[RA4] Citric Acid Stop Bath?

Discussion in 'Color: Film, Paper, and Chemistry' started by perkeleellinen, Feb 2, 2010.

  1. perkeleellinen

    perkeleellinen Member

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    I've read lots about using citric acid as a stop, but all the posts are in the B&W forums. This is just to ask is there any issues with RA4 processing?

    I currently use white vinegar diluted 1:4 as a stop and that's fine. The problem is that my wife likes to use white vinegar for cleaning and we both blame each other when the stocks are low! Last night we had only a bit left and my stop was quite weak. I got horrible blue/green streaks on my prints and now I'll have to dump my blix which I'm not happy about.

    Can I change over to a citric acid stop?
     
  2. brian_mk

    brian_mk Member

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    I wouldn't use citric acid stop bath.
    I tried it in my Nova tank once (RA-4).
    After several days, it was contaminated with what looked like some sort of scummy organic matter.

    I used to buy glacial acetic acid from Rayco, but they went out of business some time ago.
    It's pretty cheap and you only need 20ml/litre.

    I'm not sure where you can buy glacial acid these days but you can buy 80% solution from places like ag-photographic or sliverprint.
     
  3. perkeleellinen

    perkeleellinen Member

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    My plan was to use it one-shot and chuck it out after one printing session (4-6 prints).
     
  4. pentaxuser

    pentaxuser Subscriber

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    Steve I can't see why citric acid which works for B&W paper shouldn't work for RA4 unless there is something about the chemical properties of citric acid which fails to do the same job with RA4 dev. I would also have thought that if you can buy citric acid powder in bulk it would work out cheaper than white vinegar and be purer. It may be that white vinegar is pure but it may also contain ingredients that citric acid crystals do not.

    For B&W the general amount is about 15g per litre. Homebrew shops used to sell it in large quantities.


    pentaxuser
     
  5. hrst

    hrst Member

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    I always use citric acid stop for RA-4 (in trays at room temp). Never had any problems with it.
     
  6. Photo Engineer

    Photo Engineer Subscriber

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    AFAIK, Citric Acid should not be used with color products.

    Don't ask. That was a suggestion I had from years ago. I think it might be the lesser acidity of the Citric Acid in comparison to Acetic Acid.

    PE
     
  7. RPC

    RPC Member

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    IIRC the old Kodak Hobby Pacs for making prints (EP-2 process) used citric acid in the stop bath.

    RPC
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    I am not aware that the hobby pacs had citric acid. If I have one here I will check, but IIRC, they were all liquid kits and did use acetic acid. I was called from Christmas vacation in 1968 to work on a fix for the process, and had to come up with a fix. We made several variants and the 1 - 2% acetic acid won out for quality.

    Perhaps the citric acid works one shot, IDK, but the stop we designed was acetic acid.

    You see, it must be strong enough to extract the color developer from the coupler droplets so that none is retained in the coating. If CD3 is retained, the dye stability is affected. Citric acid did not acidify or extract well IIRC.

    PE
     
  9. Bob F.

    Bob F. Member

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    Confused here: Kodak say you do not need any stop, but that you should use a rinse if you do use a stop with Kodacolor RA chemicals.

    From Kodak pub. j39.pdf:
    From which I assume that as long as you don't process too many sq inches of paper you should be OK without?
     
  10. RPC

    RPC Member

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    I used the Pacs in the mid-eighties, it was the only chemistry I purchased then, and I remember the stop bath packet (liquid) saying it contained citric acid, but I could be mistaken. I didn't used them but threw them away and instead I just used acetic acid I already had mixed, so I don't know how well it would have worked, if that is what it was.

    RPC
     
  11. hrst

    hrst Member

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    Affect dye stability? This is interesting. I though the blix and a generous final wash would remove all developer from the coating. After all, there is no stop in the machine process, if I'm right?

    Anyway, I've never had any streaking with citric acid in trays used for the whole printing session and dumped after that.
     
  12. pentaxuser

    pentaxuser Subscriber

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    Well based on what I have heard here from an RA4 specialist such as PE, it would seem sensible to use acetic acid even if citric will do the job albeit at much quicker exhaustion.

    Steve, Fotospeed does a 1L pack which is quite strong, needing 1+29 dilution and at AgPhotographic's price is quite inexpensive at these dilutions. Actually cheaper than Champion which I always thought wrongly to be the cheapest but not so if you take dilutions into account.Acetic acid stop bath is what I always use for B&W and RA4 as I don't mind the smell. I think at 1+29 you might be hard pressed to beat this price-wise with white vinegar.

    pentaxuser
     
  13. Photo Engineer

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    Couplers are dissolved in coupler solvents to disperse them in droplets in the emulsion prior to coating. During processing, the CD3 migrates into the droplets as either the oxidized product forming dye, or the unoxidized free base which can remain.

    This free base, if carried through the process with bad agitation or exhausted blix or bad wash will cause a pink stain and loss of dye. The stop bath is there as a safeguard as this problem is also related to the blue/cyan stain you get in rotary processes with no stop. The rotary (drum) processes such as the Jobo are prone to this problem.

