# Availabilty of magnesium ions in phosphate buffer

Phosphate buffer is a common buffer in biological applications, it's especially popular for NMR. Magnesium ions are necessary for many biological systems to work, though the amount of magnesium ions you can add in a phosphate buffer is limited by the relatively low solubility of magnesium phosphate.

What I'm curious about is whether the phosphate buffer can affect the availability of magnesium ions even in concentrations where no visible precipitation occurs? Common phosphate buffer concentrations are around 10-100 mM, and magnesium ions are usually 5 mM or below. At those concentrations I see no visible precipitation, but is the magnesium actually fully available or is the phosphate sequestering it and thereby lowering the effective magnesium ion concentration in the solution?

Even just calculating the solubility of magnesium salts in phosphate buffer is not straightforward, as there are multiple species of magnesium phosphate and possibly even other salts like $\ce{MgKPO4}$.

Does phosphate buffer affect the availability of magnesium ions in solution compared to a non-interacting buffer? If that is so, is there a way to calculate the size of that effect?

• Could you provide a link to what phosphate buffer is and what it contains?
– F'x
Apr 29 '12 at 18:23
• @F'x Just potassium phosphate, not one of the fancier biological buffers. Apr 29 '12 at 18:38
• Technically, phosphate can and probably do form associates with magnesium ions. To find how strong, you should search databooks for relevant equilibrium constants, it is not such common info to keep it in mind if not needed. Aug 25 '12 at 10:21
• I am using a buffer which has approximately 50 mM Phosphate and 7 mM Mg, There is no visible precipitate. But my experiments are Mg sensitive and at least 5 mM Mg is needed for proper response. My preliminary experiments are indicating that in the presence of phosphate free Mg is limited.
– user862
Nov 19 '12 at 5:30