Question

An electrochemically reversible system (Fe³⁺ + e <-> Fe²⁺) gave the following table of scan rate versus peak anodic current. What is the diffusion coefficient, D, of this system, given that the area of the working electrode = 0.0201 cm², concentration of Fe³⁺ = 1.00 x 10^-6 mol/cm³?

n, V/s	Ip, Amps
0.0192	2.18 E-6
0.0489	3.46 E-6
0.0751	4.17 E-6
0.125	5.66 E-6
0.175	6.54 E-6
0.251	7.55 E-6

Answer 1

This is the Randles-Sevcik equation

i_p = (2.69×10⁵)n^3/2AD^1/2ν^1/2C

the constant is 2.69×10⁵ C mol^-1 V^-½,

i_p is the peak current (A)

n is the number of electrons in the redox reaction (here n=1)

A is the area of the working electrode (here A= 0.0201 cm²)

D is the diffusion coefficient for the electroactive species (to be found, …cm²s^-1)

ν is the scan rate (    you labeled it here as n)

C is the concentration of the electroactive species at the electrode (here Fe³⁺ = 1.00 x 10^-6mol/cm³).

You may calculate a value for D for each scan rate (and calculate the average value). But here you may also plot i_p vs ν^1/2 to find a slope that is equal to (2.69×10⁵)n^3/2AD^1/2C . Then replace n,A,C to calculate D (pay attention to measurement units).

I try to verify also your D value.