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  1. #211
    Photo Engineer's Avatar
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    Well, that is quite good Ian, but the thing is that problems with the tail end of any process, B&W or color, may not show up for 5 - 20 years!

    So, you may see a perfect print or negative when initially dry but depending on the tail end (stop, fix, wash) or (stop, blix, wash) etc. errors will not show problems for years.

    PE

  2. #212

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    Quote Originally Posted by Photo Engineer View Post
    Well, that is quite good Ian, but the thing is that problems with the tail end of any process, B&W or color, may not show up for 5 - 20 years!

    So, you may see a perfect print or negative when initially dry but depending on the tail end (stop, fix, wash) or (stop, blix, wash) etc. errors will not show problems for years.

    PE
    Are you suggesting that somehow an insufficient stop bath will cause the negatives to be unstable years after they have been fixed, washed, and dried?

    If so, is there any evidence that this happens and/or a mechanistic explanation of why it happens?

  3. #213
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    Alan;

    Yes, of course. You may have missed my post earlier about HQ and Metol.

    Metol is very soluble in acid (as compared to base), and HQ is more soluble in base than in acid. Interesting opposites these two guys! An Acetic Acid - Sulfite Stop followed by any pH fix is able to remove both products more easily by making the soluble acid salt of Metol and extracting it, and by Sulfonating the HQ and Quinone. This renders them all more soluble in any subsequent fix of any pH if they are not extracted outright by the stop bath.

    Retained HQ will oxidize to Qunone (yellow) and if stored in damp conditions it will form Quinhydrone (green) which was once used in Quink, a green ink sold years ago. Retained Metol browns.

    So, one objective of a process train is to remove the developing agents, a fact overlooked by many. And, there is no good test for retained developing agents.

    To continue, with color materials, the stop bath is intended to removed color developer (PPDs) from the color coating and as you season in the Acetic Acid stop, it turns pink. Add a dash of Sulfite and it becomes clear as it reacts with the pink oxidized PPD. If these prints are kept, they first turn pink, then brown and being fading.

    If you wish more information, I can go on with this for another page or so as I lost a Christmas vacation due to this type of problem. I was called in to troubleshoot exactly the problem related to the lack of a stop bath. So, I am not giving a fictional account, I guarantee you!

    PE

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    Quote Originally Posted by Photo Engineer View Post
    Alan;

    Yes, of course. You may have missed my post earlier about HQ and Metol.

    Metol is very soluble in acid (as compared to base), and HQ is more soluble in base than in acid. Interesting opposites these two guys! An Acetic Acid - Sulfite Stop followed by any pH fix is able to remove both products more easily by making the soluble acid salt of Metol and extracting it, and by Sulfonating the HQ and Quinone. This renders them all more soluble in any subsequent fix of any pH if they are not extracted outright by the stop bath.

    Retained HQ will oxidize to Qunone (yellow) and if stored in damp conditions it will form Quinhydrone (green) which was once used in Quink, a green ink sold years ago. Retained Metol browns.

    So, one objective of a process train is to remove the developing agents, a fact overlooked by many. And, there is no good test for retained developing agents.

    To continue, with color materials, the stop bath is intended to removed color developer (PPDs) from the color coating and as you season in the Acetic Acid stop, it turns pink. Add a dash of Sulfite and it becomes clear as it reacts with the pink oxidized PPD. If these prints are kept, they first turn pink, then brown and being fading.

    If you wish more information, I can go on with this for another page or so as I lost a Christmas vacation due to this type of problem. I was called in to troubleshoot exactly the problem related to the lack of a stop bath. So, I am not giving a fictional account, I guarantee you!

    PE
    Something to think about and discuss further. Thanks.

  5. #215
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    Quote Originally Posted by Ian Grant View Post
    A great many people who use Adox/EFKE films never see a problem.

    Ian
    I'm one of them. Efke is the only source I can find for 2x3 sheet film, and works fine for me.

  6. #216

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    ron

    what if you use developers that do not contain HQ or Elon ?
    im empty, good luck

  7. #217
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    Quote Originally Posted by jnanian View Post
    ron

    what if you use developers that do not contain HQ or Elon ?
    Depends.

