[NHCOLL-L:1290] RE: testing fluid solutions

[Julian.Carter at nmgw.ac.uk]

>>> Robin K Panza, panzar at carnegiemuseums.org23> October 2001 19:56;
What bothers me is that we put specimens in ethanol-water after fixing in
formalin.  Even after soaking, there's some formalin leaking into the
storage solution, so it's ethanol-water-formalin.  If formalin is heavier
than water and ethanol is lighter, how can a densitometer measure proof?
When the density reads 70%, is the ethanol concentration actually higher
because the water-formalin is denser?  How much formalin contamination is
"ok"?  

Ideally, we'd periodically replace the solution, rather than just "topping
off", so we'd be removing trace formalin and accurately measuring the
ethanol content, but who can afford to do that frequently?<<<<<


I think we need to remember that the formaldehyde solutions that we use to
'fix' specimens typically contain little formaldehyde - thus 10% formalin
contains about 4% formaldehyde. Essentially the fixative is mainly water and
this is what governs the bulk of the density reading. Thus when moving
formaldehyde fixed material to an alcohol preservative then the greatest
effect is going to be from dilution of the preserving solution from water in
the specimen. In this respect the density reading is going to be acccurate
enough to determine the percentage alcohol. If this reading is 1 - 5%
inaccurate then does this really matter? What could really affect this
reading could be if you use saline formaldehyde solutions (common practise
on board research boats). Salts getting into an alcohol solution could
noticably bump up the density. This could potentially give you a density
reading that is higher, suggesting a weaker alcohol solution than it acually
present. 

Overall I think when moving from formaldehyde fixation to alcohol
preservation it is important to have at least a two stage process, with at
least one soak in a weaker alcohol solution than the final preserving
solution to reduce cellular stresses and reduce the exothermic heating
effect that appears to occurs on transfer between solutions ( see The
Conservator 22: 36 - 43 ).

Jules Carter
National Museums and Galleries of Wales
Cathays Park
Cardiff
CF10 3NP
029 20573230
julian.carter at nmgw.ac.uk