From: Ryuji Suzuki (rs@silvergrain.org)
Date: 10/16/03-07:54:16 PM Z
From: Greg Schmitz <gws1@columbia.edu>
Subject: Re: pH meter
Date: Thu, 16 Oct 2003 17:34:55 -0400 (EDT)
> I have always used a Gel probe
> (Fisher Scientific in my case) because they require zero maintainence.
Gel filled pH probes are very problematic for photographic
applications. Photographic solutions are not typical of the test
solutions those manufacturers consider in designing most pH probes,
especially almost all of the low end ones. Many pH tester type
products work only for a few minutes or maybe a few days at most,
after which the reading becomes erratic, heavily biased, and
eventually becomes impossible to calibrate. This is because
photographic solutions are highly reactive with silver chloride wire
that pH probes use as their reference electrode. The easiest way I
found to avoid this problem is to use double junction pH probe with
refillable reference electrode (not gel filled stuff) and replace the
reference electrode solution (3M KCl) fairly often. Electrode should
be kept in 3M KCl solution for storage.
> The drawback to using a Gel probe is that over time the electrolite in
> them dries out and the probe must be replaced. You can maximize the
> life of the probe by keeping it capped with a wad of cotten moistened
> with neutral electrolite covering the probe tip.
That's what manufacturers usually recommend, but photographic
solutions usually kill the electrode before drying becomes the
problem.
I don't know what kind of electrode is used for pH meter sold for wine
making. But I doubt they are built to sustain difficulty of
photographic solutions.
Much more details, along with recommendation for the pH probe is found
in the web page, whose link is found in my previous post on this
thread. I also posted many emails on this topic to pure-silver list in
the past, so more details can be found there (though the web
summarises all my postings).
-- Ryuji Suzuki "Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)
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