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    Sodium metabisulfite and dichromate in reversal process

    Why can't sodium or potassium metabisulfite be used in a clearing bath following a dichromate bleach?
    Last edited by Alessandro Serrao; 06-08-2013 at 07:48 PM. Click to view previous post history.

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    The recipe I have in front of me uses Sulfite, not Bisulfite which tells me the clearing bath should be neutral or alkaline, not acidic. This doesn't surprise me given that the bleach bath is quite acidic.
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    Rudeofus: then why with permanganic acid a metabisulfite clearing bath is used?
    I ask myself: what's the purpouse of the clearing bath?
    Is it to reduce CrVI to CrIII and to form Ag-sulfates complexes that are readily soluble in water?
    Sodium sulfite is Na2SO3, sodium metabisulfite is Na2S2O5. Looking at their chemical formulas I see that metabisulfite anion has five negatively charged oxygens (three of which are more negatively charged than the other two), the same as the anion in sulfite.
    So I guess the chemical reaction in bleach must be the same.

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    Quote Originally Posted by Alessandro Serrao View Post
    Rudeofus: then why with permanganic acid a metabisulfite clearing bath is used?
    I ask myself: what's the purpouse of the clearing bath?
    I don't know for sure, but removing the products that are created when Permanganate/Dichromate are reduced seems like a plausible answer to me. Note that Dichromate is used both for intensifiers and for bleach, which means depending on exact formulation and post treatment you can leave the green stain in the emulsion or not.

    Quote Originally Posted by Alessandro Serrao View Post
    Is it to reduce CrVI to CrIII and to form Ag-sulfates complexes that are readily soluble in water?
    You don't need Silver Sulfate complexes and I'm not even sure these exist. Silver Sulfate is water soluble on its own.

    Quote Originally Posted by Alessandro Serrao View Post
    Sodium sulfite is Na2SO3, sodium metabisulfite is Na2S2O5. Looking at their chemical formulas I see that metabisulfite anion has five negatively charged oxygens (three of which are more negatively charged than the other two), the same as the anion in sulfite.
    Metabisulfite S2O52- is the anhydride form of Bisulfite HSO3-: Na2S2O5 + H2O <=====> 2 NaHSO3

    which is the half way protonized version of Sulfite SO32-: NaHSO3 + H2O <=====> Na+ + H3O+ + SO32-

    So yes, once in aqueous solution they are more or less the same except for the fact that Bisulfite/Metabisulfite is much more acidic.
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    Quote Originally Posted by Rudeofus View Post
    You don't need Silver Sulfate complexes and I'm not even sure these exist. Silver Sulfate is water soluble on its own.
    The bleach converts the Ag(0) to silver sulfates complexes that, as you say, are readily water soluble. So it "eats" away the negative image leaving the positive one to be fogged and developed later.

    Clearing bath, as I understand it, only "neutralizes" the bleach and converts Cr(IV) to Cr(III).

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    Quote Originally Posted by Alessandro Serrao View Post
    then why with permanganic acid a metabisulfite clearing bath is used?
    I ask myself: what's the purpouse of the clearing bath?
    The clearing bath is used to insure that all chromium or manganese compounds are removed from the emulsion. Manganese easily forms manganese dioxide, a black, insoluble compound. The clearing bath must be acidic to prevent this compound from forming. Once formed manganese dioxide would be impossible to remove without destroying the emulsion.

    I believe the reason why people have difficulty with permanganate bleaches is because they compound them improperly. Both the bleach itself and the clearing bath must be acidic.

    When using a permanganate bleach it is important that the manganese goes from Mn (VII) to Mn (II). If the bleach process should be stopped at manganese dioxide Mn (IV) then roughly half the oxidizing power of the bleach is lost.
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    Quote Originally Posted by Alessandro Serrao View Post
    The bleach converts the Ag(0) to silver sulfates complexes that, as you say,
    I said Silver Sulfate, not Silver Sulfate complexes ....

    And no, the Sulfate won't etch away Silver as a droplet of Potassium Bromide solution will quickly show you. It does provide an anion that at least does not precipitate Silver ions.

    Quote Originally Posted by Alessandro Serrao View Post
    Clearing bath, as I understand it, only "neutralizes" the bleach and converts Cr(IV) to Cr(III).
    If "neutralizing" and "converting CrIV to CrIII " removes the chromium stain that is created (and used by chromium intensifiers), then the term "only" is a bit of an understatement ...
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    I will answer myself.
    Metabisulfite is acidic in aqueous solution. Sulfite is alkaline.
    Potassium permanganate is reduced to potassium manganate (which is green) in an alkaline environment. In quasi-neutral environment it's reduced to manganese dioxide, which can leave brown spots embedded on the emulsion. Thus sulfite is not recommended for use with permanganate based bleach because of this.



 

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