Confused?
- Formaldehyde is CH2O, the simplest aldehyde.
- Formalin is the name for saturated (37%) formaldehyde solution. Thus, a protocol calling for 10% formalin is roughly equivalent to 4% formaldehyde. Beware though, that some solutions have methanol in them to stop polymerization but this could have a negative effect on your sample.
- Paraformaldehyde (PFA) is actually polymerized formaldehyde. "Pure", methanol-free formaldehyde can be made by heating the solid PFA. This might be called paraformaldehyde, but it actually isn't because it’s not the polymer form. You can buy EM grade formaldehyde or you can make your own . . .
Making Paraformaldehyde Solution
4% paraformaldehyde is usually made in PBS or TBS at 70 °C with several drops of 5N NaOH to help clarify the solution. Prepare 4% paraformaldehyde solution in a chemical hood if you don’t want to be slightly fixed yourself.
Often PFA stocks have insoluble impurities and it's best that these be removed via a quick spin in a table-top centrifuge or by passing the prepared solution through a filter syringe. It is also important to realize that the efficacy and impurity content of powdered PFA can vary greatly from lot number to lot number of reagent. Don’t be surprised if your fixation concentrations & conditions may need to be tweeked when you open a new bottle of PFA.
You can store the solution but all solutions go bad with time so using freshly prepared solutions that are colorless is often best. (Storing aliquotes at -20 °C and using them over a couple of months is typical).