Question

Draw a linear scan polargraphic wave. Label the axes and indicate the half-wave potential and limiting current. What is the relative concentration of reductant in the bulk solution and at the electrode surface at different points on the curve? 3.

Answer 1

Here A-B is the residual current.

B-C is the diffusion current

C-D is the limiting current.

Id is diffusion current.

at B, the concentration of reductant is greater in bulk as compared to in region C. At region C, there will be a relatively higher concentration of analyte ions( reductant) near the electrode.

residual current is something where the reaction starts happening and at B the movement of ions start taking place. residual current is basically the sum of faradaic current which is due to impurities and the non-faradaic current which is generated due to the formation of an electrical double layer around the dropping mercury electrode.

half wave potential is characteristic of that species and independent of the concentration of the analyte. half wave potential is the point at which the diffusion current is half of its maximum value.

limiting current is dependent on the concentration of the analyte.

In polarography, there are two types of current generated in the medium. one is due to the concentration gradient of analyte (diffusion current) and another is due to the difference in the potential applied across the solution (migration current). To stop the movement of analyte ions via migration current, we use a supporting electrolyte which forms a layer around the electrode so analyte ions cannot approach the electrode (so no migration current). So analyte ions approach the electrode via diffusion current to get reduced. so as the potential increases, the flow of analyte ions increases, diffusion current increases till point C.

point C gives the concentration of reducing species (analyte ions) in the medium which is mathematically given by IIKOVICH equation.

I_d= 607nFD^1/2M^2/3t^1/6

where the n= number of electrons exchanged

D is the diffusion coefficient, M is the mass flow rate of Hg from the capillary, and t is the drop lifetime in seconds.