Fixing and washing for teknogeeks
This is going to be a description gleaned from a lot of sources and a lot of personal lab work. Here goes...
Fixing is a diffusion/kinetic/diffusion process. It starts by requiring the diffusion inward of fixing chemistry and usually takes between 10 and 20 seconds at 68 degrees F depending on thickness and hardness of the coating among other factors.
At this point, when the wave front of chemistry reaches the bottom of the coating, the top has begun to fix. This is a kinetic reaction involving the formation of silver complexes with hypo and ammonium ion (if present). With sodium hypo, about 5 complexes can form, but with ammonium ion present a huge number of combinations are possible.
As soon as silver halide begins to dissolve, it begins the third phase which is outward diffusion of the complexes. This is a different speed than that at the start if swell changes path length or porosity of the gelatin. Also, the hypo complexes are larger than hypo itself and therefore may move more slowly. Alkaline fixes or fixes that cause swell will enhance this speed. Total fixation is the sum of inward diffusion (~20 seconds), reaction rate, and outward diffusion (> 20 seconds). We take this to be 2x the time it takes to clear or 2x the time it takes to diffuse in, react and then move out, so it is logical. In + react + out = fix time. React and out are influenced by the nature of the complex and the fix formula (pH and etc). No fix can be faster than the inward diffusion rate, so beware of fixes that claim to work in 15" or things like that. It might clear the film, but you have to wait for things to begin outward diffusion to properly clear. It is inevitable though that some silver hypo complexes will remain in the coating. That is one reason why we wash.
Wash, as described by many, is a non-linear exponential function based on diffusion only. It is described as the change in concentration / unit time = a constant * (the amount of complex you have - the amount leaving the coating).
In this case, the constant is related to the type of complex (sodium or ammonium), the thickness of the coating, and the pH of the fix used. However, this is ideal assuming an infinite volume of water or infinite agitation.
If you have a small container of wash water and/or don't wash, the rate of washing slows down. The equation then beocmes this.....
the change in concentration / unit time = a constant * [(the amount of complex you have - the amount leaving the coating) - the amount building up on the surface of the film or in the wash water]
Now, what this means is that washing is dependant on time in the wash water, the amount of water used, the amount of fixer building up in the water and the amount of exchange rate of water at the surface of the thing being washed.
This may go against many practices and in fact may refute some posts here but is stated by Mason from Ilford and Mees and James, both from Kodak. And, this information refutes the Ilford wash method by one of Ilford's respected researchers.
The bottom line though is the objective. Get silver to dissolve and then get the complexation products out of the film or paper. There are two test for these.
One uses sodium sulfide to test for silver halide and complexes in the coating. A drop of the test solution (Sodium Sulfide) will turn brown or black if silver ion is present in the coating in any form. The other uses silver nitrate in acetic acid and turns varying shades of yellow brown as a function of the amount of hypo retained in the coating. Using both of these will tell you what is best for you. If your coating is washed enough to pass these two tests, then it will meed ISO standards for image stability. Overwashing can cause problems as described by Ctein. Underwashing will surely fail one or another of these tests.
If you use fresh fix, there is no problem, but if you reuse fix, then you should use the fixer exhaustion test which is Potassium Iodide solution added to a small quantity dropwise to see if a yellow precipitate or cloud forms. If it does, the fixer is exhausted or has gone bad.
This entire description is generic and mentions the caveats to keep in mind when one uses acid or alkaline fixes or sodium or ammonium fixes. It generalizes water variataions and a lot of other items. What it is meant to be is a guideline used with tests to verify the effectiveness of a wash.
Lets go on and add some variables. Metol and Hydroquinone are used in many developers as is ascorbic acid. All three are soluable in base due to formation of salts in alkali. Metol is soluable in acid as well because it is both acid and base (to an extent). There is no easy test for these, and so passing the above tests will not tell you how well you have done in removing these three chemcials. All of them can do things to the silver image. In addition, having a small amount of sulfur compounds present in a coating has been seen to be useful (Ctein, Agfa Sistan, Fuji stabilzer and etc), and so overwashing has been shown to be bad.
There is also discussion on washing aids. Well, these are controversial. They do help remove hypo and silver complexes. OTOH, they can cause their own problems if you do not wash them out properly and so textbooks on this subject often caution the photographer to insure that the wash aids themselves get a good wash after their use.
Hope this helps.
I have posted other viewpoints of this in several threads as well as extended descriptions of the diffusion process and hypo build up on the surface of the film. I might add that Bill Troop's mention of HQ retention and mine are probably the first mention of this anywhere. I doubt if anyone knew of it.
I have also posted a scan of the Kodak residual hypo test kit. I mention it here for those that missed it.
I must be a technogeek - I understood every word!
-- Ole Tjugen, Luddite Elitist
Thanks PE, I really appreciate your clear, accurate, well-cited, verifiable post.
I'm bookmarking it, in case I need further information about any of what you posted, if need be, I'll use that post as a guide to the references to further my own knowledge. Many thanks, and thank you for sharing.
Many thanks, and thank you for sharing
I have a question though,
Assuming you start with fixer that tests good, is there a point where stability may suffer as the fixer approaches exhaustion?
I guess the worst case might be if the fix tests good before use and doesn't afterwards, should you refix in fresh? Extend the wash? or?
Is the relationship of stability and fixer "freshness" linear? Is it even possible (or practical) to determine this?
