A great deal of what I know about fixing was learned from Michael Guzinowicz. Dr. Guzinowicz was once a contributor this mailing list and to the Usenet photography groups. At this point I am not sure he is any longer with us. While there are others with a good deal of specialized knowlege of photographic chemistry Mike was extraordinary in being able to explain the rather complex processes of organic chemistry. This is a treatise I had archived in my files. Since my posting brought out considerable interest in the subject I decided to copy one of Mike's posts from some time ago. He wrote a great deal more. I believe much of it is still archived at Google Groups but have not tried to find it.
-- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx -------------------------------------------------------------------------------------------------------(The following are some of my notes on fixing film and papers for your personal use - not for distribution. Copyright (c)1995 by
Michael Gudzinowicz.)The basis of fixation and accompanying problems aren't treated in
depth in most texts. This oversight often leads to postponed "accidents" whenever people are tempted by a sense of false economy to save time or materials. An introduction to the underlying chemistry should help to define a more criticalapproach to film and paper preservation, which doesn't rely on rumor and the advertising literature. The following notes were taken from Grant Haist's "Modern Photographic Processing, Vol.1" (Wiley, 1979), "The Theory of the Photographic Process" edited by T. H. James (3rd & 4th ed., 1st & 2nd edited by C. E. K. Mees; Macmillan, 1966 (3rd)), "Ilford Monochrome Darkroom Practice" by
Jack Coote, and the research and technical literature. Stop Bath:Compared to a water rinse, a dilute acidic stop bath will stop development very rapidly (15 sec) due to the pH change and rapid tunneling of protons compared, to a water bath where developer diffusion takes minutes (the basis of water bath development).
The acid stop minimizes developer oxidation in fixers withdissolved silver, which prevents soluble silver reduction and the formation of dichroic fog. Likewise, the pH change retards aerial oxidation of the developer and oxidation of fixer in the emulsion by radicals generated from the developer, which degrades fixer complexes to partially oxidized insoluble products which stain
films and papers, and eventually cause fading and sulfiding. Also, acid helps to maintain the pH of the fixer in a region where it's stable, and where the hardeners are effective and don't precipitate.In addition, the use of a stop bath rather than water reduces the osmotic shock and resultant swelling of the emulsion which is seen when emulsions with high solute concentrations are placed
into water, and thereby may reduce grain clumping. Fixation:The common notion is that the fixer removes undeveloped silver
halide by a simple reaction involving the replacement of thehalide by thiosulfate to form a soluble silver complex, and then if the film or paper looks or tests "clear", the only problem is fixer removal. Unfortunately, this is not the case. When a film is "fixed", a number of complexes are formed between silver and thiosulfate, and all are in dynamic equilibrium. In addition, the accumulation of halide during fixation reduces fixer capacity with use when free silver and halide levels approach their limits
of free, non-complexed solubility.A simple table outlining the dissolution of silver in fixer, and equilibria with fixer is outlined below. The silver halide may dissociate to a very small degree in aqueous solutions, and the thiosulfate anion will form a 1:1 complex with the silver cation
(Rxn 1) or the thiosulfate may react directly with the solidsilver halide crystal (Rxn 1). In either case, the first complex
(I) is >very insoluble< and remains tightly adsorbed to the surface of the solid silver halide.A second thiosulfate anion may react with the first complex (I), to form a soluble product (II) with a silver to thiosulfate ratio of 1:2 (Rxn 2); and then if "free" thiosulfate concentrations are high, a third thiosulfate anion may react with the soluble second complex (II), creating a third complex (III) with one atom of silver and three molecules of thiosulfate which is quite soluble
(Rxn 3). Sequence of Complex Formation:Note: Charge of ions in () brackets; # of molecules []# in the complex follows brackets; TS is thiosulfate (hypo) anion; Ag,
silver; Br, bromide. <-> shows equilibrium reactions. Rxn 1) Ag (+) + TS (-2) <-> AgTS (-) (first complex (I),monoargentomonothiosulate; insoluble - remains adsorbed to the
crystal as it forms)Rxn 2) AgTS (-) + TS (-2) <-> Ag[TS]2 (-3) (aq) (second complex (II), monoargentodithiosulfate; soluble - removed from emulsion
by diffusion)Rxn 3) Ag[TS]2 (-3) (aq) + TS (-2) <-> Ag[TS]3 (-5) (aq) (third complex (III), monoargentotrithiosulfate; very soluble in aqueous
solutions)In solution, these reactions are reversible, so all complexes are present, and a small amount of Ag+ cation is not complexed in
solution. The following equilibria occur: Rxn 4) Ag (+) (aq) + TS (-2) <-> AgTS(-) (aq), where allcomponents are in solution (aq) and adsorption doesn't occur.
