Storing formaldehyde solutions

Michelle Berry <mberry [at] museum__vic__gov__au> writes

>I am seeking information regarding the storage of large volumes of
>4% formaldehyde solution. ...
>...
> ... We are
>currently identifying risks and one of them is the potential affect
>of formaldehyde vapour on the historical and technological materials
>housed in the same space. The tank will be sealed however we expect
>that there will be some evaporation into the space. ...

What kind of container is the specimen in?  What sealant was used,
and how is the lid sealed on?  If the container is made of glass,
then your biggest concern will be  evaporation through the sealants
and the container closure. If it is in an acrylic container, you
have a much more serious problem.  In either case, you will not be
able to completely prevent the evaporation of formaldehyde but you
can take steps to reduce it considerably.

Andres van Dam and his colleagues at Leiden have published several
papers (below) on the permeation of formaldehyde through acrylic and
through various sealants.  Although the rate of passage of
preservatives through acrylic is slow, the process will cause the
containers to warp, crack, and discolor over time.  As preservative
volume is lost, temperature fluctuations cause the acrylic to flex,
which produces internal stress cracks.  As the preservative diffuses
through the acrylic, it carries with it some of the dissolved lipids
and proteins, which discolor the acrylic. The diffusion of the
preservative through the acrylic cannot be stopped, but the flexing
of the container walls can be regulated with the installation of a
valve to equalize pressure; unfortunately, the use of the valve
admits air into the container, which will oxidize the preservative
(instructions for making a simple but effective valve are in van Dam
et al. 2000).

No matter what material it is made of, you are likely to have
evaporation problems on a container this size if you cannot maintain
a very stable storage temperature.  An increase in ambient
temperature will increase the air pressure in the headspace of a
sealed container, resulting in increased stress on the closure.  The
internal container pressure depends on the vapor pressure of the
fluid, as well as the thermal expansion rate of the container, the
fluid, and the air above the fluid.  Volumetric expansion is a
temperature dependent variable. For a water-based preservative such
as 4% formaldehyde, an increase in temperature means a rise in the
fluid level in the container, which causes the compression of the
air in the headspace, which puts stress on the closure and seal.

Containers of fluid-preserved specimens should be stored at a
constant temperature, preferably a few degrees lower than the place
where they were sealed in order to create a slight negative pressure
in the container.  The ratio of volume of fluid to headspace is
important. One reason to fill containers full is to reduce the
amount of air available for oxidation, but the higher the jar is
filled with fluid, the greater the compression of the air volume,
and the greater the internal pressure, which can cause the container
or the closure to break.  A decrease results in the opposite effect,
which may also crack the container or the closure.  The current
recommendation is to fill a container to 90% of its volume for
ethanol-based preservatives, and to 95% of volume for water-based
preservatives to reduce stress on the closure from temperature
fluctuations.

    Horie, V.  1994.
    Environmental control for spirit collections.
    Biology Curator's Group Newsletter
    6(6):43-44.

    van Dam, A.J.  1997.
    Conservation of fluid preserved specimens, properties of
    sealants and their effect on preservation quality.
    Bulletin of the European Association of Museums of the History
    of Medical Sciences
    23:22-28.

    van Dam, A.J.  2000.
    The interactions of preservative fluid, specimen container, and
    sealant in a fluid collection.
    Collection Forum
    14(1-2):78-92.

    van Dam, A.J., J.P. M. van der Ploeg, G.J. M. Koper and D.
    Bedeaux.  2000.
    The warping and cracking of Plexiglas specimen containers.
    Collection Forum
    14(1-2):47-56.

John E. Simmons
Museologica
1528-1-2 Puddintown Road
State College, Pennsylvania 16801
303-681-5708


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                  Conservation DistList Instance 21:25
                 Distributed: Saturday, October 6, 2007
                       Message Id: cdl-21-25-003
                                  ***