[pure-silver] Re: fixer question - wash water xchg rate
- From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Wed, 4 Jun 2008 05:42:28 -0700 (GMT-07:00)
-----Original Message-----
>From: Janet Cull <jcull@xxxxxxxxxxxxx>
>Sent: Jun 4, 2008 4:56 AM
>To: pure-silver@xxxxxxxxxxxxx
>Subject: [pure-silver] Re: fixer question - wash water xchg rate
>
>
>On Jun 3, 2008, at 6:13 PM, Richard Knoppow wrote:
>
>> The use of a sulfite wash aid will reduce both paper and film wash
>> times by a factor of about six times.
>
>
>Is that what hypo-clearing agent is?
>
It is at least what Kodak Hypo Clearing Agent and Ilford Wash Aid are. KHCA
is buffered to neutral because at that pH the hardening of hardening fixing
baths is not destroyed as it is by high pH alkaline baths and because the
swelling of most photographic gelatin is minimum there.
Kodak did extensive research in the early 1960's as part of a project to
find out why sea water was so much more efficient at washing than fresh water,
an effect known since around 1900. It was well known that an alkali bath would
accelerate washing but just being alkaline was not as effective as sea water.
Kodak researchers found out that the sulfites had an ion exchange effect with
hypo. That is, the sulfite ions displaced thiosulfate ions forcing them out of
the emulsion while being very easily washed out themselves. Ion exchangers are
used for water softening and many other processes. Sulfites were found to be
significantly more effective than other ions for thiosulfate although some
others have some effect.
This ion exchange effect makes wash aid followed by a fresh water wash even
more efficient than sea water for washing. In addition material washed in sea
water must receive a final wash in fresh water to remove the halides in the sea
water which otherwise will rapidly attack the image.
Gelatin is amphoretic, that is it has the peculiar property of not being
either acid or alkaline but having properties of both and taking on the pH of
the last bath it was treated in. However, it does have a sort of preferred pH.
This is the isoelectric point. This is where the ionic charges in the molecules
just neutralize each other. One measure of the isoelectric point is the
swelling of the gelatin. When in a pH above or below the isoelectric point the
gelatin will swell. The swelling when wet is minimum at the isoelectric point.
The charges also change depending on the pH. Both thiosulfate ions and those of
the fixer reaction products, that is the silver-thiosulfate complexes formed by
the fixing action, have an electric charge. Where the net charge of the gelatin
is opposite of these the ions will be attracted and bound to the gelatin making
them very hard to remove. By moving the pH to the alkaline side of neutral the
charges are changed to being the same polarity as the unwanted ions so that
they are repelled thus helping the washing action. Because most photographic
gelatin has an isoelectric point just a bit on the acid side of neutral
treating it in a neutral pH bath will set up the right internal charges to help
remove the thiosulfate and reaction product ions. The same will be accomplished
by a strongly alkaline bath but that is not necessary and there are some
advantages to keeping the pH near neutral. One as stated is that the hardening
action is preserved. Another is that the diffusion path is minimized by the
minimal swelling of the gelatin. The gelatin is also less vulnerable to
mechanical damage in this condition.
There is another reason to get the gelatin to neutral or alkaline: this is
the binding of the thiosulfate and reaction products by the aluminum hardener
used in most fixing baths. When in the acid condition the hardener acts as a
mordant and sticks the hypo and silver complex ions pretty tightly to the
gelatin and image silver. By moving out of the window of pH where this happens
the bond is broken. As it happens the bond is no longer effective at neutral pH
even though the hardening remains.
Kodak wash aid consists of sodium sulfite with sodium bisulfite (or
metabisulfite, they are the same in solution) as the buffer. It also has two
sequestering agents, sodium citrate and EDTA tetra-sodium salt, to prevent the
deposit of carbonates in the wash water and aluminum carbonate from the
hardener onto the surface of the material being treated.
I can't tell from the MSDS for Ilford wash aid if it is buffered, it well
may be but the constituents of the Kodak product are well known so I stick with
it.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
Los Angeles, CA, USA
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