Potentiometry

 

Is the name given to the measurement of the potential generated by an electrolytic cell under condition of minimum current. The actual measurement requires the use of a reference electrode and an indicator electrode and a high impedance voltmeter.

 

In the analytical laboratory, potentiometry is the basic measurement employed in concentration measurement and, in particular, in pH determinations,  

 

Electrochemical cells used in potentiometry are usually made up of half-cells connected by an appropriate salt bridge. A half-cell is an electrical conductor (electrode) immersed in an electrolyte solution. The chemical reaction that takes place between the electrolyte and the electrode in a half-cell has a unique potential associated with it. Under standard conditions this potential is called the standard electrode potential.

 

Consider a copper electrode immersed in a copper sulphate solution and a zinc electrode situated in a zinc sulphate solution. The transfer of copper and zinc to and from the solution can be represented by the following equations

                      Cu²⁺ +2e^–      Cu         E^o = +0.337                         (11)

                     Zn²⁺ + 2e^–      Zn         E^o = -0.783                          (12)

The measurements of (E^o) are made relative to the standard hydrogen electrode (which will be discussed in due course). If energy is taken from the cell (i.e. a current is allowed to flow from the cell) the cell is said to be galvanic, if energy is given to the cell (i.e. a current flows into the cell) the cell is said to be electrolytic. If the two half cells are combined as shown in figure 5 to produce a galvanic cell then equation (11) must be subtracted from equation (12) to provide the net emf (electromotive force) of the cell.

 

     (13)

 

The emf of a cell represents the driving force behind the reaction and it’s tendency to move towards equilibrium

         

 

 

Figure 5. A Simple Galvanic Electrochemical Cell

       Thus,              E_C  =  E_anode  –  E_cathode + E_B                           (14)

 

          Where (E_B) is the liquid junction potential of the salt bridge.