How Does Mg2+ Complex Formation Influence Ca2+ Concentration?

[Undergrad Chemist]

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Summary:: Requesting help!

Mg2+ (aq) is also likely to form a complex with the ligand or ligands in your chosen complex. Show how the formation of the Mg2+ complex will affect the concentration of Ca2+ (aq).

More details in post below

Recap of the question: Mg2+ (aq) is also likely to form a complex with the ligand or ligands in your chosen complex. Show how formation of Mg2+ complex will affect the concentration of Ca2+ (aq).

I am unsure of how the formation of Mg2+ complex will affect the concentration of Ca2+ (aq).

My chosen complex is NTA

Considering the Kstab of [Ca(NTA)]- = 10^6.41 and the Kstab [Mg(NTA)]- = 10^5.46, I see that the Ca complex will form preferentially but I don't know how the Mg ions will effect this formation as they also form very stable complexes. Any help with the question or an explanation would be highly appreciated

 

[Borek]

This is less about ligands and complexes, more about complexes and equilibrium.

First of all - if you add Mg²⁺ that competes for the same ligand, will the amount of complexed Ca²⁺ go up, or down? What will happen to the concentration of a free Ca²⁺?

Quantitatively - start with system of equations describing the system. Expressing concentration of free Ca²⁺ as a function of amount of Mg²⁺ present won't be trivial, try and see where it leads. I would try to use side-reaction-coefficients.

 

[Undergrad Chemist]

Thank you for your quick reply!

Confirming the equation is Mg2+ (aq) + calcium complex <-> Ca2+ (aq) + magnesium complex?

The adding of Mg2+ would reduce the amount of complexed Ca2+ as the equilibrium favours the product side, thus the concentration of free Ca2+ ions will increase? How can I express the concentration of free Ca2+ as a function of Mg2+?

What would happen if the complex was added to a solution of Mg2+ and Ca2+ ions assuming Ca2+ ions are preferentially complexed?

The Kstab of the Ca complex is 9 times that of the Mg complex, how does this affect the production of Mg complexes?

Sorry for all the questions, I'm just trying to understand this topic as much as I can and then hopefully answer the question!

Thank you, look forward to your reply.

 

[Borek]

Confirming the equation is Mg2+ (aq) + calcium complex <-> Ca2+ (aq) + magnesium complex?

One of many ways of skinning that cat. Note that equilibrium for this reaction can be easily calculated from stability constants of both complexes (try to do it, once you have the constant you can try to use ICE table for calculations, although it will yield a 4th degree polynomial, not something trivial to solve).

The adding of Mg2+ would reduce the amount of complexed Ca2+ as the equilibrium favours the product side, thus the concentration of free Ca2+ ions will increase?

Actually it doesn't matter which complex is stronger - adding a cation that competes for the ligand will always have this effect. How strong the effect is is a matter of the stability constants.

How can I express the concentration of free Ca2+ as a function of Mg2+?

As I told you - you need to try to derive the solution from the first principles. Treating the reaction with Mg as a side reaction should help, this approach is often discussed in analytical chemistry books.

Sorry for all the questions, I'm just trying to understand this topic as much as I can and then hopefully answer the question!

A lot depends on what kind of answer is expected, qualitative one is trivial, quantitative one much more difficult.