[pure-silver] Re: Hypo Clearing: citric acid or sodium citrate?
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 27 Apr 2007 16:54:35 -0700
----- Original Message -----
From: "Jordan Wosnick" <jwosnick@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, April 27, 2007 4:19 PM
Subject: [pure-silver] Re: Hypo Clearing: citric acid or
sodium citrate?
John,
More than a slight effect on pH. Solutions of citric acid
are acidic, whereas solutions of sodium citrate are
alkaline (salt of a weak acid). My guess is that if citric
acid is added to hypo clear, it is done for pH control, in
which case sodium citrate is not an acceptable substitute.
Bisulfite and sulfite form a buffer, but if the citric
acid concentration is high, the buffer system can easily
be overwhelmed. (Buffers have a fixed capacity.)
Jordan
The MSDS for Kodak Hypo Clearing Agent specifically
lists Citric Acid as an ingredient. The patent lists only
the EDTA Tetrasodium salt but mentions Citric Acid among
acceptable sequestering agents along with several others.
The target pH is 7.0. The formula in the patent has this
pH but, if the small amount of Citric Acid added to the
commercial product is enough to affect the pH the
proportions of Sodium Sulfite and Sodium Bisulfite are
probably adjusted to compensate. Citric acid is a
sequestering agent with a somewhat different effectiveness
than EDTA Na4. I expect Kodak found that the stuff worked
better with both. Citric Acid seems to be a sequestering
agent for Aluminum, for instance, Citric acid can not be
used to acidify fixing baths using White Alum hardener and
should not be used as a stop bath with them. So, the
purpose may be to hold carried over alum hardener which
otherwise will cause deposits on the film or paper. The EDTA
Na4 may also have this effect but is certainly present to
sequester Calcium and Magnesium salts in the water, again to
prevent deposits.
The patent lists EDTA Na4 as being present in the
working solution in the quantity of 0.5 grams/liter. This is
equivalent to 2.5grams/liter in the normal concentrate. The
MSDS shows quantity only as a _range_. Both EDTA Na4 and
Citric Acid are listed as being present in the quantity of 1
to 5 percent in the concentrate. So, its possible both are
present in the same amount but that is a guess and may be
wrong. Unless one is prepared to make a chemical analysis
probably the pH is the key to gettting the correct amounts.
The pH of KHCA is not very critical as long as it is
neutral or slightly alkaline. If no hardener has been used
the pH is even less critical. At pH 7.0 the hardening action
of White Alum is not destroyed, as it is by a higher pH but
its ability to mordant Thiosulfate and reaction complexes to
the emulsion is broken. If one does not need to preserve
hardening the pH can be higher although it should not be so
high as cause excessive swelling.
According to Ryuji Suzuki the measurement of the pH of
photographic solutions is not trivial. I believe he has an
article about this on his web site.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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