Professor Chadwick: Voltammetric Methods

Question 1

What are the basic principles of voltammetry?

Answer 1

• An increasing electrical field is applied and the current is measured.
• This leads to oxidation or (more commonly) reduction of a species at an electrode.
• Only a small portion of the material is reduced or oxidised.
• Provides both qualitative and quantitative information about
  
  • Can obtain data on the diffusion current.
  • Halfwave potential tells us the species in solution

Question 2

Why are Mercury dropping electrodes used in polarographic methods?

Answer 2

• As material is deposited onto a standard electrode, its characteristics change.
• We need an electrode that keeps clean and constantly refreshed.
• The dropping mercury electrode constantly recycles itself.

Question 3

How does the Dropping Mercury Electrode work?

Answer 3

• The applied voltage is gradually increased, and when it becomes great enough, reduction occurs at the analytical electrode causing a current.
• The reduced species rapidly saturates and alters the surface of the mercury electrode.
• The mercury surface is then renewed by knocking off a drop, refreshing the surface.

Question 4

Sketch a typical plot obtained from a DME and outline the information available from the half-wave potential and diffusion current.

Answer 4

• The diffusion current can tell us the concentration of the electrode using the Ilkovich equation.
  
  • i_d= 706 n C D^1/2 m^2/3 t^1/6
• The half-wave potential can help us identify the analyte.
  
  • The E_1/2 value is the inflection point of the curve.

Question 5

How is the hanging dropping mercury electrode employed in Stripping Voltammetry?

Answer 5

• A single mercury drop is hung from the electrode.
• An electrodeposition phase follows, where the voltage is increased for 5 minutes for 10^-7 M, 60 minutes for 10^-9 M.
• This strips all of the metal from the sample, which are all present in a single Hg drop.
• The potential is then reduced at a known rate and the anionic current is measured.
• The oxidation of the metals from mercury is measured in this step.

Question 6

What is the process of cyclic voltammetry?

Answer 6

1. CV involves sweeping the voltage between two values at fixed rates, although when the voltage reaches V₂, the scan is reversed and the voltage is swept back to V₁
2. Forward sweep produces an identical response seen in LSV.
3. When the scan is reversed, we move back through the equilibrium as M is converted back to M⁺
4. Current flow is from solution species back to the electrode.

Question 7

What are the characteristics of Cyclic Voltammetry plots?

Answer 7

1. Voltage separation between the current peaks is?
2. The positions of peak voltage do not alter as a function of voltage scan rate.
3. The ratio of the peak currents is equal to one.
4. Peak currents are proportional to the square root of the scan rate.

Question 8

What is the modified technique of Polarography known as TAST?

Answer 8

Current-sampled polarography, TAST, is where the potential is still varied linearly with time but the current is only sampled for 5-20 msec/drop near the end.

This results in smoother data.

Question 9

What is the process of Differential Pulse Polarography?

Answer 9

1. The potential is applied as a ramped square wave. The current is sampled just before and near the end of each pulse.
2. Data is recorded as a differential so concentration is represented by peak area.
3. As more data points are used, there is an increase in sensitivity.