Buffer Action and Titration: Does the Process Ever End?

[Titan97]

If H₂A²⁺ is being titrated with NaOH, a buffer forms.
The processes taking place are equilibrium between H2A²⁺ and HA⁺ ions, autoprotolysis of water and reaction of OH- ions with H₂A²⁺ ions. As NaOH is added, the concentration of acid tends to decrease but because of the equilibrium
$H_2A^{2+} <=> HA^+ + H^+$, the decrease in concentration of acid is negligible.
Adding NaOH increases the amount of conjugate base which in turn increases the concentration of acid. Does this process never end? Whatever NaOH you add, the reacted Acid will form back.

 

[Borek]

Be very, very careful about your wording.

There is a total concentration of all forms produced during the acid dissociation - so called "analytical concentration". We often speak about this kind of concentration even in solutions that don't - technically - contain an acid (like Na₃PO₄).

Then, there are equilibrium concentration of all forms, of those some (H_3A, H₂A⁺, HA^2-) are acids (produce H⁺ during dissociation).

Finally, there is concentration of dissociated H⁺, which defines how acidic the solution is.

So, which of those you mean here:

Whatever NaOH you add, the reacted Acid will form back.

(And no, as stated, this is not true, when you add NaOH to the solution concentrations of all acidic forms - H⁺, H_3A, H₂A⁺, HA^2- - decrease).

 

[Titan97]

The last sentence answers my query. I was actually solving a problem : https://www.physicsforums.com/threads/titration-of-amino-acid-with-strong-base.831012/
There, my teacher told me that the concentration of acid at any point remains the same. Maybe you have seen it.
If I take sodium acetate and titrate it slowly with NaOH, NaOH reacts with acetic acid formed by hydrolysis of sodium acetate. This decreases concentration of acid.
But according to Le Chatelier principle, as acid decreases, more acetate ions get hydrolysed to form acid. So the concentration of acid remains unchanged while concentration of salt will change.

 

[Borek]

according to Le Chatelier principle, as acid decreases, more acetate ions get hydrolysed to form acid

Not exactly. Le Chatelier's principle tells you what general direction of the shift to expect, but not what the exact change will be. When you add base, acids get neutralized and pH goes always up. It is pretty easy to show that for each acid and conjugate base pair ratio of concentrations is

\frac {[HA]}{[A^-]} = 10^{pK_a-pH}

so if the pH changes concentration of the acid (HA in this case) doesn't stay constant.

But it is all the time not clear to me what you (and your teacher) mean by "acid" in this context. Total concentration? Equilibrium concentration? As long as the volume of titrant added is negligible, total concentration of the acid doesn't change, but equilibrium concentrations do change.

 

[Titan97]

But it is all the time not clear to me what you (and your teacher) mean by "acid" in this context. Total concentration? Equilibrium concentration? As long as the volume of titrant added is negligible, total concentration of the acid doesn't change, but equilibrium concentrations do change.

I mean total concentration of acid and not equilibrium concentration.

 

[Borek]

So the total doesn't change, but the concentration of HA goes down.