Trisodium citrate is commonly used as a medical anticoagulant and works by binding divalent cations. When the concentration of free calcium falls below about 225 nmol, coagulation cannot proceed.
Citrate also binds magnesium and I have found plenty of references that state when performing regional anticoagulation with citrate, such as with a dialysis circuit, the patient will lose both Ca and Mg. What I have been unable to determine from the literature is what the relative binding affinities are. The reason I need to know is that I want to chelate enough Ca to reduce ionized calcium from the typical physiologic range of 1,100-1,400 nmols to down to a specific level of 350-400 nmols, but no further. I will have a known volume of bone marrow and the staring concentrations for Ca and Mg, so all I need to know is that amount of citrate to add to yield the desired concentration.
The problem is that I do not know how aggressively citrate will bind to Ca vs. Mg. The part that binds to Mg will be unavailable to bind to Ca, so I will need to add more than the theoretical amount required to reduce just the Ca concentration, but if Mg is a weak competitive binder I run the risk of reducing the Ca too far. I could figure this out via experimentation, but acquiring fresh bone marrow samples to play with is a non-trivial procedure.
Does anybody have a suggestion on how to estimate the amount of Ca and Mg that will be bound for a given amount of citrate? Even better, if anybody can point me to a reference article that discusses this in detail that would be great. Several hours of searching on Google and PubMed have not been fruitful.