From: Ryuji Suzuki (rs@silvergrain.org)
Date: 10/16/03-11:59:07 PM Z
From: Greg Schmitz <gws1@columbia.edu>
Subject: Re: pH meter
Date: Fri, 17 Oct 2003 00:11:35 -0400 (EDT)
> 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.
Did I ever say you used cheapo or something? All I said is that gel
filled electrodes are designed for convenience and not for widest
application areas. Understand? The price is not the
application-suitability indicator, though most of low end products are
gel filled and thus unsuitable for photographic applications. Also,
please read that web page before asking questions. I don't enjoy
repeating the same stuff over and over.
> 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.
Glass membrane used in recent pH probes are fairly improved and don't
exhibit much problems described in old literature, and also eliminated
much needs for specialized probes such as probes for high non-water
solvent samples, probes optimized for high pH ranges, etc. However,
because pH measurement relies on the Nernst equation model for H+
species, it is necessary to have a reference electrode besides the H+
sensing (glass) membrane. Most (almost all, with special exceptions
of mercury and platinum) reference electrodes use Ag(s)-AgCl(s) -
KCl/AgCl(aq solution) - (semipermeable structure) - sample solution as
the interface. In the case of double junction electrodes, it's
Ag-AgCl(s) - KCl/AgCl(aq) - (semipermeable membrane) - KCl(l) -
(semipermeable structure) - sample solution. The KCl/AgCl part is
called inner junction and KCl - sample is called outer junction.
Much of the problem is associated with the reference electrode. The
problem happens because ceramic bridge, glassfiber bridge or other
junctions like glass sleeve junctions allow a small amount of sample
solutions to interact with and diffuse into reference junction
bath. Since photographic solutions are usually very high in salt
content, and contain silver-reactive agents such as reducing agents,
sulfur-containing compounds, selenium compounds, silver ions, other
transition metal ions in case of alternative process sensitizers,
etc., these small leaks will react with silver chloride reference
electrode, and this will compromise the function of the reference
electrode. With a gel filled probes, once small amount of
contamination happens, you have no way to clean this out unless you
replace the whole electrode. With double junction electrodes, this
inner reference junction is further separated by other semipermeable
junction to another plain KCl bath with no AgCl, that is, the outer
reference junction, which contacts witht he sample solution. If the
double junction electrode is refillable, you can actually ditch the
contaminated KCl solution of the outer junction and replace it with a
clean KCl solution to prevent further contamination to the inner
junction solution. All these stuff are described in analytical
chemistry literature, and also available from technical support
departments of some DECENT pH electrode MANUFACTURERS, not all. Some
companies are simply resellers and some of them are simply
overconfident of their products -- they won't know.
Another common problem area is clogging of the junction bridge
pore. Ceramic or fiber bridge type junctions are more problematic and
requires frequent cleaning. Sleeve type junction presents much less
problems but they leak outer junction solution at a much higher rate,
so the user must know that KCl is leaking to the sample. The user also
has to fill the outer junction fluid frequently. Gel filled probes
have extra problems because reference solution cannot be replaced.
When sample solution contains reductant, transition metal ions, sulfur
compounds, etc., these stuff react with AgCl in the junction solution
to form some solid silver compounds, and may clogg the pore. Also
these agents may react at the solid-solution interface of the silver
wire electrode. Either way, the electrode functionality is quickly
compromised. In case metallic silver or silver sulfide is formed, no
amount of standard cleaning procedure will salvage the probe. One can
do some radical treatments like thiourea solution acidified with HCl
(the pH should be very low, around or below 1 -- but at that pH
thiourea decomposes fairly quickly, so you gotta act quick -- you
don't want those decomposition products in the pore either) but the
effect is limited, especially with gel filled stuff. (BY the way,
acidified thiourea solution can bleach sulfide treated silver images
because they attack silver sulfide.)
Now another approach to avoid these problems is to use non-silver
reference junction. In the past, mercury was the preferred material
when sample is suspected to react with silver chloride. This became
obsolete for obvious reasons. Some manufacturer makes their high end
probes using platinum. But this is rare, expensive and not common yet.
As you can see, silver electrode is most widely used for this sort of
applications involving stable electric contact to aquaous medium (Look
at electrophysiologists using microelectrodes; AgCl - KCl is almost
universally used.).
Similar challenge to pH probes appear in a few other non-photographic
areas. Biochemists using tris buffer has similar problem becaus tris
reacts with silver chloride. Environmental chemists who deal with
stuff like industrial effluents have the same problem -- they contain
a lot of sulfur compounds that can attack the reference electrode. In
all these cases, the solution is the same. Use refillable, double
junction glass electrodes and change the junction solution at the end
of each session. Respectable manufacturers discourage any pH probe
that is not a refillable, double junction electrode for users with
these applications in mind. Some manufacturers have product selection
charts in their catalogues clearly indicating these limitations.
Read that web page. Also check out Eastman Kodak Company's publication
H-24 Module 3.
-- Ryuji Suzuki "Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)
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