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  1. #1

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    Carbonate/bicarbonate buffer

    Does anyone use a carbonate/bicarbonate buffer? The pKa for the second dissociation of the carbonic acid/bicarbonate/carbonate system is about 10.25, or maybe it's 10.23, but anyway assuming it is 10.25, an equimolar mixture of carbonate and bicarbonate should give a pH of about 10.25.

    I gather from discussions about pH and development that a pH of 10.25 would put a developer in the active range, but not extremely active, and the buffering capacity would be high, depending of course somewhat on the absolute concentrations.

    At a molar ratio of [CO3--]/[HCO3-] the pH would be about 9.25, and the buffering capacity would be moderate, again depending somewhat on the absolute concentrations.

    If I recall correctly, a pH of 9.25 is below the threshold of strong developer activity for ascorbate and is close the the practical threshold of moderate to strong developer activity for para aminophenol.

    Taking these broad concepts to a specific developer formulation concept (i.e. raising a second topic of discussion for this thread), maybe making a pH 9.25 bicarbonate/carbonate buffer with (a super additive) para aminophenol and ascorbate developer combination could be interesting. The pH might be low enough to tame ascorbate's tendency to fog film by keeping it from being he main developer, but high enough to allow the para aminophenol to be the main developer. Because the buffering capacity of this buffering system at pH 9.25 would be relatively low, the tendency for oxidation of ascorbate to lower the pH may be only partially compensated by the buffer, especially if the total buffer concentration is kept low, so there may be a moderate compensating effect and/or edge effect when using this mixture.

    One nice thing about this proposed system is that it would be possible to make it at home using relatively common ingredients, a desirable thing, but not necessarily the main idea of this thread.

    Of course, I haven't tested this. I am just throwing out this concept for discussion.

  2. #2
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    At one time or another, I have run most of the experiments you describe above. We took, on average, that the best position for buffering at 20 deg C for the Carbonate/Bicarbonate was at 10.1. You will see that in use in the RA and C-41 developers most notably.

    Lowering pH brings many advantages but none of which are easily described, but basically the biggest example is that at lower pH, a developer becomes less sensitive to CO2 from the air, and also it becomes less sensitive to oxidation. These interesting facts bring on a lot of possibilities. Also, Carbonate is not the best choice as you lower pH.

    PE

  3. #3

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    In the case where photo chemicals are not available its probably easier to make sodium aminophenolate from acetaminophen capsule contents as in making PaRodinal then add vit C ascorbic acid to adjust the pH.But is there any real evidence that p-aminophenol and ascorbate are superadditive?

  4. #4

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    Quote Originally Posted by alanrockwood View Post
    At a molar ratio of [CO3--]/[HCO3-] the pH would be about 9.25, and the buffering capacity would be moderate, again depending somewhat on the absolute concentrations.
    My understanding from titrating thousands of water samples for alkalinity content, pH 8.3 is the endpoint. That's where you should have a 50/50 ratio of carbonate/bicarbonate.

    As far as buffering capacity, that's a function of how many moles of each you have in solution, and not the pH of that solution...
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  5. #5

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    Quote Originally Posted by Kirk Keyes View Post
    As far as buffering capacity, that's a function of how many moles of each you have in solution, and not the pH of that solution...
    Actually, buffering capacity is a function of both the pH and the number of moles. However, the pH dependence is indirect.

    The fundamental quantities that determine buffering capacity are the mole ratio of the acid and conjugate base and the total number of moles of acid and conjugate base.

    Maximum buffering capacity is achieved at equal mole ratio of acid and conjugate base. The rule of thumb chemists use is that when a mole ratio between 0.1 and 10 is the useful range.

    This ratio range corresponds to a range of +/- 1 pH unit relative to the pKa of the acid. Outside of this range the buffering capacity is dropping very fast compared to the maximum, i.e. compared to an equimolar buffer. This is why buffering capacity depends on pH, or more correctly speaking, the pH relative to the pKa of the acid.

    The buffering capacity also depends, as mentioned above, on the total concentration of the acid and conjugate base. I won't go into the reason for this because I think you agree and it is probably obvious to everyone else as well.

