Nernst Theory of Boundary Potential Difference-Solution Pressure

When a metal plate is immersed in a solution of the salt of the same metal, the positive ions from the plate go into the solution. This is due to a force exerted by the metal on the solution. It is called the electrolytic solution pressure (P) of the metal. Its magnitude depends on the nature of the metal and the solution. The more reactive metals like Zn have very high solution pressure while the less reactive metals like Cu have very low solution pressure. Further, there is a tendency of the part of the solution due to the osmotic pressure (p) to deposit positive ions from it on the metal.

(i)    If the solution pressure is more than the osmotic pressure of the solution (p>p), the plate gives away more positive ions than what it receives from the solution. It therefore indicates a negative charge on its surface. On the liquid side of the boundary between the metal and the electrolytes, the solution indicates a positive charge. Hence, an electric field exists at the boundary with the metal at a lower potential than the solution.

Example: Let an zinc plate be partly immersed in zinc sulphate solution. The solution pressure of zinc is greater than the osmotic pressure of a solution. Hence, the zinc plate is at a lower potential than the zinc sulphate solution at the boundary.



(ii)    If the solution pressure of the metal is less than the osmotic pressure of the solution (P<p), more positive ions are deposited on the metal than the number of positive ions passing from the metal to the solution. The metal is at a higher potential than the solution.

Examples: Let a copper plate be partly immersed in copper sulphate solution. The solution pressure of copper is less than the osmotic pressure of the copper sulphate solution.
So the copper plate is at a higher potential than the copper sulphate solution at the boundary between the metal and the solution.
The explanation was first give by Nernst and is known as Nernst’s theory of boundary potential difference.

Consider a porous pot containing zinc sulphate solution in which a zinc plate is dipping. If this is placed in a vessel containing copper sulphate solution with a copper plate dipping in it, the potential difference between the copper and zinc plates is given by the algebraic sum of the potential of copper and zinc are dipped sulphuric acid contained in a vessel, there will be contact potential difference existing between each of the metals and the acid.