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  1. #1
    BradS's Avatar
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    Crawley's FX-2, Glycin and Potasium Carbonate ????

    Why does FX-2 call for Potasium Carbonate instead of Sodium Carbonate? Could one substitute Sodium Carbonate for the Potasium Carbonate in the formula? At what ratio? and with what effect?


    TIA.

  2. #2

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    Not only does it call for potassium carbonate but potassium carbonate, sesquihydrate. The reason, given by Crawley in either the original BJ article or as a later note, is that the sesquihydate contains some potassium bicarbonate which acts as a buffer.

    However, there is a modification of unknown origin known as FX-2K which uses a B solution containing 97.5 g/l of sodium metaborate. This is said to produce somewhat smoother tonality with a slight loss of acutance.

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    Lachlan Young's Avatar
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    Quote Originally Posted by Gerald Koch
    Not only does it call for potassium carbonate but potassium carbonate, sesquihydrate. The reason, given by Crawley in either the original BJ article or as a later note, is that the sesquicarbonate contains some potassium bicarbonate which acts as a buffer.

    However, there is a modification of unknown origin known as FX-2K which uses a B solution containing 97.5 g/l of sodium metaborate. This is said to produce somewhat smoother tonality with a slight loss of acutance.
    I think FX-2K was a Bill Troop modification as was the suggestion that 75ml of solution A could be used with 75ml of a solution B made from 150g Kodalk in 1l of water with 30-60g of sodium chloride being added to the 1l of working solution.

    Hope this helps,

    Lachlan
    "A squid eating dough in a polyethylene bag is fast and bulbous...got me?" Captain Beefheart

  4. #4

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    Quote Originally Posted by BradS
    Why does FX-2 call for Potasium Carbonate instead of Sodium Carbonate? Could one substitute Sodium Carbonate for the Potasium Carbonate in the formula? At what ratio? and with what effect?


    TIA.
    Potassium Carbonate is more soluble than Sodium Carbonate so you can make a more concentrated stock solution with Potassium Carbonate.

    However, I use the Sodium Carbonate version and it works fine.

    The 1971 Dignan Newsletter published a sodium carbonate version of Crawley's FX-2.

    See the APUG Chemical Recipes: http://www.apug.org/forums/article.php?a=48
    Tom Hoskinson
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  5. #5

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    In my copy of "The Darkroom Cookbook", the FX-2 formula call for crystalline Pot. Carb.
    Is this the same as sesquihydrate? Sorry, I never did well in HS chemistry.

  6. #6
    BradS's Avatar
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    Thanks. The Kodalk idea is interesting - although I do not have Kodalk at hand either. It seems that solution B could, in this case be approximated with a solution of Borax and Lye - but, again in what poroportions? I think I asked this somewhere else...will go search.


    Will also give the Straight Sodium Carb formula recommended by Tom H. a go. How much base fog can be expected with this version?

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    Quote Originally Posted by Tom Hoskinson
    Potassium Carbonate is more soluble than Sodium Carbonate so you can make a more concentrated stock solution with Potassium Carbonate.
    Usually this is the reason for potassium salts being used rather than the sodium ones. However, in the case of FX-2 there is no solubility problem with the amount of carbonate needed in the developer. Potassium alkalies are somewhat more active than the sodium ones in photographic developers and this may be the reason why it was used in the original formula.

  8. #8
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    Quote Originally Posted by Gerald Koch
    Usually this is the reason for potassium salts being used rather than the sodium ones. However, in the case of FX-2 there is no solubility problem with the amount of carbonate needed in the developer. Potassium alkalies are somewhat more active than the sodium ones in photographic developers and this may be the reason why it was used in the original formula.
    I seem to remember a comment that crystalline Potassium Carbonate had some degree of buffering - is this correct?

    Thanks,

    Lachlan
    "A squid eating dough in a polyethylene bag is fast and bulbous...got me?" Captain Beefheart

  9. #9

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    Quote Originally Posted by Gerald Koch
    Usually this is the reason for potassium salts being used rather than the sodium ones. However, in the case of FX-2 there is no solubility problem with the amount of carbonate needed in the developer. Potassium alkalies are somewhat more active than the sodium ones in photographic developers and this may be the reason why it was used in the original formula.
    I believe Crawley published a concentrated FX-2 stock solution recipe in the BJPA that called out a rather large amount of Potassium Carbonate.

    In any event, I use the Dignan Sodium Carbonate Recipe (with no Pincryptol).

    I get excellent results (with Kodak TMAX, Ilford Delta and EFke) with low fog levels.
    Tom Hoskinson
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  10. #10

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    Quote Originally Posted by Lachlan Young
    I seem to remember a comment that crystalline Potassium Carbonate had some degree of buffering - is this correct?

    Thanks,

    Lachlan
    The amount of buffering would depend on the amount of potassium bicarbonate present. This would not be very large if the potassium carbonate is of reasonable purity.

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