Hi Saadia,

You don't need to fit the whole Shoup-Szabo equation. If you apply a potential step for a long enough time scale (at a diffusion-limited potential) the current should eventually reach a steady state (Iss) described by

Iss = 4nFDCr

where r is the electrode radius, C is concentration and the other symbols have their usual meaning.

Mithran Somasundrum Thank you for your reply. I am aware that I can use that equation. However, I have seen people use this to determine the radius of their electrode and then use that radius value to put into the Shoup-Szabo equation to determine the diffusion coefficient. The shoup szabo equation should be more accurate if I am working with new molecules, right?. Or should the Iss=4nFDCr equation be enough?

Hi Saadia

Radial diffusion is measurable even for millimeter radius electrodes

give recommendations on the size of the macro electrodes for the quantitative study and must exceed 4 mm radius in

aqueous solution.

Hi Saadia

The 4nFCDr expression will be good enough if you do actually observe a steady state current.

Thnk you Mithran Somasundrum How do we define the diffusion-limited potential? If I am working with a 3-electrode arrangement (not RDE) and stirred my cell at 50 rpm, could still apply the Iss equation?

I think so far people are not answering the question. The advantage of the Shoup and Szabo equation is that you can fit for BOTH D and nC. This may be advantageous when concentrations are not known (species that are known to be tricky to dissolve or "new" species).

This is possible because the equation fits to BOTH timescales for a freely-diffusing species: The steady state at long times when i is proportional to D and nC, and the Cottrell equation at low timescales when i is proportional to D^1/2 and nC.

For fitting to the Shoup and Szabo equation, you will need to perform a non-linear fit to your chronoamperometric data in software like Origin using their derived equation.