[pure-silver] Re: Darkroom Chemistry... Question
- From: `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Mon, 02 Mar 2015 22:33:47 -0800
On 3/2/2015 3:31 PM, Bogdan Karasek wrote:
Hello,
I'm turning to the Pure Silver group as I can't find the answer on the
Internet and it's been over 50 years since I did my high school
Chemistry.
I decided to switch to an all alkaline based procedure for developing
my film and as a fixer, I am using the TF-2 formula as presented in
the "DARKROOM COOKBOOK".
To make 1L of TF-2, the formula calls for the addition of 10 grams of
Sodium Metaborate (Kodak Balanced Alkali). I'm starting to run low
on the Kodak balanced Alkali and have read that I can replace it with
Sodium Tetraborate (Twenty Mule Team Borax, 99.5% pure).
The question is how many grams of Sodium Tetraborate should I add to
get the equivalent of 10 grams of Sodium Metaborate?
Any help would be appreciated. Thanks in advance!
Regards,
Bogdan K.
As Brian says a mixture of sodium hydroxide and borax in solution
becomes the equivalent of Kodalk or sodium metaborate. However, its
not necessary to add any alkali to fixing baths. Fixer works
regardless of pH. The common acid fixing bath has the acid for two
purposes: 1, to stop development and prevent the developer from being
active in the fixing bath; 2, to activate the metallic hardeners used
in most fixing baths, usually white alum or potassium aluminum
sulfate to give it its proper name. If you do not use a hardener you
don't need the acid providing you wash out the developer in a good
water rinse before fixing and have enough sodium sulfite in the fixing
bath to prevent staining from whatever developer is left. If no acid
is added to a fixing bath about 5 grams per liter of sulfite is enough
although more won't hurt. The usual acid bath has about 15 grams per
liter to preven the acid from decomposing the thiosulfate.
One reason given for using an alkaline fixing bath is that it
makes washing faster. Actually, if a sulfite wash aid like the now
discontinued Kodak Hypo Clearing Agent or its equivalent Ilford wash
aid, is used the pH of the fixing bath will have no effect on wash
rate. Hypo Clearing Agent as made by Kodak and presumably also the
stuff from Ilford is neutral pH. This serves two purposes: 1, it leaves
the emulsion in a neutral state which eliminates the ionic binding of
thiosulfate and its reaction products to the gelatin due to its ionic
charge. Gelatin is neither acid or alkaline but is amphoretic, that is
it takes the state of the last bath it was in. When that bath is neutral
the gelatin is neutral. Gelatin has a however does have a sort of
preferred state called the isoelectric point. For most photographic
gelatin this is slightly on the acid side of neutral. When its neutral
its on the alkaline side and the ionic charges are such that they
_repel_ the thiosulfate and reaction product ions where when its more
acid, as is the case with an acid fixing bath, it attracts them. The
neutral pH also breaks the binding forces of the alum hardener where its
used. Any alkaline bath will do so but if more alkaline than about
neutral will also undo the cross-linking of the hardener. This is, of
course, of no significance if no hardener is used.
Sulfite also has another property which most other alkaline agents
do not have, namely its an ion exchange agent for the thiosulfate and
its reaction products so it very much accelerates washing action. This
ion exchange feature is effective whether a hardener is used or not.
The washing rate when a neutralized sulfite bath is used is the
same regardless of whether the fixing bath is acid or alkaline and wash
rate is faster than if an all alkaline process is used because of the
ion exchange property.
I have to look up the formula for wash aid because I am drawing a
blank on it right now but its sodium sulfite neutralized with sodium
metabisulfite, the Kodak version has two sequestering agents in it
mostly to prevent sludging from aluminum products from alum harderners
and also to sequester minerals in the wash water.
Wash aid is very effective for emulsions but less so for untreated
paper support and somewhat less for the Baryta layer which is used on
"fiber" paper. However, Kodak's recommendation of three minutes
treatment after a rinse and before washing will pull the residual
thiosulfate and reaction products in double-weight paper to archival
levels. Note that the sulfite will also make some otherwise insoluble
reaction products soluble so will somewhat extend the working life of
fixing baths.
I have citations to the research papers from Kodak Research Labs
and the patents but it will take me some time to find them due to
breaking in a new computer.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
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