I was challanged by a molecular biologist in my lab, today.  He is
saying that once paraformaldehyde is made into solution, it should work
the same as formalin.  I always make a fresh paraformaldehyde before the
perfusion fixation for lacZ staining in a mice tissue.  I always make a
fresh one to make sure the every tissue are fixed in the same condition.

1) What is the difference between fresh made paraformaldehyde and
formalin, chemically?
2) Why does the red bible recommend to use 4% paraformaldehyde at 4 C.
Is it to make the light fixation?

Thank you in advance.

-- 
Masayuki Miyagishima, MD
University of Pittsburgh, 
Department of Surgery
412-647-2345, and ask the hospital operator to page 3228, please.