Cyclic Voltammetry: is an electrochemical technique used to measure the current 
      that develops in an electrochemical cell when the voltage is varied.
      •  The voltage applied is controlled by a potentiometer  (you have to programme 
         the potentiometer).  
      •  The potential is swept through a defined potential range over time (as shown by 
         section 1 of the graph opposite).
      •  This linear increase is known as ramping.
      •  The scan rate is the time over which the potential is changing before returning to 
         the original voltage, i.e. the time to complete one cycle.
      •  The laptop will record current (I/amps) readings at intervals over the scan.
      •  The data obtained is plotted as current against voltage and known as a cyclic 
         voltammogram. An example is shown and explained on the next slide.
                                                                           1
               Current vs Potential Graph:                  Potential ≡ Voltage
                        Explained
   The characteristic shape of a voltammogram for a solution phase redox couple is explained 
   by the effect of diffusion: at potentials beyond the peak potential the electroactive species 
   at the surface of the electrode is depleted and the kinetics of the reaction (the current) is 
   limited by the rate of diffusion of the species to the electrode surface from the bulk of the 
   solution. Hence the current shows a peak and then a decrease before leveling out slightly.
    As time increases, the                            C
    potential increases, (shown               B
    by the ramping on the graph 
    in slide 2 and on the x-axis of 
    this graph). The current in 
    region A remains unchanged              A                 D
                                       F
                                                     E
                              Zero 
                              Current  
                              line
                           Current vs Potential Graph:
                                       Explained
  Region B shows increasing current as the potential difference is increased. This is due to the 
  oxidation of 
  the species in solution. The current in this region is limited by the transfer of electrons between 
  the species and the electrode. 
  In region C the current then                                           C
  peaks before dropping off as the rate of                     B
  reaction is limited by the 
  diffusion rate of the species in solution.
   Region D shows the potential difference                  A                       D
   being decreased. As the potential 
   difference decreases, the current                   F
   decreases because the reaction rate                                  E
   decreases. In Region E the species 
   produced in the forward scan is being    Zero 
   reduced.                                 Current  
                                            line
 Region F shows the reaction being limited by the diffusion of the oxidised species to the 
 electrode.
 In this way, the Current vs Potential Graph can be read with time in a clockwise direction even 
 though time is not a feature of the graph.
        Current vs Potential Graph: Uses
        •  The graph can be used to determine the number of moles (of copper in this case) 
           that are reduced/oxidised at the electrode during the reaction. 
        •  Find the area under the curve of the cyclic voltammogram (CV) 
        •  Divide the area by the scan rate this will give the charge (Q/As) that is associated 
           with the electron transfer in a reaction. 
        •  The charge is then directly related to the number of moles by the equation:
                                                  Q = nFm
         
             where:     Q is the charge (C)
                   n is the number of electrons transferred in the reaction 
                   F the Faraday constant (96485 C mol-1) 
                   m the number of moles (mol)
      Remember 1 Coulomb is 1 Amp second                                C = As
                          The Randles-Sevcik equation 
                         T   he Randles-Sevcik  equation relates the peak current, ip, (measured as the 
                         distance from the baseline to the peak) to the scan rate allowing determination 
                         of the diffusion coefficient. The equation is written as: 
                          
                                                                                                                                                        5         3/2                   1/21/2
                                                                                                        i  = (2.69 x 10 )n                                               AcD                                      (eq.1)
                                                                                                          p
                          
                         Where:    ip is the peak current (A)
                                           n is the number of electrons transferred in the reaction 
                                           A is the area of the electrode (m2) 
                                           c is the concentration of the solution used (mol m-3) 
                                                                                                                                                          2  -1
                                           D is the diffusion coefficient (m s )
                                           is the scan rate (V s-1) 
                         The constant has units of A s V-1/2 mol-1
                         The tendency observed is that the faster the scan rate the larger the current.
                         The equation needs to be rearranged to determine the diffusion coefficient.