From: Greg Schmitz (gws1@columbia.edu)
Date: 10/16/03-10:11:35 PM Z
I'm not using one of those "cheapo" $29.95 jobs. I'm using a Fisher
Scientific Accumet Model 955 meter with a 13-620-108 probe. I don't
know what the probe is selling for now but 6 or 7 years ago it was
about $75.00 with our institutional discount. I've owned 2 of these
probes (over the last 15 or so years) and never had one fail. I have
occasionally checked it against a standard probe in a variety of
solutions, including photographic soups of many varieties, and it's
always been dead on.
I was not aware of the problems you've mentioned with gel cell probes
(perhaps the membrane used makes a difference). Could you provide a
reference regarding the problem, I'd really like to read it.
Thanks and best. -greg
On Thu, 16 Oct 2003, Ryuji Suzuki wrote:
> 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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