What buffer are you using? EDTA complexes Ca and Mg, these ions
render it uneffective.
You write that your EDTA dissolves at 6%. For pure EDTA (the acid)
this is not true if you are using neutral range pH buffer (like Tris
or PBS). I assume that you have the sodium salt (as one obtaines when
neutralizing EDTA acid with NaOH in order to dissolve it. The pH
usually then is somewhere between 8 and 10).
If your protocol says "dissolve EDTA by adding NaOH (solid) until
the solution is clear and you are using sodium EDTA, the pH may be
the issue, esp. for the calcium ions going into solution.
Just my 2 ZL/
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