Electrochemical cells are a core part of IB Chemistry. They allow chemical energy to be converted into electrical energy through redox reactions. One of the most important components of a galvanic cell is the salt bridge, yet many students struggle to explain exactly what it does. This article breaks down the purpose of a salt bridge, how it works, and why a cell cannot function without one.
What Is a Salt Bridge?
A salt bridge is a device, usually a U-shaped tube or soaked strip, containing an inert electrolyte solution such as:
- KNO₃
- KCl
- NaNO₃
The electrolyte must:
- Be ionic, to supply free ions
- Be non-reactive with the electrodes
- Not form precipitates with the ions in the cell
Common materials for salt bridges include filter paper soaked in electrolyte or glass tubes filled with gel and electrolyte solution.
What Does the Salt Bridge Do?
The salt bridge maintains electrical neutrality in both half-cells by allowing ion flow.
When electrons flow through the external wire from the anode to the cathode, charges inside the solutions shift and must be balanced.
Without a salt bridge, charge imbalance would stop electron flow almost instantly.
Why Is Charge Balance Important?
In a galvanic cell:
- Oxidation occurs at the anode, producing positive ions (cations).
- Reduction occurs at the cathode, consuming positive ions or producing negative ions.
If nothing replaced or balanced these charges:
- The anode compartment would become too positive
