Working life of permanganate bleach?

Discussion in 'B&W: Film, Paper, Chemistry' started by pdeeh, Jun 25, 2014.

  1. pdeeh

    pdeeh Member

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    One bleach used for black&white reversal is a mixture of Sulfuric acid and Potassium permanganate.

    The general recommendation in the literature I have read (both online and in older printed material, e.g. BJP Annuals) is that the Sulfuric and Permanganate solutions are made up separately and only combined just before use, as the mixed bleach has a very short working life.

    However, I haven't found anywhere a quantification of "short", or a description of why the working life would be short.

    Is it a matter of a few minutes, or tens of minutes or an hour or so?

    The reason for enquiring is that I am about to try reversal processing of (RC) paper negatives, and it would be convenient not to have to make up a fresh batch of bleach for each negative.

    I also wondered about the capacity of this bleach (i.e. if the working life were long enough, could I re-use the bleach for a few sheets?)

    I do realise, by the by, that a dichromate bleach has fewer shortcomings (potential for staining, emulsion damage etc) than permanganate, but I am unwilling to handle dichromate because of its toxicity, and permanganate is anyway what I have to hand.

    If I have to make bleach for every sheet I process, so be it, but it would add to my stock of knowledge to have an understanding of the limitations of the material I use, hence the question.

    (if someone wants to explain why the working life is short without using reaction equations, that'd be good :smile:)
     
  2. Gerald C Koch

    Gerald C Koch Member

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    The useful life of this bleach is short but it should last for an hour or so. Long enough for use a session of making paper negatives. if it appears to be no longer working or only partially working then it should be discarded. Permanganate bleaches soften the emulsion so treat your paper negatives with care. It is important that the bleach not become alkaline as this may cause staining.
     
  3. pdeeh

    pdeeh Member

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    Thank you Gerald. Your point about the bleach not becoming alkaline prompts the following thought:

    In one of the threads I have read in researching this, PE suggested that after 1st development (as well as after bleaching), a Sodium sulfite clearing bath should be used (this was however in the context of using a dichromate bleach).

    If I am to avoid any alkali carryover into the bleach, would it be better for me to follow the 1st developer with a mild acid stop and then wash before bleaching, and omit a clearing bath at this stage ?

    Or to use a metabisulfite as a combined stop&clear, and then wash before bleaching (as I will have a metabisulfite clearing bath prepared anyway for after the permanganate bleach)?
     
  4. Gerald C Koch

    Gerald C Koch Member

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    Just follow Ilford's instructions. I would not use the sulfite clearing bath with the permanganate bleach.