The Principle of the Cell: Anode and Cathode

Separating to Force the Current Through a Circuit

In a cell, the oxidizing agent and the reducing agent are placed in two separate compartments (two half-cells), each containing an electrode dipped in a solution. The two electrodes are connected by an external conducting wire, and the two solutions are connected by a salt bridge (or a porous wall), which allows ions to flow without the solutions mixing.

Anode and Cathode

• The anode is the electrode where oxidation occurs (the reducing agent gives up its electrons). In a cell, it is the negative terminal: this is where the electrons leave toward the external circuit.
• The cathode is the electrode where reduction occurs (the oxidizing agent captures electrons). In a cell, it is the positive terminal: this is where the electrons arrive from the external circuit.

Two Directions of Flow Not to Be Confused

• The electrons flow through the external conducting wire, from the anode (-) to the cathode (+).
• The conventional electric current, by historical convention, flows in the direction opposite to the electrons: outside the cell, it therefore goes from the cathode (+) to the anode (-).
• Inside the cell, in the solutions, it is ions that migrate (via the salt bridge) to maintain the electroneutrality of each compartment: cations migrate toward the cathode, anions toward the anode.

Common Mistake

Don't confuse the direction of the electrons with the direction of the conventional current: this is the most common mistake in electrochemistry. Remember the rule: electrons always flow in the direction opposite to the conventional current, both in a cell and in any electrical circuit.