EGTA seems to have much higher affinity for calcium ions, whereas the more popular EDTA seems to bind with many divalent ions like calcium, magnesium, etc. So if a wastewater sample already been contaminated with EDTA, will adding EGTA cause calcium ions already chelated to EDTA to unbind and go for EGTA? If so, what is the scale we are talking about here? I appreciate your assistance in helping me understand this.
Basically, you have a multicomponent equilibria problem.
Keq 1 = [Ca•EGTA]/[Ca][EGTA]
Keq 2 = [Ca•EDTA]/[Ca][EDTA]
where; [Ca•EGTA] and [Ca•EDTA] are the concentrations of the two bound calcium complexes, [Ca] is the concentration of unbound calcium, and [EGTA] and [EDTA] are the concentrations of the the calcium-free chelates. All concentrations are those at equilibrium. You can apply the material balance to convert these equilibrium relations to the total concentrations with a little algebra (i.e.,
[EDTA]total = [Ca•EDTA]+[EDTA]
[EGTA]total = [EGTA] + [Ca•EGTA]
[CA]total = [Ca•EDTA]+[Ca•EGTA]+[Ca] )
Bottom line is that no matter how high the Keq constant, there will always be some free Ca, and that Ca will become bound to the EGTA. By mass action the EDTA will then re-equilibrate, liberating more free Ca.
Luke V Schneider Thanks for breaking down the problem and giving me the keywords to better research this.
Luke laid out the main analysis nicely; however I wonder about the time rate of the re-equilibration reactions? It is my understanding that EDTA binds calcium quite tightly, so I wonder how slowly the re-equilibration with EGTA will be approached after EGTA is added?
Also, in his analysis Luke did not account for the occupation of some of the total EDTA with other cations such as magnesium, etc.-- the addition of similar equilibrium expressions such as Luke has shown, for each of these other cations, can address the infuence of these other cations on calcium speciation with EDTA and EGTA.
The kinetics of EDTA aren't instant, but over an hour you will see displacement occur. This is easily seen in Capillary Electrophoresis where the a Ca•EDTA solution injected into a column containing a running buffer based on Mg2+. You can vary the speed of elution with the field strength to allow more or less equilibration time, but you will get two EDTA peaks at the end that shift in heights, and if you run with imidazol you will see both the free Ca2+ and Mg2+ species as negative peaks in the imidizolium-based buffer system.
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