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  1. #21
    Hexavalent's Avatar
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    Quote Originally Posted by Ray Rogers View Post
    ...
    As far as the compound crystallizing I thought I had seen a possible reason / solution the last time we discussed it, but I haven't tested my hypothesis.
    (I don't have the compound in question.)
    ....Ray

    What is the "compound in question"?
    - Ian

  2. #22
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    PE,

    I hope a good bit of the book is devoted to discussing the difficulties of a Chloro-Bromide emulsion because it is interesting in itself. Personally, I learn a lot by being exposed to something I don't understand.
    If you are the big tree, we are the small axe

  3. #23
    Photo Engineer's Avatar
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    Ok, summary here:

    Cl = contact emulsions, low speed, high UV sensitivity < excellent keeping of AZO is an example >

    Br = enlarging speed, good visible light sensitivity < moderate keeping >

    Cl/I = variant on Cl with more visible sensitivity < poor to mediium keeping unless heavily doctored >

    Br/I = variant on Br with more visible sensitivity < moderate keeping >

    Cl/Br = enlarging speed with reduced keeping

    Cl/Br/I = variant on above. < poor keeping unless heavily doctored >

    Example: Keeping of Cl/Br (Brovira) is poor due to inherent nature and need to use Rhodium Chloride to get good curve shape. Rhodium does the same in Br and Br/I but with less keeping problem but also with less contrast effect so G is only half right.

    BIG OVERSIMPLIFICATION HERE:

    Now, imagine a pure AgCl in equillibrium with the environment. If a chloride leaves to the surround it returns without changing the emulsion. The same is true for AgBr. The emulsion tends to keep well. AgBrI and AgClI tend to keep well because the Iodide is LOCKED in place. Nothing much changes the crystal. HOWEVER for an AgClBr emulsion, both Cl and Br can leave and return in equilibrium. They do NOT need to return to the same place and if they do not, they stress the crystal and cause changes!

    OVERSIMPLIFICATION ENDS!

    All keeping I report for raw emulsion is from emulsion held at 4 deg C. All keeping for coatings is for coatings held at 20 C and 50% RH as much as possible.

    PE

  4. #24

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    You could have at least mentioned solubility products and made it less of a "big oversimplification". ;^)
    Kirk

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

  5. #25

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    Quote Originally Posted by Photo Engineer View Post
    HOWEVER for an AgClBr emulsion, both Cl and Br can leave and return in equilibrium. They do NOT need to return to the same place and if they do not, they stress the crystal and cause changes!
    Quote Originally Posted by holmburgers View Post
    Personally, I learn a lot by being exposed to something I don't understand.
    Adding in a little bit of the chemistry here - the diameters of a chloride ion and a bromide ion are about 181 and 196 picometers, respectively. So they are close in size, and are able to substitute for each other in the crystalline matrix, but the size difference causes stresses and hence reduce the stability of the crystals.
    Kirk

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

  6. #26
    Photo Engineer's Avatar
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    Kirk;

    Let those interested try this then: http://www.chem.purdue.edu/gchelp/ho...y_products.htm

    And PKSP or pKsp = the negative log of the Ksp.

    And you are correct with the size causing the stress to the extent of the ratio of the sizes times the pKsp values involved factoring in the excess halide outside of the crystal and etc......... It goes on and on and on, but a mixed Cl/Br is tough to calculate and was the weakest point in our model as the sizes were close but no match and the pKsps were close but no match. So, this is a difficult subject and I hate to go into detail. I could not afford the experiments that would be needed to work one out by trial and error.

    PE

  7. #27
    Photo Engineer's Avatar
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    Try this reference and see if you like the math there. If you do, I might consider going on.

    http://www.d.umn.edu/~psiders/course...ions/AgNH3.pdf

    PE

  8. #28

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    Speaking strictly for myself,
    I would be interested in a book that goes into such matters in detail,
    with the math worked out and full explanations.

    So despite the anticipated difficulty, I think there is a need.

    If not a whole book, maybe a few pages?
    Last edited by Ray Rogers; 03-13-2011 at 10:55 PM. Click to view previous post history.

  9. #29
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    Ray;

    A dilemma exists here though. There are many books on pKSp calculations including some on photography, but there are few on making emulsions with examples and formulas. There you have it. Ok, the book is now pushed back to Christmas 2011 and I will add the math.

    Howzzat? Opinions please.

    PE

  10. #30
    rmazzullo's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    Ray;

    A dilemma exists here though. There are many books on pKSp calculations including some on photography, but there are few on making emulsions with examples and formulas. There you have it. Ok, the book is now pushed back to Christmas 2011 and I will add the math.

    Howzzat? Opinions please.

    PE
    I am very interested in seeing the extra material. It would certainly help us to fill in the gaps in our understanding.

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