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.

Learn how IR drop and polarization affect the potential across electrodes in electrochemical cells, and how this impacts the operation of electrolytic and galvanic cells.