Potassium Dichromate Bleach?

Discussion in 'Color: Film, Paper, and Chemistry' started by Athiril, Apr 4, 2009.

  1. Athiril

    Athiril Subscriber

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    On one of my trips scouring the net I came across this reciple for b&w slide processing (http://personal.riverusers.com/~jdf/todd_walker/reversal.html)

    which lists a bleach recipe:
    water 1000.0 milliliters
    potassium dichromate 10.0 grams
    sulfuric acid 12.0 milliliters


    Can someone shed some light on this, if this bleach performs the same function as a colour bleach? and in effect can be used to bleach colour film (regardless of colour shifts).

    I was just thinking for us frugal digital darkroom users that this in terms of economy beats the pants off of even a copper sulfate bleach.
     
  2. Ian Grant

    Ian Grant Subscriber

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    It would work but it's not used for Colour work as it's gross overkill, it's a Black & White reversal bleach. It's more hazardous than Ferricyanide or EDTA based bleaches, the strong acidity is likely to cause a colour shift

    It's function is slightly different in that it converts all the silver to soluable Silver Chromates while conventional colour bleaches convert silver to a halide which is then removed by fixing.

    Ian
     
    Last edited by a moderator: Apr 4, 2009
  3. stefan4u

    stefan4u Member

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    As Ian told, this is the “classic“ BW reversal bleach. It does dissolve developed silver areas after first developing but does not touch undeveloped silver halides, which shall form the positive image after light exposing and second development.

    It’s very effective, re-usable, quite stable and does not harm film emulsion as bad as its eco-replacement (Potassium permanganate). But it’s toxic for almost every kind of living organism and a carcinogen, despite the fact it was used much more in “former” times.

    Be very careful if ever working with this and dispose it properly, never do a “sewer disposal” !!

    Regards,
    Stefan
     
  4. Athiril

    Athiril Subscriber

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    carcinogen? so if i wear a dust mask, and rubber gloves it's still a bad idea to handle?

    I guess copper sulphate bleach would be much better than.

    What kind of disposal? patch of lawn in the corner?
     
  5. stefan4u

    stefan4u Member

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  6. RobertV

    RobertV Member

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    The stuff is dangerous in that way the EU obliged manufactureres to switch over to Potassium Permanganate in the B&W reversal kits from Tetenal (in the mean time discontinued) and Foma.
     
  7. stefan4u

    stefan4u Member

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    High athiril

    Definitely it is dangerous, take a break and read that data sheet from “sciencelab”, please.

    What the EU says and the way they set priority is somehow relative. They just banned Mercury for thermometers etc. in April and now spreading it over whole Europe through CFL’s because they see a risk in incandescent bulbs. Strange world…

    Off topic, I know, but couldn’t resist :smile:

    Regards,
    Stefan
     
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  8. Ian Grant

    Ian Grant Subscriber

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    It's not THAT dangerous as long as you're careful. If it gets near a cut or scratch it will cause the wound to not heal properly & ulcerate, so use gloves.

    There are far worse chemicals that can be used for photography, I was responsible for running a laboratory (precious metal testing) and had to draw up the Health & Safety (COSSH) procedures to meet EU Regulations, they were quite sensible but thorough.

    Ian
     
  9. pentaxuser

    pentaxuser Subscriber

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    I haven't used it but Tim Rudman mentions its use several times in his excellent: The Photographer's Master Prining Course" It clearly forms a valuable weapon in his printing armoury. His book was first printed in 1994 so not in "the dark ages" as far as omitting to mention the need to take sensible precautions against harmful chemicals. He mentions it as being corrosive and causes skin ulceration as do several darkroom chemicals that require sensible precautions but to the best of my knowledge he is still in the rudest of health.

    So a sense of proportion may be needed here. Oh and as far as I am aware his title of Doctor refers to his medical background so he is likely to know what substances are just too dangerous to even think about using for the amateur darkroom enthusiast. It wouldn't appear to fall into that list.

    pentaxuser
     
  10. sanking

    sanking Member

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    Dichromate is definitely not very dangerous if used with reasonable care. In fact, in the dilute solutions we use it photographic applications it represents almost no threat after it goes into solution. Also, it is used in very minute amounts in photographic applications.

    Main precautions, take care in mixing the dry chemical into solution, don't drink the solution, and wear gloves to avoid dermal absorption.

    Sandy King








     
  11. Photo Engineer

    Photo Engineer Subscriber

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    Dichromate is not all that dangerous. I agree with Ian and Sandy. It does have problems, but it will remove silver and form the Silver Sulfate (soluable) salt and that is why the bleach is a mix of Dichromate and Sulfuric Acid.

    The problem is that it cannot be used in the C-41 process cycle unless you do the folowing process:

    Color develop as usual
    Stop + Sodium Sulfite
    Wash
    Bleach with Dichromate or Permanganate or Ferricyanide bleach - your choice.
    Clear bath of Sodium Sulfite
    Wash
    Fix
    Wash
    Stabilize.

    If you do not use the stop and clear as noted above you are going to have problems.

    Also, there is no guarantee that the C-41 dyes will survive the extreme acid of the Dichromate or Permanganate bleach baths. That is your problem.

    The same statements can be said about E6 use of these bleaches.

    PE
     
  12. stefan4u

    stefan4u Member

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    I don’t wanna add fuel to the flames, but still tend to disagree; it’s described to be quite poisonous, carcinogenetic, mutagen and teratogenic. It was "upgraded" in the mid 90's as far as I know. But OK, these nasty risks can be widely reduced by safe and accurate handling. Wearing nitrile rubber gloves, avoiding any skin contact and never ever inhale dust are the way to handle it. But it is stays potentially dangerous.

