We make up 4% paraformaldehyde in PBS (Dulbecco's), store at 4C, for
months.  If any white precipitate forms, the solution is discarded.  I am
not sure what solubility of PFA is and it may not stay in solution.  We
prefer 4% and dilute with DPBS to 1 or 2% when needed, this way the
concentration of buffer is always the same, ready for dilution with same
buffer (we store some DPBS in cold) rather than have conc PFA in conc buffer. 



Gayle Callis
Veterinary Molecular Biology
Montana State University - Bozeman
Bozeman MT 59717-3610

406 994-6367
404 994-4303 (FAX)