Why is an electrodes standard reduction potential does not depend on amount of chemicals?

The standard reduction potential is fixed for the type of chemical reaction. Why is it so? Why is an emf of a cell or an electrode's Standard reduction potential does not depend on say, quantity of chemical, concentration of chemical present in cell etc. If there are more chemicals, then more ions. Then why does the potential increase with increase in amount of chemicals?

Suppose we have some cell with a reaction:

$$A + B \rightleftharpoons C + D$$

the the cell emf does depend on the concentrations of all the reagents. Specifically we can write it as:

$$E = E^\circ - \frac{RT}{zF}\ln\left(\frac{a_Ca_D}{a_Aa_B}\right)$$

where the $$a_A$$ etc are the activities of the chemicals in the cell and $$E^\circ$$ is a constant. This constant $$E^\circ$$ is the standard cell emf, and it's just the cell emf when everything is as unit activity. It's a constant because it's defined that way.

We use the standard emf because it's a nice way to separate out concentration effects from the fundamental properties of the cell. We use the same idea for electrode potentials i.e. we write them as a constant plus a term that depends on concentration, and the constant part is then the standard electrode potential.

• Thank you. Is there a way to calculate Eo from a chemical reaction's fundamental properties? I am just looking at a way to understand what gives a cell it's emf – Karthick S Mar 26 '19 at 18:21
• @KarthickS yes there is because it is related to the Gibbs free energy change $\Delta G$ for the reaction. The Gibbs free energy is basically a form of potential energy, and the cell EMF is also a potential energy, and the two are proportional. $\Delta G = -|z|FE$ where $E$ is the cell potential and $F$ is a constant of proportionality called Faraday's constant. $z$ is the number of electrons transferred in the reaction. – John Rennie Mar 26 '19 at 18:33
• @Karthick S basically it is really intrinsic to the species involved. Microscopically you can see it as potential between particles. All the rest brings quantities to a easy to handle and practical amounts. – Alchimista Mar 27 '19 at 10:07