Rxn 5) AgTS (-) (aq) + TS (-2) <-> Ag[TS]2 (-3) (aq) where the
monoargentomonothiosulfate is in solution and not adsorbed.Rxn 6) Ag[TS]2 (-3) (aq) + TS (-2) <-> Ag[TS]3 (-5) (aq) where both the monoargentodithiosulfate and monoargentotrithiosulfate
complexes are in solution.As more silver is put into solution with fixer use, more complex
II & III are formed, and the level of the less soluble 1:1complex (I) and free silver ion are also increased. After a few
uses of fresh fixer, the less soluble complex (I) and silver halide are left in the paper or film at low, but destructivelevels, although the film appears to clear. Also, thiosulfate is adsorbed to developed silver grains in papers (iodide tends to
displace it from films). Residual complex I and residual thiosulfate adsorbed to silver grains are converted totrithionite and higher thionites in a few days, and degrade and
react with silver giving stains (sulfiding) and fog. (Brownsilver sulfide is seen after bleaching the silver grains, and is
proportional to the developed silver.)With progressive use of the fixer, levels of bromide rise, as
well as chloride from papers and iodide from films. Silver halides have low solubility, and as the level of bromide oriodide rises, it forms silver halide in solution and the fixer will no longer dissolve silver halide. A number of complexes and equilibria occur with each halide and mixtures. On a relative
basis, silver chloride is more soluble than bromide and haslittle effect on fixer capacity; silver bromide is less soluble and determines fixer activity to a significant degree, unless films with low levels of iodide are fixed, in which case fixer
capacity is reduced significantly due to silver iodide insolubility (a problem with T-Max films, treated later). Ininstances where silver is removed to "regenerate" fixers, iodide
accumulation may interfere. Also, in two-bath fixation whichfollows, carry-over occurs, which requires periodic replacement
of both baths.The only way to ensure that little AgBr or the insoluble first complex is left in the paper, is to use fresh fixer with little accumulated silver and halide, and an excess of non-complexed thiosulfate to remove it. This approach to archival fixing when used with one bath is fairly wasteful, though effective. Rather than using one bath, the same results can be obtained with two baths, and the capacity of the fixer is far greater. Essentially, the first bath removes the bulk of the silver and halide, and leaves traces in the emulsion and paper. This amount when carried
over to a second bath, is insignificant, so the second bathalways acts as "fresh" fixer with high non-complexed thiosulfate levels to react with the small amounts of silver halide and less
soluble complexes to speed their complete removal. More on Fixing - One and Two Bath Fixation:Haist cites the following maximal permissible values for one-bath film and paper fixers for commercial and archival processing:
One-bath fixation: Commercial Archival Film: Max. Ag conc.: 1.5g/l 0.2g/l Max rolls/gal: 25 rolls/gal 2 rolls/gal Non-image Ag in film: 0.01mg/in^2 0 Paper: Max. Ag conc.: 0.3g/l 0.05g/l Max. sheets/gal: 30 8x10 5 8x10 Non-image Ag in paper: 0.005mg/in^2 0 Essentially, as fixer total silver (free and complexes) andhalide concentrations rise, the fixer's ability to remove all of the silver from the paper diminishes markedly, as indicated by
the very limited capacity of one-bath to remove silver to archival levels.The solution to the limited capacity is to use a fresh second
fixer bath to maintain a very low total silver level, with a
water rinse between the first and second baths to minimize
fixer/silver carry-over.
