[Rutherford]

In a galvanic cell I have two separate solutions, so the potential is calculated by subtracting the potentials of the solutions. In potentiometric titration I got one solution, so its potential is calculated by using only on process (ox. or red. depends what is in excess, when ox. is in excess I get a positive vault, when red. is in excess I get a negative vault). The equivalent point is obtained when both processes make the same potential (not when equal amounts of ions reacted). Now I am asking, what means the excess when the potential is calculated before the EP? It shouldn't be the excess in concentrations, right? If something I wrote is wrong, please correct it.

[Borek]

The equivalent point is obtained when both processes make the same potential

No. Both processes MUST have the same potential always. You can't have two different potentials in a single solution.

http://www.titrations.info/potentiometric-titration-curve-calculation

[Rutherford]

I see that at the equivalence point they should be equal.
"Both processes MUST have the same potential always. You can't have two different potentials in a single solution."
For instance, if I start titrating Fe³⁺ with Ti³⁺ and I add just a drop of the titanium solution (Fe is in excess) then the potentials can't be equal for both processes. It should be much lower for the titanium oxidation.

[Borek]

For instance, if I start titrating Fe³⁺ with Ti³⁺ and I add just a drop of the titanium solution (Fe is in excess) then the potentials can't be equal for both processes. It should be much lower for the titanium oxidation.

Then you don't understand what it is all about and what equilibrium means. Both potentials will be the same, no matter how many times you will repeat they can't. They have to. You can't have two different potentials in a single solution.

As if I have not already wrote it.

[Rutherford]

I mean the potential before establishing the equilibrium (when I use Q in Nernst equation and not K). Those two potentials before the equilibrium can't be same because the standard electrode potentials for both substances aren't same, and their concentrations aren't same, so E₁≠E₂. I don't understand how can they be equal, when math show that they can't.

[Borek]

Substances react till potential for both half cells is the same. Starting potentials in separate solutions are different, but when you mix them, they can't be different any longer.

[Rutherford]

But one half cell is a burette  .

[Borek]

But one half cell is a burette  .

In potentiometric titration I got one solution

So, what is the question - about separate potentials of initial solutions, or about potential in the titrated solution after some of the titrant was added?

[Rutherford]

"potential in the titrated solution after some of the titrant was added"
This. After only a drop of the titrant is added, by maths, the potentials of oxidation and reduction in the solution can't be same.

[Borek]

"potential in the titrated solution after some of the titrant was added"
This. After only a drop of the titrant is added, by maths, the potentials of oxidation and reduction in the solution can't be same.

Then your math is wrong, they are the same.

Actually fact that the potentials are the same is what allows us to calculate concentrations, not the other way around.

[Rutherford]

You are probably right, but I still don't understand so I will write the equation too see what is wrong, E₁ is for Fe, E₂ is for Ti:
E₁=0.77-0.059log[Fe²⁺]/[Fe³⁺], let's say that 25% of iron was titrated and its initial concentration is 0.01 M, then:
E₁=0.77-0.059log(25/75)=0.8V
Now for titanium (I think that one equivalent if the c=2.5*10^-3M was needed for the 25% ):
E₂=-0.13-0.059log[Ti⁴⁺]/[Ti³⁺]=-0.13-0.059log(x/2.5*10^-3)
Now, you mean that the concentration x is an amount that when putted in must make that E₁=E₂? It can't be calculated through K_eq because this is not an equilibrium?

[Borek]

It is an equilibrium. Otherwise you are right - you calculate E₁ from known concentrations of Fe³⁺ and Fe²⁺, then you use this potential to calculate x (concentration of Ti⁴⁺). Actually you use this potential to calculate ratio of Ti⁴⁺/Ti³⁺ concentrations and you use known total amount of Ti to calculate [Ti⁴⁺], but that's just a technical detail (besides, before equivalence point you can usually safely assume [Ti⁴⁺] is close to zero and [Ti³⁺] is that of added titrant).

Which is exactly a procedure described on the page I linked to in my first post in the thread.

[Rutherford]

Okay, thanks for explaining, I started to understand this.
Only the potential will be - when using Ti and + when using Fe, but the amount is same.