    LC50 for a rat is 0.094mg/L/4h inhaled…

    Lowest Published Lethal Dose:
    LDL [Man] - Route: Oral; Dose: 143 mg/kg
    LDL [Child] - Route: Oral; Dose 26 mg/kg

    Other sources claim that 0.5g till 1g swallowed kill a man (most likely a child), the truth will be somewhere between. Store it well and safe !
    Absolutely nothing to use without need, a color bleach does not have to be that poisons. The old stlyle Potassium ferricynide looks nearly healthy, compared: LDL [Rat] - route: Oral; Dose: 1600 mg/kg, (LD50) [Mouse]: Oral: 2970 mg/kg (Potassium ferricyanide)
    Being aware that there are more noxious substances and no one intentionally “messes around” with chemicals, common darkroom chemicals can be described as “fountain of youth” if compared. Well, maybe accidently hydrogen cyanide releasing compounds excluded…

    Further concern is putting this into the drain after usage; Chromates tend to slip through sewage plants or can harm the microbes for biological treatment. ChromVI is very poisonous for flora and fauna in rivers and seas and does not vanish due sedimentation, it too soluble…maybe a biologist can describe this better…

    These are my information and this stuff is one of the few (hobby) lab chemicals I give to hazardous waste collection after (very rare) usage.

    Regards,
    Stefan
     
  13. Athiril

    Athiril Subscriber

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    Thanks for the tips Photo Engineer,

    So, I can always make the working solution weaker for colour?

    Hmm I'll do more research, vanbar has all of these different chemicals for sale, so I do have a choice of what I can make
     
  14. mts

    mts Subscriber

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    Your Pot. dichromate/sulfuric acid "bleach" is called tray cleaner in the old Kodak B&W formulas booklet. It works to clean out badly stained trays and bottles but isn't good for the environment. A similar chromic acid compound was used as boiler cleaner to clean steam lines for many years until it too was banned for the above mentioned environmental and health reasons. It's certainly not acceptable as bleach for film. The ferricyanide formula can be used as PE notes, but it is by no means ideal for modern films. It was the original C-22 bleach for Kodacolor-I, II when those emulsions were introduced well before C-41 came to life.
     
  15. Alessandro Serrao

    Alessandro Serrao Member

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    Not all dust masks are approved to be effective with chromates dust. Wear only approved dust mask or integral respirator (consult your local authority since dust mask types vary with countries).
    You should wear NIRTILE gloves (not simple rubber ones) and safety goggles, apron ecc...

    Mix the required amount in open space to minimize airborne dust.

    Permanganate doesn't damage emulsion if use half-strenght of what's recommended.

    However the dichromate bleach won't work for your intentions...

    :smile:
     
  16. Photo Engineer

    Photo Engineer Subscriber

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    I didn't say that.

    PE
     
  17. DarkroomExperimente

    DarkroomExperimente Member

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    I used Kodak's "system cleaner" for my BW reversal bleach...easier than mixing it from scratch

    re:toxicity ....doesn't the sodium sulfite bath turn the hexavalent chromium into a less nasty valence of chromium?

    the first time I used this bleach it was about midnight and I realized I had the dichromate, but no acid...I went out to my car with an eyedropper and a huge grin...took 12ml of acid out of my car battery...worked perfect
     
  18. Photo Engineer

    Photo Engineer Subscriber

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    Battery acid is about 35% sulfuric acid or thereabouts. It will work.

    Chromium is chromium and can be toxic in several valence states so as I said above... It is not as bad as all that, but it is not benign by any means and in any state. Be careful.

    The sulfite is more to reduce the dichromate so that it does not destroy the hypo in the fixer and it does not stain the film with chromium salt mixes.

    PE
     
  19. RPC

    RPC Member

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    Is it necessary to use a clearing sulfite bath when using ferricyanide? I have used a ferricyanide bleach along with the stop + sodium sulfite, but not the clearing sulfite bath after the bleach step, and have not noticed any problems.

    RPC
     
  20. Photo Engineer

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    Ferricyanide will oxidize hypo very rapidly and form sulfur, sulfur dioxide and hydrogen sulfide. The sulfur can cause a haze in your film and the overall oxidation can reduce the life and capacity of the fixer.

    The choice of what to do is yours. If it works, what can I say but let you know what may happen.

    PE
     
  21. RPC

    RPC Member

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    Wouldn't the wash after the bleach remove enough ferricyanide? If I decide to try the clearing bath, how many g/liter of sulfite should be used?

    RPC
     
  22. Photo Engineer

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    There are several clearing bath formulas posted here and there. It is the one used for reversal B&W processing.

    The wash after the bleach is iffy due to the variable nature of water supplies and your method of washing such as flow rate and agitation. So, I can't really tell. All I can do is tell you what you "MIGHT" get.

    PE
     
  23. Ricardoleite

    Ricardoleite Member

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    I've ben using potassium bichromate for 15 years now, i've mixed maybe more than 200 doses of bleach since i began working with film.

    Some months ago my doctor found a lump of 2cm in my thyroid, i must say i got scared, after all there are some studies about Thyroid Hyperplasia to people exposed to chromium, i'm waiting for the results of the tests,

    But tell me Ian, what other chemistry do you think is more dangerous? I've been working in my lab for years and now i'm switching all toxic chemistry to low or non-toxic components like ascorbic acid, phenidone, etc.
     
  24. Photo Engineer

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    We had over 2,500 people in the Research Labs at Kodak. Studies showed that the causes of death or illness of these people were about identical to the other people that were tracked in such statistics.

    In fact, I took blood tests every 6 months keeping track of the status of my health while I was a bench chemist or pkoto engineer.

    PE