    Regarding the end point of a titration, that is not where the acid and conjugate base are in 50/50 ratio, but rather the point at which one or the other (depending on the direction of the titration) is virtually completely used up.

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    I should add one other comment. You are correct when you say that buffering capacity depends on the number of moles of each (acid and conjugate base) present. This is an alternative and equivalent description to my description of using the mole ratio and the total number of moles.

  7. #7

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    Quote Originally Posted by alanrockwood View Post
    Actually, buffering capacity is a function of both the pH and the number of moles. However, the pH dependence is indirect.
    I'd say the pH is a function of the the ratio and number of moles of buffer present, the capacity is a function of the number of moles of that buffer ratio present.

    Buffer first, pH second.

    Keep in mind that alkalinity is not buffering capacity, but does measure the amount of carbonate, bicarbonate, and hydroxyl ion as well.

    I stand corrected with your statement that the endpoint is where one ion has been consumed and is converted into the other ion. In other words:
    H+(aq) + CO3=(aq) --> HCO3-(aq) at pH 8.3

    I had 5 hours of sleep last night after flying 1700 miles...
    Last edited by Kirk Keyes; 11-29-2009 at 05:54 PM. Click to view previous post history.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  8. #8
    Murray Kelly's Avatar
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    The last half of your original post reminds me of Pat Gainer's 'Begone Fog!'
    contribution in the Unblinking Eye article 'Appreciating Rodinal'.
    http://unblinkingeye.com/Articles/Rodinal/rodinal.html
    I can attest to the extreme fog of PaRodinal and ascorbate mixed - just haven't gotten around to adding some borax. Initially I would expect any free caustic to form metaborate with added borax but with persistence, enough borax would pull the pH down to about 9.2. An excellent buffer, too.
    Doesn't add anything to your carbonate/bicarbonate conjecture but the general idea is the same.
    Murray


    Quote Originally Posted by alanrockwood View Post
    Big SNIP ----

    If I recall correctly, a pH of 9.25 is below the threshold of strong developer activity for ascorbate and is close the the practical threshold of moderate to strong developer activity for para aminophenol.

    Taking these broad concepts to a specific developer formulation concept (i.e. raising a second topic of discussion for this thread), maybe making a pH 9.25 bicarbonate/carbonate buffer with (a super additive) para aminophenol and ascorbate developer combination could be interesting. The pH might be low enough to tame ascorbate's tendency to fog film by keeping it from being he main developer, but high enough to allow the para aminophenol to be the main developer. Because the buffering capacity of this buffering system at pH 9.25 would be relatively low, the tendency for oxidation of ascorbate to lower the pH may be only partially compensated by the buffer, especially if the total buffer concentration is kept low, so there may be a moderate compensating effect and/or edge effect when using this mixture.
    More SNIP ----

  9. #9

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    I just tried adding ascorbic acid to PaRodinal 1:50, pH with given weights of ascorbic acid added to 500ml was: +0g >12, +1g >12, +2g=11.8, +2.5g=10,+3g=8.2.
    T-max 100 developed in the 1:50 with 3g ascorbic acid/500ml 10m 68F gave very thin negatives.
    Adding ascorbic acid without a buffer causes IMO unacceptable pH changes.

    I wonder roughly how much sodium carbonate I should put in with the PaRodinal 1:50 +3g ascorbic acid before trying to adjust the pH to say 9.25 with sodium bicarbonate,ie enough buffer to fix the pH so that one could suggest a formula for use,eg would 10g/L sodium carbonate be OK?
    All these except sodium sulfite are locally available chemicals.I cannot get borax from local pharmacies,some sell boric acid and do not know the difference.

  10. #10

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    The pKa2 of cabonic acid (bicarbonate going to carbonate) is at pH 10.25 at 25 °C. So a 50/50 mix of them should give that pH. If you add more bicarbonate to the mixture, you can force the pH lower, about 1 pH unit and still have reasonable buffering.

    If you add the ascorbic and lower the pH down to 8.2, and then add carbonate to bring the pH back to where you want, you will have some buffering from that.

    There are online buffering calculators that you can use to help determine how much you need to add.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

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