A few years ago I compared different methods of measuring pH of different
materials (eg. paper, textiles, leather etc). I agree with Karin, I
personally do not rely on surface pH measurements, for one it depends on
surface properties, sometimes the pH on the surface can be quite different
than the bulk. And it has the added problem of causing tidelines - as you
need water and time to extract.
The best method of pH measurement is cold water extraction, you can use a
microextraction method or make good use of the wash water - if deionized
water is used for washing. When that is not possible, like Karin, I also
like using the non-bleeding pH strips as an alternative. I usually wet the
pH strip thoroughly with deionized water, shake off the excess water and
apply the wet strip onto the textile (or paper) and apply a bit of finger
pressure for a minute or so. Some paper conservators put a piece of mylar
on top of the pH strip and put a weights on top to ensure proper contact.
This gives a reasonable estimate of the bulk pH of the textile. This
method works best when the object is very acidic or very alkaline. It is
most ambiguous when it is neutral or if there is very little extractable
materials, as the pH strips are not very sensitive at low ionic strength
environments.
The CCI is in the process of publishing a research report: "pH Measurement
of Diverse Media", it should be available very soon. In the report there
is a comparison of different measuring devices including portable pH meters
and pH strips. It also talks about measuring pH of deionized water.
Season
Season Tse
Conservation Scientist/Chemist
Conservation Processes and Materials Research
Canadian Conservation Institute
Dept. of Canadian Heritage
1030 Innes Rd.
Ottawa, Ontario
K1A 0M5
Tel. (613) 998-3721 (x-187)
Fax. (613) 998-4721
email: [log in to unmask]
Visit the CCI Web site at http://www.cci-icc.gc.ca/
"Karin von
Lerber, Prevart To: [log in to unmask]
GmbH" cc:
<karin.vonlerber@ Subject: Re: Portable pH meters
PREVART.CH>
Sent by: Textile
Conservators
<[log in to unmask]
EDU>
08/21/02 03:50 AM
Please respond to
Textile
Conservators
I have made the same experience as Janet. Moreover ph-meters seem to be
unable to measure pH propperly in deionized water, unles dirt has
solubilized and re-establishes coductivity of the water.
I use ph-papers by Merck (the kind which does not bleed) in a combination
of
several types and with a standardized method as well for measuring pH in
the
liquid as for measuring surface pH. With a pH meter you have to use a real
drop of water in order to make them work, and the error margin is still
rather big with surface measurements. With the papers, the textile does not
become copletely wet, and you still get good readings (sometimes only in
tiny spots on your paper; so it takes some experience, but it works fine).
Make sure you use water with a pH of 7 (so don't use deionized water, which
is always below seven). E.g. tap water or "evian" bottled water when you
are
working on site.
There has been an interesting student paper at the "Fachhochschule Köln"
(Diane Lanz, ca. 1992?)comparing exactness of ph-measuring methods
(surface-diode; wet extraction; ph-papers) and the papers turned out to be
the much better equivilant to the wet extraction than the surface-diode.
Hope this helps. Sincerely, Karin von Lerber
Karin von Lerber
______________________________
Karin von Lerber
Prevart GmbH
Konzepte für die Kulturgütererhaltung
Oberseenerstrasse 93
CH-8405 Winterthur
Telefon + 41 (0)52 233 12 54
Fax + 41 (0)52 233 12 57
e-mail: [log in to unmask]
www.prevart.ch
----- Original Message -----
From: "Janet Farnsworth" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, August 20, 2002 6:44 AM
Subject: Re: Portable pH meters
> I would just like to add a word of caution: My experience of pH meters
> is very mixed and I suggest that before you buy a flat electrode meter,
> you test it on all the surfaces you might want to measure the pH of, [in
> case it need to make the surfaces so wet it is unusable] and ask about
> recalibration and indeed about drying out problems.
>
> I would be very interested in the results of your research; I am sure
> that pH meters have improved since I last looked at them.
>
> message <[log in to unmask]>
> , Ellis, Shirley <[log in to unmask]> writes
> >Hello,
> >We are looking for a portable pH meter with a flat electrode that can
read
> >the pH of textiles or paper, or liquids. Does anyone have or know of
one
> >that they can recommend?
> >
> >thanks,
> >
> >Shirley
> >
> >Shirley Ellis, MAC, CAPC
> >FSO/Conservator
> >Department of Human Ecology
> >B-20 Human Ecology Building
> >University of Alberta
> >Edmonton, Alberta T6G 2N1
> >ph (780) 492-7678 fax (780) 492-4821
> >email: [log in to unmask]
>
> --
> Janet Farnsworth
>
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