I understand the theory behind not freezing paraformaldehyde but I have been
storing aliquots at -20 in a NON frost free freezer for years (not the same
aliquot for years). According to Hogan et al "Manipulating the Mouse Embryo"
the repetitive freeze/thaw causes the solution to oxidize. But otherwise
they say go ahead and freeze. I have never seen a ppt in the fresh thawed
paraformaldehyde (4%) and don't have a problem with thorough fixation of the
tissue hmmmm. I do use phosphate buffered saline which is pH'd. anita