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  1. #11
    Ole
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    And while I'm at it - I myself would never under any circumstances use concentrated sulfuric acid if I could get by with a 10% solution. Concentrated sulfuric acid is a truly horribly nasty stuff! The only acid I fear more (and have used) is hydrofluoric.
    -- Ole Tjugen, Luddite Elitist
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    ...and about bichromates

    Quote Originally Posted by Ole
    And while I'm at it - I myself would never under any circumstances use concentrated sulfuric acid if I could get by with a 10% solution. Concentrated sulfuric acid is a truly horribly nasty stuff! The only acid I fear more (and have used) is hydrofluoric.
    It seems that here in Italy, where I live, I cannot get access to potassium permanganate, because of local laws that forbid the selling.
    So I've searched around and found a substitute for it: potassium bichromate.
    Probably one of the nastiest stuff out there: [COLOR=Navy]http://chemdat.merck.de/pls/pi03/web...78-50-9&lang=4[/COLOR]
    is [COLOR=DarkRed]carcinogenic[/COLOR], [COLOR=DarkRed]mutagenic by inhalation[/COLOR], it's classified [COLOR=DarkRed]T+[/COLOR] and has a [COLOR=DarkRed]R-phase 49-46 [/COLOR] (the nastiest).
    So I'm at a loss: what I'm gonna do?

  3. #13
    Ole
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    Alessandro, is that bichromate or dichromate?

    Potassium dichromate (K2Cr2O7) is used in many bleaches, while I've never heard of a use for Potassium dichromate (KHCrO4)!

    Anyway: The DIchromate is nasty, oxidizing, possibly carcinogenic, and quite safe if handled carefully - just don't throw it around. Potassium permanganate is all of the above, in addition it's strongly oxidizing, explosive when mixed with some other chemicals, stains everything, etc...

    If you can get K2Cr2O7 but not K2MnO4, there's probably a reason for it: Dichromate is safer!
    -- Ole Tjugen, Luddite Elitist
    Norway

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    Quote Originally Posted by Ole
    Alessandro, is that bichromate or dichromate?

    Potassium dichromate (K2Cr2O7) is used in many bleaches, while I've never heard of a use for Potassium dichromate (KHCrO4)!

    Anyway: The DIchromate is nasty, oxidizing, possibly carcinogenic, and quite safe if handled carefully - just don't throw it around. Potassium permanganate is all of the above, in addition it's strongly oxidizing, explosive when mixed with some other chemicals, stains everything, etc...

    If you can get K2Cr2O7 but not K2MnO4, there's probably a reason for it: Dichromate is safer!
    Potassium dichromate is the same as potassium bichromate: they are merely synonym, as you can see from the Merck web page.

    And potassium permanganate is way less toxic that the bichromate counterpart (is not carcinogenic nor mutagenic for example), again refer to http://chemdat.merck.de/pls/pi03/web...nganate&lang=4

  5. #15
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    You might try Edwal Tray Cleaner. It will bleach silver images to a form that is soluble in sulfite solution without dissolving the undeveloped silver halide. It has been a long time since I tried it, so can't remember a particular dilution, but that part of the process goes to completion anyway, so it shouldn't matter too much. In any case, try it on something less than precious.
    Gadget Gainer

  6. #16

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    Quote Originally Posted by gainer
    You might try Edwal Tray Cleaner. It will bleach silver images to a form that is soluble in sulfite solution without dissolving the undeveloped silver halide. It has been a long time since I tried it, so can't remember a particular dilution, but that part of the process goes to completion anyway, so it shouldn't matter too much. In any case, try it on something less than precious.
    Thanks Patrick but Edwal is not accessible in Italy. I think I'll stick with the Foma b&w slide kit, available at 19€ per 8 rolls. Too nasty the bleaches are to me . Thanks anyway.

  7. #17
    Ole
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    Quote Originally Posted by Alessandro Serrao
    Potassium dichromate is the same as potassium bichromate: they are merely synonym, as you can see from the Merck web page.

    And potassium permanganate is way less toxic that the bichromate counterpart (is not carcinogenic nor mutagenic for example), again refer to http://chemdat.merck.de/pls/pi03/web...nganate&lang=4

    Ehrm .... In this case, I disagree. Di- is not Bi- , in this case; in almost all others they're equivalent. I make an exception for Chromium and Silicon.

