What other cations are present?  Any divalent cations like Mg++ or Ca++?

The Ksp of magnesium phosphate is about 10^-24, so even if you have a very small amount present, say as a contaminant with citrate or EDTA, it will crystallize.

On Feb 21, 2011, at 1:22 PM, Yibin Lin wrote:

> Dear all,
>
> I got a lot of salt crystals in reservior solution (well solution), which contains 0.1 M phosphate/citrate ph 4.2, PEG200 47%, EDTA-2Na 0-22mM. Reservior solution appears crystals from 12mM EDTA. Could someone help me to explain why?
>
> Thank you very much!
>
> Yibin
>

William G. Scott

Contact info:
http://chemistry.ucsc.edu/~wgscott/