[pioyi]

Sorry for the trouble. I think I figured it out!
The equation that I was using was already based on the fact that the total potential was zero, the cell was forced to be at equilibrium so that the reverse reaction would start happening...
n (Ε_working^o - E_ref - Ε_applied) / 0.05916 = log (1/ [Cu²⁺]).

Thus there is indeed an onset potential, which depends on the conditions of the experiment, and can be found be solving for E(applied) in the equation above.

I think that's it... I accept any objections to that

One of the first things that I learn about chemistry was the Nernst equation. I didn't ask anything more related to that and just accepted it (together with an oversimplified derivation).

Now, I know that I really haven't understood the most basic part of electrochemistry...
 
It all started when I was reading about linear sweep voltammetry.
Let there be a solution of copper ions (1M for example). We are starting with a positive potential and we scan to a more negative one until the Cu²⁺ are reduced in a pre-determined percentage. Skoog says that the potential needs to surpass a certain value for the reduction to begin.

That's where I object. Why does the potential needs to surpass a certain value? Why must it be digital in nature (the reaction doesn't happen before but now it is suddenly happening) and not analogue?

If the reaction was reversible and fast I would assume that the nernst equation would always hold:
n (Ε_working^o - E_ref - Ε_applied) / 0.05916 = log (1/ [Cu²⁺])
And the current would be governed by it.

I do understand that the external potential needs to surpass that of the E_cell^o so that the reaction goes in the inverse direction, just like batteries. But what about the Nernst equation? Why wouldn't it hold for lesser (by absolute value) external potentials? It is not specified...

Even in a galvanic cell, if I have placed too many products in the solution, shouldn't the inverse reaction start taking place? (in a dynamic matter). In that way the reaction quotient would be transformed to the equilibrium constant, and equilibrium would be achieved.

I really should read Bard sometime, but there is just too much to learn! (That's an excuse, I should really study harder...)