    If Merck claim that Permanganate is not carcinogenic or mutagenic they're ignoring 30 years of evidence to the contrary.
    -- Ole Tjugen, Luddite Elitist
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  8. #18
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    Quote Originally Posted by Adrian Twiss
    Slightly off thread but I cannot get Sulphuric Acid in concentrations of greater than 10%.?
    go to B&Q, (UK market) they have a drain cleaner which is concentrated Sulphuric Acid.

    It did clean my drains !

    Or buy from Scientific & Chemical Supplies in Wolverhampton but you need to order on company headed notepaper.

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    Quote Originally Posted by Ole
    Ehrm .... In this case, I disagree. Di- is not Bi- , in this case; in almost all others they're equivalent. I make an exception for Chromium and Silicon.

    If Merck claim that Permanganate is not carcinogenic or mutagenic they're ignoring 30 years of evidence to the contrary.
    Well, all msds sheets on the net state that KMnO4 is not carcinogenic nor mutagenic: if you have other evidences I'm very interested, so please post them here.
    I miss your explanation of bi- not being di-: could you be more precise?

  10. #20

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    Quote Originally Posted by Alessandro Serrao
    Well, all msds sheets on the net state that KMnO4 is not carcinogenic nor mutagenic: if you have other evidences I'm very interested, so please post them here.
    I miss your explanation of bi- not being di-: could you be more precise?
    Bi- vs. di- first. The use of 'bisulfate', 'bicarbonate' and names like them is not considered 'proper' chemical form nowadays because of this very ambiguity. Those names have a lot of historical precedent behind them, though, which is why we use them. They come from simple stoichiometry -- think of it this way -- sodium bisulfate (NaHSO4) has twice the number of sulfate ions per sodium ion that sodium sulfate has (Na2SO4). This is where the 'bi' comes from. The 'di' notation would refer to the relative 'numbers' of each ion.

    It is likely that in other languages 'bi' and 'di' are used interchangeably or 'bi' is used where 'di' would be used in English. I know this to be the case in French. This may be where Alessandro and Ole's disagreement stems from. I have never heard of 'acid chromate' salts as there is a dichromate-chromate equilibrium that is pH-dependent.

    To complicate matters even further, there is an additional nomenclature that uses 'acid salt' terminology. NaHSO4 would then be called 'sodium acid sulfate'. So NaHSO4 = sodium bisulfate = sodium hydrogen sulfate = sodium acid sulfate. Confused yet?

    There are specific reasons why bleaches are acidified with sulfuric acid (or sources of it like sodium hydrogen sulfate). The acid required has to be strong enough to generate the oxidizing species ('permanganic acid' in the case of permanganate bleaches and chromic acid in chromium bleaches) and I doubt highly that acetic acid is up to the job -- it is simply too weak an acid (in terms of acidity constants). The acid must provide an anion that forms a soluble salt with silver, which sulfuric acid does (silver sulfate is water- soluble), so that the oxidized silver dissolves into solution rather than remaining in the film. Finally, the acid must itself not have a detrimental effect on the remaining silver halide in the film. Sulfuric acid seems to fit the bill nicely for all these requirements. I've never seen a reversal bleach that is not acidified with sulfuric acid or one of its derivatives (I would be very interested in hearing of any examples that are).

    Ole, you are right about my posts. I was right the first time. I agree with you that sulfates without hydrogen form alkaline solutions, but this effect is very weak, as the 'second' proton in sulfuric acid is still pretty strong (though not as strong as the 'first'). People bathe in magnesium sulfate solutions as a muscle relaxant (Epsom salts).

    Re: toxicity -- there was a vigorous debate about this a few months ago in another thread which I will not re-hash. Suffice it to say that I am very surprised that dichromate is available for sale in Italy while permanganate is not for toxicity reasons. Dichromate is a potent carcinogen and persistent, while permanganate tends to decompose pretty quickly and is not nearly as toxic (it is used in dermatological preparations). AFAIK Kodak replaced the dichromate in their TMX reversal kit with permanganate for that very reason.

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