Electroanalytical Chemistry HELP

[Lady Woodie]

hey I am a third year chemistry student out on a gap year and I've been given an assignment on electroanalytical chemistry only some things don't make sense!

questions are:
2a) the concerted 2 electron 2 proton oxidation of the neurotransmitter serotonin is studied using several different electrode geometries. assume all experimental variables other than the electrode geometry is constant. explaining your reasoning which of the following exhibits the smallest current?
1. a 4.0um x 4.0um square electrode
2. a 15.0 nm x 3.0 um band electrode
3. a 2.0um diameter disc electrode
4. a ring electrode with outer radius of 1.0um and inner radius of 0.5um
??

2b) the reversible oxidation of dopamine is a 2 electron process. a cyclic voltammetric oxidative peak current of 2.2uA is observed for a 0.4mM solution of dopamine in phosphate buffer at a glassy carbon disc electrode of 2.6mm2 area with a scan rate of 25mV s-1. what will the peak current be under similar conditions except with a scan rate of 100mV s-1 and a concentration of 1.2mM??

any help would be fab thanks!

 

[Borek]

2a - there are equations that show how the current changes for different geometries, but at first approximation it will be just electrode surface dependent.

2b - there is a simple equation that shows how the CV peak current depends on the concentration, number of electrons, diffusion coefficients, electrode area and a scan rate. Use it.

 

[Lady Woodie]

rite okies...well for 2a i am using the equation i = nFSj. is that correct?
i am however using 1 for the flux as it is not stated neither is the time to work the flux out. so say 2a i) i = nFS = 2 x 96485.4 x 16 = 3087532.8mV?

and for the CV equation i am using ip = SD0.5 n1.5 v0.5 c
there is no D stated so i am leaving that out giving me an answer of
ip= 2.6 x 2^1.5 x 100^0.5 x 1.2 = 88.246uA?

am i on the rite track do you think?

thanks

 

[Borek]

rite okies...well for 2a i am using the equation i = nFSj. is that correct?

Do you understand what "compare surfaces" mean?

3087532.8mV?

You have tried to calculate current. Your answer is 3 kV. Neither units nor value range makes sense. If your plan was to shock me with the most absurd answer possible - don't bother, I have seen what students can do

In 2b - express the current in terms of i=constant*things that are given in correct powers. Calculate this constant value - and use it to calculate current for the other case.