Electrochemical cells are a core part of IB Chemistry Topic 9 and Topic 19. 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
- The cathode compartment would become too negative
This charge buildup would oppose further electron movement, stopping the reaction.
The salt bridge prevents this by offsetting the charge changes.
How the Salt Bridge Maintains Charge Neutrality
The salt bridge contains both cations and anions that move depending on charge changes.
At the anode (oxidation site)
Positive ions accumulate because metal atoms lose electrons.
To balance this, anions from the salt bridge flow into the anode solution.
At the cathode (reduction site)
Positive ions are consumed (or negative ions may form).
To balance this, cations from the salt bridge flow into the cathode solution.
This continuous ion movement ensures both solutions remain electrically neutral.
What Happens If There Is No Salt Bridge?
Without a salt bridge:
- Electrons would initially flow from anode to cathode
- Charge imbalances would quickly develop
- The reactions would stop within seconds
- The cell’s voltage would drop to zero
The salt bridge is therefore essential for the sustained operation of a galvanic cell.
Salt Bridge vs. Porous Separator
In some commercial cells, a porous separator replaces the salt bridge.
Similarities:
- Allow ions to move
- Prevent mixing of whole solutions
- Maintain charge neutrality
Differences:
- Porous separators are solid membranes
- Salt bridges are liquid-based
- Porous separators are more compact and practical for batteries
For IB purposes, both serve the same functional purpose.
Characteristics of a Good Salt Bridge
A suitable salt bridge must:
- Contain ions that do not react with the half-cell components
- Allow ions to move freely
- Prevent mixing of the solutions
- Maintain electrical neutrality
- Complete the internal circuit
This ensures stable and predictable cell behavior.
IB Exam Tip
When asked the purpose of a salt bridge, IB examiners expect a full explanation:
“To maintain electrical neutrality by allowing the movement of ions between half-cells, completing the circuit and allowing electron flow to continue.”
Avoid vague answers like “it completes the circuit” without explaining charge balance.
FAQs
Why are KNO₃ and KCl common salt bridge solutions?
Their ions (K⁺, NO₃⁻, Cl⁻) are usually inert and do not form precipitates or participate in the redox reactions occurring in the cell.
Can pure water be used as a salt bridge?
No. Pure water has extremely low conductivity because it contains almost no ions.
Do electrons travel through the salt bridge?
No. Electrons travel through the external wire.
The salt bridge carries ions, not electrons.
Conclusion
The purpose of a salt bridge is to maintain electrical neutrality in both half-cells by allowing ion movement. This prevents charge buildup, ensures the redox reactions continue, and enables a galvanic cell to produce a steady flow of electrons. Without a salt bridge, an electrochemical cell would stop functioning almost immediately. Understanding this principle is essential for mastering electrochemistry in IB Chemistry.