Two bath fixation: Commercial Archival
Film:
Bath 1:
Max. Ag conc.: 6g/l 3.5g/l
Max. rolls/gal: 60-70 40
Bath 2:
Max. Ag conc.: 0 .5-1.5g/l 0.02g/l
after 60-70 after 40
Non-image Ag in film: 0.01mg/in^2 0
Paper:
Bath 1:Max. Ag conc.: 2g/l 0.8g/l
Max. sheets/gal: 200 8x10 70 8x10
Bath 2:
Max. Ag conc.: 0.3g/l 0.05g/l
after 200 after 70
Non-image Ag in paper: 0.005mg/in^2 0
The first fixer gets rid of most of the silver, and the
second
maintains a very low silver concentration and relatively
high
free thiosulfate concentration to remove the remainder of
the
insoluble complexes and non-image silver present in the
emulsion
after the first fixation.The first bath is used for the maximum number of sheets or rolls indicated, and then discarded after silver recovery. The second bath is substituted for the first, and a fresh second bath is prepared. After 5 cycles (substitutions), or one week if exposed to air in tanks, both baths are replaced. Compare the capacity for commercial or archival standards using two baths to that for one... it is far more economical than using one bath, but also
avoids the temptation to over-use fixer, resulting in under- fixation and difficult removal of insoluble complexes. Films:With films, the fixation time in the first fixer should be at least twice the clearing time... likewise for the second bath. The clearing time should be checked often if that approach is used, however, Kodak recommends 5-10 minute fixation with non- rapid fixers and most films. Since there is no danger in longer fixing times, incorporating a five minute fix in each bath into a "normal" development procedure may avoid problems and provide some security. Agitation should be constant to remove fixer from
the surface of the film to facilitate diffusion, however,increased agitation never can replace adequate fixing time or counteract the cumulative effects of re-using fixer. With rapid
fixers, there is little "danger" of bleaching film with 5-10minute fixation... also, if standard procedures are used, any
minimal bleaching would never be noticed, since it would beincorporated into tests for contrast and development time. With T-Max films, Kodak recommends longer times. For instance, they suggest that it is "safe" to check clearing at five minutes with standard fixers or three minutes with rapid fixers... and total fixing time should be twice the clearing time. (Kodak's "advice" on T-Max varies from simplistic on 35 mm film boxes, to warnings in detailed technical literature, not only on times, but also on
fixer replenishment rates for processors.) T-Max Films:With some films, such as Kodak's T-Max series, fixer capacity is reduced to one-half of what one normally expect, and fixing times
are extended to twice the usual time, since silver iodide isresistant to fixation. In Kodak publication F-32 on T-Max films, Kodak indicates that a magenta stain may be left in the emulsion with inadequate fixing, and recommends further fixing with fresh
fixer to remedy the problem. (The magenta sensitizing dye isadsorbed to the silver halide (EKC statement - not speculation); when the halide is fully dissolved, the dye is removed.) In some instances, the dye can be removed by treatment with hypo-clear, which may contain sulfite or high salt concentrations which can
act as weak fixers in addition to displacing hypo, or withprolonged water washes. The "stain" problem isn't whether it will interfere with variable contrast paper filtration or not, but its
indication that the film isn't fixed properly. Papers: For paper fixation, do not use fixer which has been used forfilm. It is difficult to track capacity accurately (see table
above... silver capacity differs for film and paper), fixerdilution may vary between paper and film fixers, and the "sudden" accumulation of iodide after developing films may greatly prolong paper fixation or may leave insoluble silver iodide behind unless
one tests clearing times at every session.The clearing time for papers may be determined experimentally or by manufacturer recommendation (for Ilford, see below). Fixing times for most fiber papers is on the order of five minutes for each bath, with an intervening water rinse and storage in water. To save time, prints can be fixed in the first bath, rinsed and
held in water, then fixed in the second bath at the end of asession. As mentioned, long contact with fixer can cause problems if fixer enters the paper fibers (not between them). Papers and
fixers vary, and it is best to use at least the minimum timerecommended by the paper manufacturer. Kodak recommends 10 min for fiber base and 2 min for RC in one bath, or half that time for each of two baths. The RC time is optimistic, though five
minutes per bath is reasonable for fiber papers. Prolongedcontact with rapid fixers will slowly bleach an image or cause uneven bleaching if prints remain in rapid fix without agitation
for prolonged times (1/2 hr+).In any case, paper and film should be promptly removed from the second fixer, rinsed, and placed in a water bath until treated
with a hypo clearing solution to displace free thiosulfate. Rapid Fixer:Rapid fix has the advantage of a shorter contact time, and that
may minimize the penetration or degradation of fixer in the paper's fibers. Also, the useful capacity of rapid fixers isfairly high... 10-15 g/l silver vs. 6g/l for films or 2 g/l for papers using regular fixers (James; Haist table above for bath 1
of a two bath sequence). However, there is little data toextrapolate those numbers into increased capacity _without risk_ of problems. In that regard, Kodak's recommendation for capacity of rapid fix and other fixers is nearly the same (100-120 sheets
or rolls), which is optimistic for one bath commercial processing. Hardeners:For film, a hardening fixer is often preferred to minimize any emulsion damage in handling. The only advantage of rapid fix with film is decreased processing time and perhaps, decreased rinse time. Non-hardening fixers are also preferred for development of
the stain with pyro developers.For paper, rapid fix without hardener is often preferred, and
gives better results with toning. Paper curl seems to beminimized and there is less danger of "breaking" the emulsion when prints are flattened or mounted. Also, the avoidance of alum may reduce silver complexes bound in the emulsion. (To remove hardener for toning: household ammonia diluted 1:10 (0.3%) for 2 min with 45 min wash; or 5 min in 2% solution of Kodalk or sodium carbonate, then wash).
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