What is exactly junction potential?

[adianadiadi]

We know that use of salt bridge is to avoid junction potential. However, exactly what is this junction potential?

 

[Billy T]

Different metals have different Fermi levels - that is the top of the filled band, or perhaps some where in the conduction band if metal is hot. Think of it as the highest energy electrons.
Both metals are uncharged when separated, but when contacted, electrons from the one with the higher Fermi level will flow into the metal with the lower Fermi level until they are equal.
Now the one that got electrons from the other is negatively charged and the one they came from is postively charged - That difference is the junction potential.

 

[exchemist]

Different metals have different Fermi levels - that is the top of the filled band, or perhaps some where in the conduction band if metal is hot. Think of it as the highest energy electrons.
Both metals are uncharged when separated, but when contacted, electrons from the one with the higher Fermi level will flow into the metal with the lower Fermi level until they are equal.
Now the one that got electrons from the other is negatively charged and the one they came from is postively charged - That difference is the junction potential.

Nice explanation of the junction potential of two metals in contact.

However since the questioner asks about salt bridges, my suspicion is that what he or she is really asking about is liquid junction potentials, i.e. those that occur between two solutions with a shared interface, as a result of differing rates of ionic diffusion. This is explained here: https://en.wikipedia.org/wiki/Liquid_junction_potential

Maybe if the questioner returns we can find out a bit more about what the issue is.

 

[adianadiadi]

Thank you. I am actually trying to understand junction potential in galvanic cells. Views from others will enhance the understanding and that is why posted here. Thanks for the link.

Nice explanation of the junction potential of two metals in contact.

However since the questioner asks about salt bridges, my suspicion is that what he or she is really asking about is liquid junction potentials, i.e. those that occur between two solutions with a shared interface, as a result of differing rates of ionic diffusion. This is explained here: https://en.wikipedia.org/wiki/Liquid_junction_potential

Maybe if the questioner returns we can find out a bit more about what the issue is.

 

[adianadiadi]

Thank you. It is new for me.

Different metals have different Fermi levels - that is the top of the filled band, or perhaps some where in the conduction band if metal is hot. Think of it as the highest energy electrons.
Both metals are uncharged when separated, but when contacted, electrons from the one with the higher Fermi level will flow into the metal with the lower Fermi level until they are equal.
Now the one that got electrons from the other is negatively charged and the one they came from is postively charged - That difference is the junction potential.