Fixatives difference - Formalin Vs Formaldehyde Vs Paraformaldehyde (Dec/16/2009 )

Dear all

I recently resolved to approach the confusing world of bacterial cells fixation for DNA staining. Among all the fixatives, three of them caught my interest (especially because we have them on stock in the cabinet).

Formaldehyde
Formalin (which is a saturated solution of Formaldehyde)
Paraformaldehyde (which is a polimer of Formaldehyde)

In the methods I have been reading, it looks like they are used quite interchangeably. Would anybody know if there is any substantial difference between these three and whether one of them is in any way superior to the other two?

Many thanks

If my memory serves me right, Formaldehyde is actually a gaz, Formalin, the liquid form and paraformaldehyde is a solid (a powder).

Yes, they are all basically the same. If you are interested in fixing stuff you need either formalin or formaldehyde. As soon as you dissolve paraformaldehyde in PBS it turns to monomers though (AKA formaldehyde), so anything will do really.

One thing to note about the naming is the concentrations. This mistake happened recently on our lab. A fully saturated formalin solution contains about 40 % (v/v) formaldehyde. Most fixing requires a 4 % Formaldehyde concentration, so that is 10% of a fully saturated formalin solution (and not 4%). Bottom line: Use formaldehyde percentages.

Formalin and Formaldehyde are the same thing just a difference in concentration. 10% Formalin is 3.7% Formaldehyde. 10% formalin is like a misnomer.

Solution of Paraformaldehyde is formaldehyde.Formaldehyde = solution of paraformaldehyde.

So, all the 3 are the same thing. For experiments where we need purest form of Formaldehyde, we make fresh solution of formaldehyde by dissolving paraformaldehyde. Stored formaldehyde might not be pure and might have small amounts of alcohol in it.

(Correct me where I am wrong)