[agent1594]

This complexometric analysis is done to determine Al³⁺ concentration of a sample. A back titration is conducted since Al-EDTA complex formation is too slow to determine an end-point.

For the back titration of excess EDTA and Zn²⁺, Al-EDTA stability constant should be higher than that of Zn-EDTA. Else, Zn²⁺ can displace EDTA from Al-EDTA complex.

In a back titration an excess of EDTA is added to the metal ion solution, and the
excess EDTA is titrated with a known concentration of a second metal ion. The second
metal ion must form a weaker complex with EDTA than the analyte ion so the second
metal does not displace the analyte ion from its complex with EDTA.

But Al-EDTA stability constant(16.13) is lower than Zn-EDTA stability constant(16.50).

How come this titration still possible?
Thanks.

[Borek]

Isn't it a matter of kinetics? EDTA/Al reaction is so slow (in both directions) you can reach the end point before substantial amount of Al is replaced by Zn.

[agent1594]

Isn't it a matter of kinetics? EDTA/Al reaction is so slow (in both directions) you can reach the end point before substantial amount of Al is replaced by Zn.

So does it mean the saying, "The second metal ion must form a weaker complex with EDTA than the analyte ion so the second metal does not displace the analyte ion from its complex with EDTA." is not always true, that even though the second complex is more stable than the first one, the titration is still possible?

[Borek]

So does it mean the saying, "The second metal ion must form a weaker complex with EDTA than the analyte ion so the second metal does not displace the analyte ion from its complex with EDTA." is not always true, that even though the second complex is more stable than the first one, the titration is still possible?

Can be possible, in some specific cases - yes.

[agent1594]

Can be possible, in some specific cases - yes.

Another side question...
Does the pH of the solution has a relation with the pKa4 of EDTA?
For instance, someone told me that we use the buffer of pH 10 in the titration to find the total hardness of water since it is closer to the pKa4 of EDTA which is 10.3 .
I feel that the above statement make no sense. There is a relation between the pH of the solution and the stability constant. I think that this relation is based on experimental data. For instance, the pH is selected to be 10 since Mg²⁺ does not precipitate as Mg(OH)₂ at that pH and both Ca²⁺ and Mg²⁺ forms sufficiently stable complexes in that high pH range. There is no mathematical relationship there where we can calculate a suitable pH for the titration. Am I correct?

[Borek]

Does the pH of the solution has a relation with the pKa4 of EDTA?

In a way.

For instance, someone told me that we use the buffer of pH 10 in the titration to find the total hardness of water since it is closer to the pKa4 of EDTA which is 10.3 .

I wouldn't put it that way.

There is a relation between the pH of the solution and the stability constant. I think that this relation is based on experimental data.

Technically it is all based on the experimental data. However, when we know all pKa values and stability constant for the EDTA^4- + Yⁿ⁺  YEDTA^n-4 reaction, we can calculate conditional constant for any pH, as knowledge of pH allows us to find the concentration of EDTA^4- in the solution (which is a function of the total EDTA concentration and pH).

For instance, the pH is selected to be 10 since Mg²⁺ does not precipitate as Mg(OH)₂ at that pH and both Ca²⁺ and Mg²⁺ forms sufficiently stable complexes in that high pH range.

Yes, but these things can be calculated from the conditional stability constant and the solubility product for Mg(OH)₂.

There is no mathematical relationship there where we can calculate a suitable pH for the titration. Am I correct?

This final statement is incorrect - we can definitely calculate optimal pH based on known Ksp, Kf and pKa values.

[agent1594]

This final statement is incorrect - we can definitely calculate optimal pH based on known Ksp, Kf and pKa values.

Is there an equation connnecting Ksp, Kf, pKa, and optimal pH?
What it is? This is beyond our scope but I really want to know.
Thanks.

[Borek]

There is no simple equation - system at equilibrium is described by a system of simultaneous equations. You define what effect you need (ie: you define what you mean by optimal pH - in this case it will be probably the lowest amount of non-complexed Ca²⁺ and Mg²⁺) and then you solve the system with a condition given. Rather basic undergraduate math.