Chromotropic acid tests

Will Jeffers <wjeffers [at] mfa__org> writes

>In testing a range of materials, I'm finding that while the solution
>does not turn purple within the specified 30 minute period, a purple
>species will appear if I continue to run the test for a period of 2
>to 28 hours.  Purple species do not appear in any blind controls run
>concurrently with these tests.  Which brings me to my questions:
>
>    Has anyone else had similar experiences when running
>    chromotropic acid tests?
>
>    Is the appearance of this purple species--even though it is
>    occurring well beyond the specified test period--significant?

I think the effect described is due to the concentration of
formaldehyde in the solution.  A lot of materials emit small amounts
of formaldehyde (or other aldehydes that the chromotropic acid acid
solution will react to).  The emission rate determines the airborne
concentration after a particular time (in the sealed glass vessel
used).  Some of the airborne formaldehyde partitions into the
solution, hence the solution concentration of formaldehyde will
slowly increase, if the materials is emitting any formaldehyde at
all. When the solution concentration hits a particular value the
purple colouration becomes visible to the naked eye.  It can be
detected at lower levels (about 0.01 micrograms per litre) with
visible spectroscopy.  Formaldehyde solutions of concentrations
0.640000, 0.012800, 0.002560 and 0.000512% (vol/vol) cause the test
to changes colour after 5, 10, 40 and 40 minutes respectively. By
looking at the test after 30 minutes what we are in effect doing is
stating an 'allowable' limit of formaldehyde concentration in the
air (or a emission rate from the material).

The 30 minutes was determined by testing a number of materials with
this test and accelerated corrosion tests with lead and trying to
calibrate the two sets of results against each other.  Several of
the chromotropic acid tests changed colour after longer than 30
minutes, but these materials caused no visible corrosion in the
accelerated aging tests.  This assumes that if lead survives
exposure to a material at 100% RH and 60 degrees C for 28 days then
the material will be safe for use, which has been our experience
over the past 27 years.

David Thickett
Conservation Scientist
Department of Conservation
The British Museum
London WC1B 3DG
+44 20 7323 8174
Fax: +44 20 7323 8636


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                  Conservation DistList Instance 14:38
                Distributed: Saturday, January 13, 2001
                       Message Id: cdl-14-38-010
                                  ***