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  1. #21

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    Potassium ion only becomes a problem when you are dealing with rather high concentrations. The amount of carry over is negliable when using a stopbath or rinse. IIRC, the problem with potassium ion slowing the rate of fixation was first noticed with certain Agfa fixer formulas which contained potassium thiosulfate instead of the sodium or ammonium salt.

  2. #22
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    Quote Originally Posted by Gerald Koch
    Potassium ion only becomes a problem when you are dealing with rather high concentrations. The amount of carry over is negliable when using a stopbath or rinse. IIRC, the problem with potassium ion slowing the rate of fixation was first noticed with certain Agfa fixer formulas which contained potassium thiosulfate instead of the sodium or ammonium salt.
    Gerald;

    I'm not sure about the Agfa fixer you mention above, but Mees reported on the effects of potassium salts on fixation as early as the 1954 edition of his book, and internal Kodak reports warned of it in the design of developers (no potassium carbonate) and the design of fixers (no potassium salts).

    AFAIK, that is true today in all Kodak formulations.

    Mees specifically stated that use of potassium hypo reduced fix rates by about 4x. Therefore, some level of reduction in fix rate could be expected as a fixer 'seasons' by carried over developer if the amount of potassium carbonate were to be high enough to survive the stop or rinse in a reasonable amount.

    My opinion, with no concrete data is this: "Why take a chance?" Besides, sodium salts are less expensive and easier to get and have no significant adverse effect on fixation. This does not mean that they cannot be used, or that low levels are to be avoided. I have used KBr in developers as an antifoggant with no concernt about any effect on fixation due to the low level used. Potassium Sulfite might be useful in developers as well.

    PE
    Last edited by Photo Engineer; 06-05-2006 at 05:44 PM. Click to view previous post history.

  3. #23

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    Quote Originally Posted by Photo Engineer
    Mees specifically stated that use of
    potassium hypo reduced fix rates by ...
    PE
    Reduced fix rates! Well there goes my theory. I've never
    seen it spelled out; how does potassium negatively affect
    fixation. So I theorised.

    In solution we have the ammonium and/or sodium ions and
    the silver complexed with thiosulfate; the argentothiosulfate
    ion. Now the ammonium and sodium salts of that complex ion
    are very soluble while the potassium salt is only sparingly
    soluble. I suppose that would affect the "rate" but I'd
    think Capacity would be the issue. Dan

  4. #24
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    Quote Originally Posted by dancqu
    Reduced fix rates! Well there goes my theory. I've never
    seen it spelled out; how does potassium negatively affect
    fixation. So I theorised.

    In solution we have the ammonium and/or sodium ions and
    the silver complexed with thiosulfate; the argentothiosulfate
    ion. Now the ammonium and sodium salts of that complex ion
    are very soluble while the potassium salt is only sparingly
    soluble. I suppose that would affect the "rate" but I'd
    think Capacity would be the issue. Dan
    What theory was that Dan?

    But yes, both rate and capacity may be influenced, or it may just seem that capacity is influenced by potassium as the rate becomes (or is) so slow.

    IDK the reason, but it could be that the counter ion is closely associated with the many silver thiosulfate complexes.

    Mees cites a phase rule diagram and shows that 4 different sodium argentothiosulfates vary from very slightly soluable through sparingly soluable, to readily soluable. The same is probably true of potassium salts, but with ammonium salts, the ammonium ion can become part of the acutal complex and influence rate, being very much smaller than a thiosulfate ion. This can influence diffusion rates.

    It is very complex (pun intended), and I studied it for several years working on blixes and fixes for color materials.

    PE

  5. #25

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    Perfectly ignorant question

    Please excuse my laziness in asking a chemistry question. It's been many years since college chemistry and am not finding a lot of help in answering my question in the chemical forums. I just finished baking sodium bicarbonate to get my carbonate ready for when my metol gets here so I can dive in to my Buetlers developer testing.
    I started out with 230 grams of bicarb from the grocery store, and now have 173g of powder after baking for an hour at 350 or so. Was my conversion complete, or do I need to get out a cookpot on the stove and cook it a little longer? Thanks.

  6. #26

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    The reaction is 2NaHCO3 --> Na2CO3 + H2O + CO2

    From the balanced equation and the molecular weights 2 x 84 g of bicarbonate should yield 106 g of carbonate or a yield of 63.1%. If your yield is greater than this then the conversion was not complete. Looks like you need to bake a bit more. Did you stir the bicarbonate a few times during the baking. Using a shallow baking pan also helps.
    Last edited by Gerald C Koch; 09-11-2013 at 10:10 PM. Click to view previous post history.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  7. #27

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    You can also get sodium carbonate monohydrate from a pool supply store. One of the brands is called pH Plus. It is used to adjust the pool pH.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  8. #28

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    Thank you. No, I did not stir it. Baking dish was shallow to where I was afraid I'd spill some if I stirred. I'm just going to put it in a cookpot and turn the burner up and finish it. Sure wish my Elon would hurry up and get here. Can't wait to mix up my Beutler's. I just know it's the answer I've been researching for. I'm going to have this Xray film looking like the prettiest continuous-tone film you ever saw.

  9. #29

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    Be sure to use a glass, stainless steel or enamel ware pot and not aluminum. The carbonate will attack the aluminum.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  10. #30
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    You can overdo the reaction and give off CO2 forming Na OH, which is why it is best to buy the Carbonate and Bicarbonate.

    PE

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