    Phenidones are hydrazine derivatives but AA is the only freebie here. Unfortunately, AA has a bad rep. IDK if it is deserved or not, but it is not the best developing agent IMHO or it would be all over the place in developers. It is tricky and messy.

    So, go formulate a good developer. I know of a few.

    PE

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    PE, you are diplomatic, knowledgeable, intelligent, generous and experienced. Thank you for the effort you devote to this website.

  9. #219

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    Quote Originally Posted by Photo Engineer View Post
    To continue, with color materials, the stop bath is intended to removed color developer (PPDs) from the color coating and as you season in the Acetic Acid stop, it turns pink. Add a dash of Sulfite and it becomes clear as it reacts with the pink oxidized PPD. If these prints are kept, they first turn pink, then brown and being fading.
    Does this apply to color film, too? The Kodak publications I've seen don't mention a stop bath per se. For instance, my Kodak Color Darkroom Dataguide (1996) specifies C-41 steps of developer, bleach, wash, fixer, wash, and stabilizer (p. 22). The description of RA-4 (p. 48; also p. 49) does mention stop bath, but says it's "optional." If a stop bath is likely to increase the longevity of my C-41 negatives, I'll certainly add that to my procedure.

  10. #220

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    Quote Originally Posted by Photo Engineer View Post
    ...
    1. You don't have to diffuse the acid through the emulsion, only the proton, which is the smallest of all ions. I have seen the rate of pH drop when using acid on alkaline films. It is virtually instantaneous on a wet film. Today's thinner films make acidification very rapid. How was this tested? Among other things we coated /gel/dummy emulsion/indicator dye/support and then looked at the spectrum of the indicator dye vs time. We also used a surface pH meter but that was too slow!
    ...
    PE
    Except under very special conditions you cannot have protons diffusing alone without also having diffusion of a counter ion (an ion of negative charge) in the same direction or diffusion of an ion of the same charge in the opposite direction.

    The reason is because you must maintain electrical neutrality, or at least a very close approximation to electrical neutrality. For example, if diffusion of the proton gets too far ahead of diffusion of the counter ion what happens is the build up of an electrical field with opposes the concentration gradient and cuts off further diffusion. (To put it in terms the electrical engineers use when discussing transport of charge carriers in a semiconductor, the drift current opposes the diffusion current.)

    One can estimate how much unipolar diffusion is allowed. Suppose that an emulsion were 10 microns thick. Further suppose that the relative dielectric constant of the wet emulsion approximates that of water (i.e. ~80). Further suppose that the pH of the emulsion drops from about 9 to about 7. I think these are all conservative assumptions as emulsions are, I think, usually a little thicker than this, the dielectric constant of the emulsion will be somewhat less than the dielectric constant of water, and the drop in pH for an acid wash will be greater than indicated above.

    Next, suppose for sake of discussion that the diffusion is nearly unipolar, i.e. dominated by proton transport. under the assumptions discussed above one can calculate the charge imbalance built up in the emulsion and estimate the voltage induced by this charge imbalance. It works out to about 10 volts.

    It is impossible for such a large voltage to be built up by diffusion. The most you can get is something of the order of thermal energy (about 0.025 electron, or about 40 times lower than the 10 volts implied by the calculation above). Therefore, unipolar diffusion of hydrogen ions cannot be a dominant process in a stop bath process. Therefore, diffusion must be ambipolar and therefore determined in large part by the diffusion coefficient of the counter ion. This diffusion coefficient will not be vastly different from the diffusion coefficients of developer molecules. Therefore although diffusion of the acid into the emulsion may be faster than diffusion of developer from the emulsion, it is not likely to be even an order of magnitude faster than diffusion of the developer, or in other words it will be on the same general time scale as diffusion of the acid.

    I am not contradicting your claim that diffusion of acid into the emulsion will be fast. What I am saying is that diffusion of developer out of the emulsion will also be fast. However, there is also a possible curve ball mother nature may throw at us. If the developer becomes adsorbed or bound to the gelatin molecules or other emulsion components this could slow the transport of developer from the emulsion.
    Last edited by alanrockwood; 07-22-2010 at 03:21 AM. Click to view previous post history. Reason: corrected a typo and added an additional comment



 

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