4. Potentials and Electrodes

A reference electrodes is an electrode acting as a ...

reference against which other potentials can be measured.

Desired characteristics for reference electrodes:

 Known, constant and reproducible potential/voltage (E ref)
 Reversible interfacial electrode reaction
 Insensitive to chemical species in the electrolyte solution where they are used in.
 Follow Nernst equation
 Robust and easy to use

Different types of reference electrodes

The primary reference electrode:
- Normal hydrogen electrode (NHE)
- Provides the zero point for the electrochemical potential scale.

Secondary reference electrodes:
- Their potentials are not zero but exactly known in relation to the NHE.
-Ag/AgCl electrode
- Calomel electrode (seldom used, contains mercury)

Normal hydrogen electrode (NHE)

Is a electrode with stable and reproducible potential with a potential of 0 V by definition.

Liquid junction potential (E'j)
where it develops?
Where is it estimated?
what does it depend on?

 Develops at the interface between two dissimilar solutions.
 Can be estimated with the Henderson equation.
 The liquid junction potential depends on the ion mobilities.

Liquid junction potential (wikipedia)

occurs when two solutions of electrolytes of different concentrations are in contact with each other. The more concentrated solution will have a tendency to diffuse into the comparatively less concentrated one.

Which is the cathode and anode?

Xa(s)│Xa2+(aq) || Fo+(aq)│Fo(s)

THe left is the anode? and the right is the cathode

Case 1 for liquid junction potential

Two solutions of the same electrolyte with different concentrations.

Case 2 for liquid junction potential

Two solutions with the same concentration having a common ion.

Case 3 for liquid junction potential

Two different solutions with different concentrations.

The liquid junction potential (E 'j) depends on...

charge, mobility and concentration of ions