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Formaldehyde
based fixatives are the most widely used histological fixatives for routine
histopathology. Formaldehyde is a gas which is highly soluble in water and is
commercially available as an aqueous solution containing up to 40% formaldehyde.
This solution is usually referred to by its common trade name, “formalin”.
Thus, 40% formaldehyde is considered to be 100% formalin. Histological fixatives
contain formaldehyde at a concentration of 4%, usually referred to as 10%
formalin.
Formaldehyde
is most effective as a fixative when used at neutral pH and a buffer is added to
hold the solution close to pH 7.0. Over time the solution will become acidic due
to progressive oxidation, resulting in the formation of formic acid. It is
therefore best used within three months of receipt and stocks of specimen
containers should be rotated.
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it
is not necessary to store containers of formalin in the refrigerator |
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it
is not necessary to refrigerate the fixative once a specimen has been placed
in it - room temperature is indeed preferable |
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specimens
should be placed in fixative as soon as they are taken |
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larger specimens should be placed in larger containers - ideally there should be at least 20 times the volume of fixative to specimen. |
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specimens
may be kept for days, even weeks without seriously hampering subsequent laboratory procedures
or diagnosis |
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after
several hours in formalin the specimen will change colour. This is due to
chemical changes caused by the fixative and not an indication that something
is wrong |
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if
you do not have any formalin in which to place a specimen please contact the
laboratory for advice |
Attached
is a Material Safety Data Sheet for formaldehyde. While the degree of hazard
associated with 4% buffered formaldehyde is much less than with the concentrated
form (40%) in terms of acute or chronic toxicity, adequate
precautions should be taken to prevent skin or eye contact, and inhalation of vapour.
It is
advisable to have a means of absorbing and neutralising minor spills.
We are able to supply an absorbent towel impregnated with a
neutralising agent which would adequately deal with the spilled contents of a
30ml specimen container. Please contact
Mark
Daymon or
Margaret
Mount for more
information.