Lots of questions. Well, the 2 fix method is best to eliminate a fixer going bad during treatment. I don't use one for space reasons, so I do the best I can by increasing time as the fixer becomes more used. That does not always help though.
Refixing is best, but extending wash generally will not help.
Fix can go bad while being used and form sulfur or H2S gas. I have personally only seen this in acidic fixes, but it can happen. Fixers stay fresh as long as the sulfite is there to prevent aerial oxidation (this is a generalization, of course). So, fixer will be good to go as long as there is no haziness or actual precipitate formed. I have even used bad fixer by filtering it and adding more sulfite when I had a lot go bad, but I did test fixing time beforehand.
Having experimented a lot with fixers, I would say use it if it smells good (ammonia if alkaline, sulfite if acidic, no odor if neutral) and is clear. If in doubt, run a fix test on a strip of film. Don't use paper, it is too hard to observe clearing and fixing times.
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PE, thanks for the interesting post.
By 'wash aids', are you referring to hypo clear?
Anyway, I would be interested to read a description of what happens in the hypo clear process, similar to your fix item.
Also, re different types of film - what is it that makes film such as Tmax take longer to clear than Tri-X? I have put pieces of both films together in a beaker of fix & watched until they clear. Tmax takes at least twice as long as Tri-X. It's as if it has a coating of pink concrete on it.
Firstoff, let me thank you all for your comments. I'm about to make a major addition to this thread on super additivity and 'super subtractivity' .
By wash aids, I mean any product used after fixation with the exception of toner. These all have proven good effects, but only in the face of sufficient washing to remove them. They are, after all, chemicals themselves and can have deleterious effects on the images. So all textbooks say that you must wash well after using any of them. Unfortunately, there is no test for the retention of any of these materials in coatings, nor are there tests for what they do. It is just a precaution given in the texts.
For products that are called stabilzers, such as the Fuji product and Sistan, follow the recommendations of the manufacturers.
As for differences in fixing times, this depends on silver quantity in the coating, thickness of the gelatin, hardness of the coating and quantity of iodide in the emulsion just to name a few. So, don't be surprised if films fix at different rates. I do a clearing time on the tabs of my 35mm film cartriges to verify before I fix, but I usually overfix just a tad to be on the safe side if I have not run a test. If I see a visible problem (cloudiness in the film) I zip it right back into the fixer.
Superadditivity in fixing
I've mentioned this before but never delved into it. Textbooks have been discussing it for years without coming right out and calling it this.
Let me explain. You know about superadditivity in development, where you mix 2 developers and get more than 1+1=2, that is, the activity goes way up. Well, the same can happen with fixer solutions.
If you use sodium thiosulfate (sodium hypo) fix goes at rate N. If you use ammonia to fix, (yes this is possible) the rate is slow but is M. But if you use ammonium thiosulfate (ammonium hypo) the rate is not N + M or anything close because you are getting some very strange complexes that involve Silver ion and Ammonium ion with thiosulfate.
To demonstrate this affinity of ammonia for silver, you can take some silver nitrate in a beaker of water and add drops of 28% ammonium hydroxide. You first form black or brown silver hydroxide precipitate, but gradually the solution clears and you make a silver ammonia complex that dissolves in the water.
So, picture Sodium ion, Silver ion and Hypo in a large tangle of ions that is X in size. Now picture Ammonium ion, Silver ion and Hypo together. Actually both the Ammonium ion and the Silver ion bind (complex) as well as the Hypo making a smaller tighter molecule (complex) that dissolves more rapidly than either of the two complexing agents make alone, and so Ammonium Hypo is much faster than expected even though the Ammonium ion itself is about the same size as the Sodium ion. (18 vs 23 in atomic weight as a rough approximation).
Potassium (39) OTOH, when used as Potassium Hypo drastically slows down fixing mainly due to its size and this causes diffusion effects and also complexing (kinetic) effects. It is hard for the Potassium, Silver and Hypo to form a good diffusible complex, and therefore the fixation is largely paralyzed.
So, we can show in the lab that increasing the size and weight of the metal ion used (Potassium, Calcium or Magnesium) will slow fixation due to size and thereby mainly diffusion but also complexation, we can show that Ammonium ion speeds it up far more than the size can account for.
The bottom line is this, referring to the OP, Size of the molecule mainly interferes with diffusion in and out, whereas the reactivity of the ions relate to the kinetics of the reaction.
At this time, Kodak has experimented with a wide variety of superadditive fixing agents that increase fixation. They are mainly directed towards the ability to speed up the kinetic step, but this leads me to another factor.
Agents which increase swell also increase fix rate by affecting outward diffusion, but they increase path length. If one were to combine a swelling agent with a fixing agent, this would achieve the best of both worlds and lead to a very high speed fix.
And, I have not addressed the effects of ternary or quaternary mixes of fixing agents. Yes, these are possible and fix rate keeps climbing while wash rate can go down! This is an amazing property of fixation that has been largely ignored except in TF-4 among others. It was the first. It will obviously not be the last.
Thank you, PE,
I am actually going to print out these posts and put them in a binder. This is solid technical information and will come in very handy.
The superadditivity is very obvious when comparing a slightly alkaline sodium thiosulfate fixer with the same fixer with a little added ammonium chloride.
Yes, that's what OF-1 is. I have tried both with and without Ammonium chloride (I managet to forget it once - thought it smelled a little funny!), and the difference in fixing speed is surprising.
-- Ole Tjugen, Luddite Elitist