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  1. #1
    hrst's Avatar
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    Washing and fogging; thoughts and questions

    As we are going to make our next emulsion experiment very soon (after a long pause), some thoughts have arisen concerning washing and fogging.

    My understanding about latent image may be a bit incomplete, but I'm presuming that single metallic Ag atoms work as development sites and without them there is no latent image and thus no fog.

    Basically, I can understand that overwashing causes the bromide level to go too low and that a bit of extra bromide salt is needed to avoid fogging. Is this because extra bromide can prevent Ag from reducing from AgBr, or even by reacting with reduced Ag to form AgBr again?

    Then, what are the concequences of underwashing? Is it just problem with adhesion to film base because of the salts precipitating when drying? Or is there something else? Can underwashing also cause fogging? As I see it, IMO fog is quite an evil enemy, so I'd rather under- than overwash. Does this make sense?

    Well, then, about the washing. If it's just the bromide level that has to remain at a certain level, and other products (like NO3 ions) can be safely washed completely out, wouldn't it be a good idea to add a tad of NaBr to wash water? As long as the noodles have higher bromide, diffusion would work quite the same way as with pure water (well a bit slower). But as bromide goes lower in emulsion, a low level of bromide in water would stop the diffusion at the desired point. Washing of other salts would continue, though. Is this sensible at all?

    If this is correct, I would guess that adding between 0.029 and 0.192 g/l NaBr to the wash water (maybe after the initial 0.5 hour that is safe anyway without the bromide) would be a great thing to do as a guarantee against
    overwashing. The above-mentioned figures was calculated by PE for me when I asked about adding bromide to an already overwashed emulsion. But now, I would be adding it to wash water beforehand. It would go into the emulsion by diffusion if already overwashed, or just prevent the overwashing by stopping the diffusion at the desired level.

    Q#3, could ferricyanide+bromide bleach be used to remove any latent image / fog (convert any silver atoms to AgBr)?


    Thanks.

  2. #2
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    Overwashing does cause fog, but not instantly in most cases. If you go much over +50 mv in vAg during making, you are pretty much fogged, but if you do this during the wash, there is little immediate problem.

    Underwashing causes crystals to form after drydown of the coating (unless it is a FB paper coating - many early paper emulsions were unwashed). Underwashing also slows down sensitization with sulfur or sulfur + gold. More heat and/or longer time will generally fix this problem.

    Ferricyanide + bromide is a rather rare method used in this situation and can cause problems. I think that there is a patent on it by J. Hoppe of EK. The first version of Kodacolor 400 used a method similar to this for sensitization and fog supression. However, the oxidation process did something else as well. I am vague on the details as this was over 40 years ago that John Hoppe discussed this with me. So, I may be way off. If so, sorry.

    The bottom line is to aim for 50 mv at the end of the was, or adjust to this point if you are more positive in vAg.

    PE

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    hrst -

    I've got the feeling that you're in a university setting, am I correct?

    If so, have someone dig around in the chemsitry department and look for a pH reference electrode that you can borrow, and then with a 10 cm length of silver wire (I recommend 99.99% pure silver at 12 gauge or so, not thin wire) you can make yourself a silver ion specific electrode and combined with the pH reference electrode and a digital voltmeter or a pH meter that can read in mV, you can make readings of the wash water for bromide concentration.

    PE and I have both made silver ion specific electrodes out of silver wire, glass tubing, and a little epoxy glue. It takes just a few minutes to do. (Making a reference electrode could be fairly easy, but neither of use have done so yet so that's still to be proven how easy it is!)

    There's a few tricks like plating the silver wire before use, but we can cover that if you decide to go that route.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  4. #4
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    I have posted photos of my electrodes in another thread with a description of the plating and data on measuring vAg.

    PE

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    hrst's Avatar
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    Yes Kirk you are right. I could probably borrow a pH reference electrode. Or if not borrow, then maybe buy from VWR.

    Using the information provided by PE and others I could measure vAg and this would probably solve many of the uncertainties around the washing sequence in a professional way.

    Thanks for your answers and tips.

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    Agfa used to use resistance to measure the final wash status. IDK what the proper values are though, but it can be done.

    For vAg the proper reference electrode is a double junction Ag/AgCl electrode, and you should use a bridge as I showed in my previous posts on this subject.

    PE

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    hrst -

    If you're going to buy a reference electrode, look for something like the Orion Ag/AgCl Double Junction Reference half cell model 90-02 (Orion Cat# 900200). PE has a Hanna cell that he uses and I'm sure he can pass along that part number.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!



 

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