[Histonet] Re: CBG recyler and recycled Formalin

[John Kiernan]

Bob Richmond makes an important point that should also be made clear by any company selling apparatus to recover formaldehyde by distillation of diluted formalin. The recovered product recovered from an aqueous fixative will be an aqueous solution of formaldehyde (and its low polymers), of uncertain concentration. 

Distillation of full-strength formalin (37%w/w = 40%w/v formaldehyde), done at atmospheric pressure, yields 20-30% of formaldehyde in the distillate, and leaves a higher concentration of formaldehyde and polymerization products in the still. Vacuum distillation or pressure distillation can change the yields to favour the collection of hydrated formaldehyde (methylene glycol) in the distillate. I'm summarizing Chapter 8 in Walker, JF (1964) Formaldehyde, 3rd edition. ISBN 0882752189. Walker's Chapter 8 tabulates conditions for distilling formaldehyde from various concentrations, and takes into account the methanol (about 5%, included in formalin to retard polymerization). 

The notion of an expensive machine for recycling formalin in a histology lab makes little sense. Why not simply filter (if necessary) and re-use the fixative after removing the specimen? It is still neutral, buffered formaldehyde. 

John Kiernan
London, Canada
= = =
On 28/02/15, Bob Richmond  <rsrichmond <@t> gmail.com> wrote:
> Ryan Roy HTL (ASCP) in Manchester NH asks:
> 
> >>We are getting a new CBG that recycles xylene , alcohol, and formalin. We
> purchase buffered formalin. Does anyone know if after recycling the
> recycled formalin would or would not need be re-buffered?<<
> 
> If you distill buffered formalin, the formaldehyde is going to distill
> over, but not the buffer phosphate, which will remain in the still pot. I
> suppose you can buy phosphate mixtures to make Lillie's neutral buffered
> formalin anew, using your recycled formaldehyde.
> 
> I think you also have to measure the concentration of formalehyde in the
> distillate, and dilute accordingly.
> 
> Bob Richmond
> Samurai Pathologist
> Maryville TN
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