Precipitate on natural history specimens

Alfred VanDeGeer <a.vandegeer [at] reading__ac__uk> writes

>On behalf of the curator of the Cole Museum of Zoology in Reading
>UK:
>
>    We are undertaking conservation work of natural history
>    specimens in liquid preservatives, many of which have been
>    neglected for several years, sometimes decades. When topping up
>    specimens stored in 4% buffered formalin, we have noted that
>    sometimes a white precipitate forms. The pH of the old formalin
>    is usually between 5 and 7 and the pH of the new formalin is
>    neutral. This happens more frequently if we use formalin we have
>    made up ourselves, rather than formalin purchased already
>    buffered, although it has happened with commercial formalin as
>    well. We suspect that we are seeing salt precipitation, but are
>    not sure what exactly is causing this. Does anyone have any
>    suggestions for what is happening?
The cause of this is temperature fluctuation which causes
polymerisation of the formaldehyde molecules in solution linking to
form a mix of polymers: meta- and para-formaldehyde which appear as
a white suspension becoming more heavy and, in extreme cases, I have
seen an entire (small) jar with its entire contents solidified in
this manner. The polymers are unfortunately, insoluble in water and
have to be taken through an alcohol ladder to about 70% strength and
this gradually dissolves away the polymer. Sometimes this can also
occur by reaction with buffer salts and I still use the sodium
hydrogen phosphates mixture as a buffer. With all good wishes,

Simon Moore, MIScT, FLS, ACR,
Senior Conservator of Natural Sciences.
Hampshire County Council
Recreation and Heritage Department,
Museums and Archives Service,
Chilcomb House, Chilcomb Lane,
Winchester SO23 8RD. UK.
+44 1962 826737


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                  Conservation DistList Instance 21:35
                Distributed: Tuesday, December 18, 2007
                       Message Id: cdl-21-35-004
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