    We wanted to take no chance with bad dye stability and so if you see streaking on any prints or a cyan dmin it is indicative of poor removal of developing agent in the paper and needs a good stop. Acetic acid is the best choice for this IMHO having worked on the problem through vacation to make dye stability better for you. This is not something you would hear about generally because I feel bad about the problem. It cropped up after release and we had missed it due to our high quality processing. Medium to poor processes show this problem.

    So, why take a chance.

    Besides, a good stop gives better whites, longer life to the blix and improves the wash rate.

    So, there you are! Remember that all processes, water supplies and peoples work flows are not created equal. The process is aimed at the worst possible conditions now, one that I personally missed way back then and am very very aware of!

    BTW, one of the reasons it was missed was that EP2 was to have been EP3 with a final stabilzer. This pH 4.5 stabilzer with Citric Acid, was the final bath and did do the same job after the blix had first shot at the CD3. Most customers did not want the stabilizer so it was eliminated from the process thereby revealing the problem too late.

    Interestingly, this pink stain appears and then vanishes so that was another reason why we missed it. But, once the process has begun, it does not stop even though the prints may appear normal.

    If you wish to see the pink stain, use an acetic acid stop repeatedly without dumping it, until it is nearly exhausted, and it turns bright pink. Addition of sulfte (which is in the blix) eliminiates this pink color. If you add a pinch of sulfite to the stop, the pink color vanishes. Same reason as it vanishes in the paper. It is being sulfonated in this case by the Sulfite. Good blix was intended to prevent this, and so a well replenished fresh blix is also another reason why the photofinishers see little of this type of problem.

    I hope now that you see the ramifications of color system development and the common problems you may experience if you make unwarranted substitutions. Good luck.

    PE
     
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  15. perkeleellinen

    perkeleellinen Member

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    Acetic acid may be the way to go. I hope the smell isn't too offensive. My darkroom is a cupboard right next to the living room and smells emerging may cause some concern with my wife. Stop bath is my Achilles Heel with RA4: my heated Nova is two-slot necessitating a transfer into a tray of stop and then back into the processor. Works fine, vinegar stop wasn't too smelly, citric acid I hoped would be less so. Acetic acid stop from memory is stronger smelling.
     
  16. hrst

    hrst Member

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    PE, thank you again very much for this deep insight. The odor problem with acetic acid is not that bad. Good to know it's better for RA-4. As you say, why take a chance.
     
  17. Alessandro Serrao

    Alessandro Serrao Member

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    Here comes the doubt: I knew citric acid is more acidic than acetic acid I guess, since it has a higher pKa than acetic acid (first dissociation).
    Am I wrong?
     
  18. Photo Engineer

    Photo Engineer Subscriber

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    Actually, at equal Normality, both have about the same pH in solution and about the same concentration or about 5 g - 6 g/L for a 0.1 N solution. However, I have never heard of anyone getting citric acid burns from contact with concentrated citric acid solution, but I have gotten burns from concentrated acetic acid solutions.

    Yes, acetic acid can be more acidic, or be effectively more acidic in solution.

    Remember that 1 N acetic = 1 M acetic, but only 1/3 M Citric.

    PE
     
  19. Kirk Keyes

    Kirk Keyes Member

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    Keep in mind there's a difference between corrosiveness and acidity.
     
  20. Photo Engineer

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    Yes.

    There is also solubility in fat and skin and diffusion rate. These are related to activity in swollen gelatin as well as in contacts with the human body. Citric acid is much more polar and a very large molecule compared to Acdtic Acid. All of these were factors in the design of this process.

    Although corrosiveness and acidity are hard to separate out and measure easily.

    PE
     
  21. brian_mk

    brian_mk Member

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    Just discovered you can buy 500ml glacial acetic acid online for £8.96 in the UK from Abbey Chemicals:-
    http://www.abbey-chemicals.co.uk/

    Not sure what the delivery charge is or if prices exclude VAT.
    500ml should make about 25 litres of stop bath.
    Works out less than 50p/litre when diluted.
     
  22. perkeleellinen

    perkeleellinen Member

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    I just checked the postage on the glacial acetic acid, it's £12.50 with a 10-14 day delivery period "to ensure where possible they will be used for appropriate experiments".
     
  23. brian_mk

    brian_mk Member

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    Ouch - that's a bit pricey!
     
  24. brian_mk

    brian_mk Member

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  25. brian_mk

    brian_mk Member

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  26. Tom Taylor

    Tom Taylor Member

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    I just checked my Kodak Publication on Tray, Drum and Rotary Tube RA-4Processing. While the stop and wash steps are indeed optional for tray and drum processing, it is apparently required for rotary tube processing.

    Boy, I was really suprised to see the prices on has to pay for glacial in Europe! I last paid $27.95US for 1 gallon of Kodak Glacial which will make 64 US Quarts of stop. That's less that 50 cents a quart. Kodak recently announced they were no longer going to make Glacial so it's a good time to buy another gallon.