Electrochemical Cells and Potential

Electrochemical Cells

• When a net current is present in an electrochemical cell, the measured potential across the two electrodes is not simply the difference between the two electrode potentials calculated from the Nernst equation.
• Two additional phenomena, IR drop and polarization, must be considered when current is present.

IR Drop

• IR drop is a result of the resistance of the cell to the flow of charge.
• Ohm's law describes the effect of this resistance on the magnitude of the current in the cell.
• The product of the resistance R of a cell in ohms (V) and the current I in amperes (A) is called the ohmic potential or the IR drop of the cell.
• To generate a current of I amperes in the cell, a potential that is IR volts more negative than the thermodynamic cell potential must be applied.

Electrolytic Cell

• An electrolytic cell requires a potential larger than the thermodynamic potential to operate.
• The cell consists of a working electrode (cathode) and a reference electrode.
• The working electrode is where the reduction of ions occurs.
• The reference electrode has a constant potential during the analysis.

Example Electrolytic Cell

• The cell consists of Ag | AgCl(s), Cl2(0.200 M), Cd21(0.00500 M) | Cd.
• This cell can be used to determine cadmium(II) in hydrochloric acid solutions by electrogravimetry or coulometry.
• Similar cells can be used to determine Cu(II) and Zn(II) in acid solution.

Thermodynamic Potential

• The thermodynamic potential of the example cell is 20.734 V.
• The negative sign indicates that the spontaneous reaction is not the reduction of Cd21 on the right and the oxidation of Ag on the left.

Constructing the Electrolytic Cell

• To reduce Cd21 to Cd, a potential more negative than the thermodynamic potential must be applied.
• The cell becomes reversible, and in the absence of an external voltage source, the spontaneous cell reaction occurs in the right-to-left direction (oxidation of Cd(s) to Cd21).
• Short-circuiting the galvanic cell makes the Cd electrode the anode.