here is one who roughly knows the difference (but unfortunately is not
a good Enslish speaker/writer).
SHORT STORY: protocols reading '4% paraformaldehyde' just indicate a 4%
formaldehyde solution that has been prepared freshly from
NOVEL: Receipies reading "...4% paraformaldehyde in PBS..." are a
little confusing since although you prepare it by adding 4%
paraformaldehyde to your PBS solution, what you obtain is in fact a
(approx.) 4% formaldehyde solution.
As you will know, formaldehyde is a gas readily soluble in H2O.
Paraformaldehyde, however, is solid. It simply is a polymer, and
formaldehyde is the corresponding monomer. Paraformaldehyde forms
spontaneously over time in a concentrated formaldehyde solution,
preferrably at low temperatures. It's the junk you might find in older
stock solutions. In this case you would call the stock 'spoiled'. If
you need it just for rather coarse purposes, you can dissolve the stuff
by heating the solution - Iwouldn't recommend to use this for
As opposed to the junk that forms in your bottles, commercial
paraformaldehyde (a powder) is a basis to prepare very pure
formaldehyde solutions. Immunocytochemical protocols recommend
paraformaldehyde, and most protocols stress that it should be prepared
immediately before use. I don't know what kind of reactions else take
place in a formaldehyde solution besides the formation of the polymer,
nor do I know whether those by-products -or the polymer itself- have
any impact on immunocytochemistry.
However, if you prepare your formaldehyde solution freshly before you
use it, all the paraformaldehyde will be dissolved and you will have a
very pure and fine fluid.