon-ate/bicarbonate buffer. Suggest, qualitatively, a reason for the altered behaviour and increased currents, as compared to the data at pH 13. 

The data shown for a pH of 9.65 is seen to follow the simple one-electron behaviour at very fast flow rates, but tends towards two-electron behaviour at low flow rates. Thus the effective number of electrons transferred, n ff, changes in a manner expected of an ECE or DISP type process where the one-electron product (S ) is rapidly removed from the electrode at fast flows. At low flow rate it is able to react further, leading to the second electron transfer. 

It is thought that a DISP 1 process operates in this system  

Note that the protonation of S will be faster at a higher proton concentration (lower pH), thus explaining the change in behaviour between pH 13 and pH 9.65. 

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