[Histonet] Crystallized formalin

[John Kiernan]

The formaldehyde in formalin is present as low polymers. The depolymerize when the liquid is diluted. With storage, especially at lower temperatures, the small polymers join together anf form large insoluble polymers, known as paraformaldehyde. This reduces the effective concentration of formaldehyde in the fixative solution. (At a guess, it might be down from 4% to 3%. ). This probably doesn't matter. 

Read about formalin and formaldehyde at Microscopy Today 08-01: 8-12 (2000). This is a free download from Microscopy Today's web site, and it includes references to more significant publications. 

Paraformaldehyde is easily depolymerized. Increase the pH to about 8 with a bead or two of sodium hydroxide, and heat to 60-65C. The paraformaldehyde depolymerizes ("dissolves") and you go ahead with dilution and buffering to make a 4% NBF. This information is in every histotechnical textbook published since about 1965. 

Your description of "crystals" in old formalin puzzles me. The paraformaldehyde usually settles out as as a finely granular white powder. 

John Kiernan
London, Canada
= = =
On 26/05/15, Julio Benavides  <j.benavides at eae.csic.es> wrote:
> Hi there,
> 
> we have a bottle of formaline 40% with crystals at the bottom. Can we used it? is there any way to re-suspend the crystals? Better dispose of it? and, BTW, do you know why it crystallized? by its side there is another bottle, same batch, with no crystals.
> 
> thanks a lot your comments
> 
> Regards
> 
> Julio
> 
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