Application of Nernst Equation in Potentiometric titration of Fe2+ with Ce4+

It is well Known that to find $$E_{cell}$$ during titration of $$Fe^{2+}$$ with $$Ce^{4+}$$ we consider 3 domains:

Before Equivalance point , $$E_+ = E_{Fe^{3+}/Fe^{2+}}^o+\frac{RT}{nF}log\frac{[Fe^{3+}]}{[Fe^{2+}]} \tag{1}$$

At Equivalance point , $$E_+=\frac{E_{Fe^{3+}/Fe^{2+}}+E_{Ce^{4+}/Ce^{3+}}}{2} \tag{2}$$

After Equivalance point ,$$E_+ = E_{Ce^{4+}/Ce^{3+}}^o+\frac{RT}{nF}log\frac{[Ce^{4+}]}{[Ce^{3+}]} \tag{3}$$

$$E_+$$ is the electrode potential of the positive electrode where the negative electrode is a fixed reference electrode.

In the book Quantitative Chemical Analysis by Daniel C Harris, Chapter 15 says that both equation 1 and 3 can be used either of the two domains and the reason why one is used instead of the other is due to the fact that the ratio of the concentration of a particular metal ion in different oxidation state is much easily found in that domain while the other ratio usually requires a complicted equilibrium equation to be solved to obtain the solution.

my questions:

1. Does this mean that if by some means I knew the ratio of concentrations of both metals I could substitute it in eqn 1 and eqn 3 and end up getting the same value for $$E_+$$.

2. If not does it only hold near the Equivalance point?

3. If one is True , The only explanation I have is that both $$Fe^{2+}$$ and $$Ce ^{4+}$$ are in equilibrium with the same electrode. But this still doesn't make sense as usually when 2 reactions are occurring at the same container the $$\Delta G_{net}=\Delta G_{1}+\Delta G_{2}$$ So in some sense shouldn't $$E_+ =E_{due-to-Fe}+E_{due-to-Ce}$$ be the equation we require?

If I'm missing some essential concept it would be Great if someone could point it out and probably provide some links for me to look up.

• The two closing votes say this is off topic because it is not related to chemistry. This is quite absurd and strange. Potentiometry is a standard course material in modern analytical chemistry curricula. Sep 20 '19 at 17:22