[Big-Daddy]

I have heard that I might be asked to manipulate acid-base equilibria at the same time as complexation equilibria (i.e. within a single system). Are there any examples of scenarios where this could be necessary? I have heard EDTA used as an example before. Can you outline the equilibria I might have to deal with (for instance, is it only the fully deprotonated form of the acid which is complexed one ligand at a time, or do I somehow have to create multiple formation constants for every form of the acid? if formation constants are given without it being said directly what reaction they refer to, should I assume Kf1 is the first ligand being added to the fully deprotonated form of the acid, Kf2 is the second ligand, etc., assuming the "ligand", i.e. what is being added to the acid-base mixture, is a metal) and say what forms of acid and ligand they refer to, and then I will attempt to produce the abstract polynomial formula from pH for the system you provide me with. This is practice for the problems I may get.

[fledarmus]

For the EDTA example, look at the structure of EDTA - http://en.wikipedia.org/wiki/File:EDTA.svg

When all four carboxylic acid groups on the structure are deprotonated, they can each form a strong coordinate bonds to the same large metal ion. Lone pairs on the nitrogen atoms can also form strong coordinate bonds to the metal ion, and the EDTA can wrap itself tightly around the metal ion with six points of attachment.

As you protonate the carboxylic acid groups, however, the bonds they can form to the metal ion are much weaker. EDTA^4- binds very tightly to large metal ions, H₄EDTA doesn't bind very tightly at all, and if you acidify the solution further you can protonate the amine groups as well, giving H₆EDTA²⁺, which being positively charged, actively repels metal ions. The four pKa values for EDTA are 1.99, 2.67, 6.16, 10.26, so you can see that the complex formation of EDTA with metals will depend on the pH of the solution.

[Big-Daddy]

For the EDTA example, look at the structure of EDTA - http://en.wikipedia.org/wiki/File:EDTA.svg

When all four carboxylic acid groups on the structure are deprotonated, they can each form a strong coordinate bonds to the same large metal ion. Lone pairs on the nitrogen atoms can also form strong coordinate bonds to the metal ion, and the EDTA can wrap itself tightly around the metal ion with six points of attachment.

As you protonate the carboxylic acid groups, however, the bonds they can form to the metal ion are much weaker. EDTA^4- binds very tightly to large metal ions, H₄EDTA doesn't bind very tightly at all, and if you acidify the solution further you can protonate the amine groups as well, giving H₆EDTA²⁺, which being positively charged, actively repels metal ions. The four pKa values for EDTA are 1.99, 2.67, 6.16, 10.26, so you can see that the complex formation of EDTA with metals will depend on the pH of the solution.

It is obvious that the complex formation will depend on the pH since EDTA is an acid, but does that mean that the complex formation constants (K_f values) will change depending on the pH? They would cease to be constants then!

So; are you saying that all forms of the acid from EDTA^4- to H₆EDTA²⁺ will form ligands with metals, but 1) the complex formation constant for complexes formed involving H6EDTA or even H4EDTA will be far smaller than the complex formation constant for EDTA4-, with the same metal ion doing the complexing, and 2) because EDTA is hexadentate in no form of the EDTA acid can a metal bind to more than 1 EDTA group?

Is it also accurate to say that the same is true for any acid or base - the fully deprotonated form of the acid (the anion A at the end) or the form of the base fully devoid of hydroxides (the cation B) will always be better at binding to metals (anion A) or ligands (cation B) respectively than forms with more protons (in the case of the acid) or more hydroxides (in the case of the base)? (So, whilst the formation constants may exist for other forms, they will be much smaller.)

[fledarmus]

They are still constants, but the constants are in reference to specific structures. Each protonated state will have a different binding constant to each metal.

Metal ions can bind to more than one EDTA, but due to entropy and sterics, it is usually not an energetically favorable outcome.

I'm not sure exactly what you are saying in your last statement, but in general, I believe negatively charged ligands form tighter bonds to cationic metals than neutral ligands do.

[Big-Daddy]

They are still constants, but the constants are in reference to specific structures. Each protonated state will have a different binding constant to each metal.

Metal ions can bind to more than one EDTA, but due to entropy and sterics, it is usually not an energetically favorable outcome.

I'm not sure exactly what you are saying in your last statement, but in general, I believe negatively charged ligands form tighter bonds to cationic metals than neutral ligands do.

Got it - and if you are instead titrating a base with ligands? The fewer hydroxides the base's cation has (i.e. the greater its positive charge) the more tightly it will bond to the ligands being introduced, but even forms with many hydroxides can bond to ligands (though of course they will do so much less frequently). Right?