Electrochemical Salt Bridge: Definition & Purpose

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  • 0:04 Galvanic Cell
  • 1:08 Salt Bridge vs No Salt Bridge
  • 2:56 Salt Bridge Added
  • 3:54 Lesson Summary
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Lesson Transcript
Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Electrochemical cells generate electric current and are similar to a battery. A salt bridge is a vital component to the cell, and the cell won't function without it. In this lesson, we will go through what purpose the salt bridge serves in the electrochemical cell.

Galvanic Cell

There was a television show in the 80's that had a Jack of all trades as the main character. In one episode, he created a welding machine out of a generator, jumper cables, and two half-dollar coins. We can be our own Jack of all trades and create a battery out of some things laying around in the chemistry lab. Let's see how to do this.

Here is a list of materials we will need to create our battery.

  1. Zinc bar
  2. Copper bar
  3. Two beakers
  4. Conducting wire
  5. Zinc sulfate solution
  6. Copper sulfate solution
  7. Salt bridge consisting of sodium sulfate
  8. Voltmeter (this isn't required to make the electrochemical cell work, but it proves that it does work)

A galvanic cell is one where an electric current is generated by a redox reaction. A redox reaction occurs when electrons are transferred in the reaction. Let's look at the salt bridge and see why it is critical in the function of a galvanic cell.

Salt Bridge vs No Salt Bridge

A salt bridge is a u-shaped tube plugged at each end with cotton wool that contains an aqueous solution of an ionic salt. The salt bridge can even contain a gel material containing the ionic salt solution. We don't want the solutions to openly mix. A standard bridge is a pathway for cars or pedestrians over something like a body of water or a road. A salt bridge is a pathway for ions to flow between the beakers. Let's see what happens without a salt bridge in a galvanic cell.

Before the wires are connected, the solutions in each beaker are neutral. They have equal numbers of positive and negative charge. The zinc bar (anode) gives up two electrons that flow through the wires to the copper bar. The zinc atom that lost two electrons is now a zinc ion (Zn+2), which is now in the solution. This makes the solution have a net positive charge.

The copper bar (cathode) takes the two electrons causing one positive copper ion (Cu+2) to leave the solution and accept the two electrons. When this happens, an atom of copper is deposited on the copper bar. This leaves the solution in this beaker with a net negative charge.

An easy way to remember which metal bar is the anode and cathode is to use the mnemonic device PA-NIC. ''Positive is anode, negative is cathode.''

Now we have a voltage in two locations in the galvanic cell. One between the metal bars, and the other between the charged solutions. The voltage between the metal bars is positive, and the voltage between the two solutions, which is called the internal circuit, is negative. This causes the voltages to cancel out, and no current flows. Without the salt bridge, you have two pieces of metal in an ionic solution, nothing more!

Salt Bridge Added

When the salt bridge is added to the galvanic cell, the internal circuit disappears because there is no net voltage between the solutions. The instant the solution surrounding the copper bar becomes negative because it lost the positive copper ion, two positive sodium ions enter the solution from the salt bridge.

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