[Nhcoll-l] long-term storage of amphibian larvae in formalin

[John E Simmons]

Dries,
As always I appreciate your observations based on your experience. A few
comments:

On Thu, Jul 3, 2014 at 1:46 AM, <A.J.van_Dam at lumc.nl> wrote:

>  Formalin is not stable!
>
Of course formalin is not stable, that is why it has to be buffered. If it
was stable, we could merely neutralize it and be done. In any case, I said
the buffering system was stable, not the formaldehyde.

We noticed in our fetus collection stored on phosphate buffered formalin
> (pH 7.3) that after ten years pH is around 6, after 20 years around 5, and
> after 30 years around 4 (close to unbuffered formalin).
>
What you are describing is acidification of a formaldehyde solution; what
Greg described as a solution becoming sufficiently alkaline to cause
clearing of tissues. One has to be cautious about extrapolating from one
sort of preserved specimen to another. A human fetus has a very different
surface-to-volume ratio than do amphibian larvae, more lipids (your
formaldehyde may be acidifying due to the breakdown of lipids into fatty
acids), and the overall specimen volume to preservative volume is likely to
be very different for a human fetus and a clutch of amphibian larvae--all
of these can be factors in pH change in formaldehyde solutions.

Therefore, when using buffered formalin as a preservation fluid, I
> recommend to change the fluid every 10 years.
>
 I advise monitoring the pH of the formaldehyde solution as well as
visually watching for signs of clearing (which would indicate a shift to
alkalinity) or evidence of acidification. If there are problems, then
either adjust the buffer or, in worse case scenarios, change the
formaldehyde solution.  I do not recommend changing preservatives unless
you have identified some problem that a fresh preservative solution is
likely to resolve. Changing to fresh preservative solutions (whether
formaldehyde-based or alcohol-based) can create other problems.

--John
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