Single Electrode Potential

SINGLE ELECTRODE POTENTIAL

An electrochemical cell consists of two half-cells. With an open-circuit, the metal electrode in each half-cell transfers its ions into solution. Thus an individual electrode develops a potential with respect to the solution. The potential of a single electrode in a half-cell is called the Single electrode potential. Thus in Daniel cell in which the electrodes are not connected externally, the anode Zn/Zn²⁺ develops a negative charge and the cathode Cu/Cu²⁺, a positive charge. The amount of the charge produced on an individual electrode determines its single electrode potential.

The single electrode potential of a half-cell depends on : (a) concentration of ions in solution ; (b) tendency to form ions ; and (c) temperature.

Standard emf of a cell

The emf generated by an electrochemical cell is given by the symbol E. It can be measured with the help of a potentiometer. The value of emf varies with the concentration of the reactants and products in the cell solutions and the temperature of the cell. When the emf of a cell is determined under standard conditions, it is called the standard emf. The standard conditions are : (a) 1 M solutions of reactants and products ; and (b) temperature of 25^o C. Thus standard emf may be defined as the emf of a cell with 1 M solutions of reactants and products in solution measured at 25^o C. Standard emf of a cell is represented by the symbol E^o. For gases 1 atm. pressure is a standard condition instead of concentration. For Zn-Cu voltaic cell, the standard emf, E^o is 1.10V.

Zn | Zn²⁺_{(aq, 1M)}       ||  Cu²⁺_{(aq, 1M)}  | Cu E^o = 1.10 V

Determination of emf of a half-cell

By a single electrode potential, we also mean the emf of an isolated half-cell or its half-reaction. The emf of a cell that is made of two half-cells can be determined by connecting them to a voltmeter. However, there is no way of measuring the emf of a single half-cell directly. The emf of the newly constructed cell, E is determined with a voltmeter. The emf of the unknown half-cell E^o can then be calculated from the expression

^Emeasured      ^{=  E}R ^{- E} L

If the standard half-cell acts as anode, the equation becomes

^E_R    ^{=      E}_measured             ⁽Q ^E_L ^{= 0)}

On the other hand, if standard half-cell is cathode, the equation takes the form

^E_L    ^{= - E} _measured        ⁽Q ^E_R ^{= 0)}

The standard hydrogen half-cell or Standard Hydrogen Electrode (SHE), is selected for coupling with the unknown half-cell. It consists of a platinum electrode immersed in a 1 M solution of H⁺ ions maintained at 25 ^oC. Hydrogen gas at one atmosphere enters the glass hood and bubbles over the platinum electrode. The hydrogen gas at the platinum electrode passes into solution, forming H⁺ ions and electrons.

The emf of the standard hydrogen electrode is arbitrarily assigned the value of zero volts. So, SHE can be used as a standard for other electrodes.

The half-cell whose potential is desired, is combined with the hydrogen electrode and the emf of the complete cell determined with a voltmeter. The emf of the cell is the emf of the half-cell.

For example, it is desired to determine the emf of the zinc electrode, Zn | Zn²⁺. It is connected with the SHE. The complete electrochemical cell may be represented as :

Zn  |  Zn²⁺   ||   H⁺  |   H₂ (1 atm), Pt

The emf of the cell has been found to be -0.76 V which is the emf the zinc half-cell. Similarly, the emf of the copper electrode, Cu²⁺ | Cu can be determined by pairing it with the SHE when the electrochemical cell can be represented as :

Pt, H₂ (1 atm) | H₊ || Cu²⁺ | Cu

The emf of this cell has been determined to be 0.34 V which is the emf of the copper half-cell.

E^o_cell  = E^o_Cu/Cu²⁺ - E⁰_SHE

=0.34 - Zero

=0.34 V