1. Calculate the value of Eo’ for Fe(CN)6 3-/4-. You used an Ag/AgCl reference e

Question

1. Calculate the value of Eo’ for Fe(CN)6 3-/4-. You used an Ag/AgCl reference electrode with 3 M KCl. Correct the Eo’ value to that versus a standard hydrogen electrode. You will need to look up the potential for the Ag/AgCl electrode.

2. Compare the cyclic voltammogram before and after the polishing step. What are the peak splitting values? Why is the ideal peak splitting value 59 mV (hint: see the Nernst equation)?

3. What would be an ideal splitting for a redox couple changing two electrons?

4. Explain the general principles of stripping voltammetry.

5. What is the composition of a typical solution employed in stripping voltammetry and what is the role of each component in the solution?

6. Estimate the surface area of the glassy carbon electrode and then calculate the thickness of the Hg layer formed during the initial “plating out” part of the experiment (assume the density of Hg is 13.546 g/cm3 ).

7. Construct calibration curves for each of the elements by plotting both area and peak height vs. concentration. Fit linear regression lines to each data set and determine the slope, intercept and correlation coefficient.

8. Do the peak heights or peak areas give a better fit to a straight line?

9. Using your calibration curves, calculate the concentration of Pb2+, Cd2+, and Cu2+ in the tap water. Remember that only 1 mL of tap water was analyzed so take this into account when determining the metal concentrations. Give an estimate of what you think the error in these concentrations would be.

Allowable Levels of Metals in Drinking Water MCL 50 ppb 2 ppm 4 ppb 5 ppb 0.1 ppm 1.3 ppm 2 ppb 15 ppb 6 ppb 50 ppb 2 ppb Element As MCLG Be Cd Cr Cu Hg Pb Sb 2 ppm 4 ppb 5 ppb 0.1 ppm 1.3 ppm 2 ppb 6 ppb 50 ppb 0.5 ppb Th maximum concentration level goal. The level which the EPA considers the element has an insignificant effect on human health maximum concentration level. The maximum allowable level that can be enforced by the EPA under the US Clean Water Act of 1995. States may set lower, but not higher, limits than these. MCLG MCL

Explanation / Answer

1) The value of EAg/AgCl for Fe(CN)6 3-/4- redox couple is

EAg/AgCl = ( Epa + Epc)/2 (Epa = potential at peak anodic current, Epc = potential at peak cathodic current)

=> EAg/AgCl = -0.57 V vs Ag/AgCl (3 M KCl)

Therefore, E0 = EAg/AgCl + 0.210 (calibration constant vs SHE is 0.210)

=> E0 = -0.36 vs. SHE

2) The difference in the peak potential of anodic and cathodic peak decreases after polishing of the electrode. Peak splitting value is around 0.61 V. Ideal peak splitting value of 59 mV represents that reaction occurs via one electron transfer.

3) An ideal splitting for a redox couple changing two electrons will be 0.059V/2 = 29.5 mV. (Using Nernst equation).

4) Stripping voltammetry is an electrochemical technique where an analyte can be analyzed quantitatively. The analyte is deposited by applying a particular potential. Then at another potential, the analyte is oxidised and removed off the substrate. Current is measured during the potential. The magnitude of current is directly proportional to the amount of analyte.

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