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Hexavalent
03-13-2011, 12:23 PM
...
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"?

holmburgers
03-13-2011, 01:20 PM
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.

Photo Engineer
03-13-2011, 01:56 PM
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! :D

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

Kirk Keyes
03-13-2011, 06:30 PM
You could have at least mentioned solubility products and made it less of a "big oversimplification". ;^)

Kirk Keyes
03-13-2011, 06:36 PM
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!


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.

Photo Engineer
03-13-2011, 07:14 PM
Kirk;

Let those interested try this then: http://www.chem.purdue.edu/gchelp/howtosolveit/equilibrium/solubility_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

Photo Engineer
03-13-2011, 07:45 PM
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/courses/chem4633/labinstructions/AgNH3.pdf

PE

Ray Rogers
03-13-2011, 08:21 PM
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?

Photo Engineer
03-13-2011, 09:03 PM
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

rmazzullo
03-13-2011, 09:20 PM
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.

Photo Engineer
03-13-2011, 09:23 PM
Did any of you look at the references I gave? That is essentially what I would have to wrestle with to incorporate into the book.

PE

holmburgers
03-13-2011, 09:44 PM
PE,

Just include a hyperlink in your book to this thread. Is that possible these days with modern publishers?

:sideways:

Hexavalent
03-13-2011, 09:44 PM
Ray;

.....There you have it. Ok, the book is now pushed back to Christmas 2011 and I will add the math.

Howzzat? Opinions please.

PE

If software design, this is called "feature creep", more and more bells get added, and the project never gets finished ;)

Photo Engineer
03-13-2011, 10:15 PM
Exactly Ian. So, the book may be published about 10 years after I'm gone to the great darkroom in the sky! ;)

Chris, I already published a link here and no one liked it, and there is loads of this in most texts on Photographic Science but they seem to want me to repeat it! So............

PE

holmburgers
03-13-2011, 10:17 PM
I know, I was just joking. :D

My final thought is, don't feel like you have to go into great depth about these murky emulsions, but give it a little lip service so that a reader is at least aware of its existence and potential pitfalls.

Ray Rogers
03-14-2011, 03:58 AM
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

That is true, but I think the issues mentioned in modling are interesting and relates to real emulsions more closely than a lot of the scientific studies that use ideal or special parameters. Well, thats mostly a guess.

Actually, I was really talking about a separate book.

Ray

Kirk Keyes
03-14-2011, 08:40 AM
PE,

Just include a hyperlink in your book to this thread. Is that possible these days with modern publishers?

:sideways:


Hyperlinks come and go - we need something in the book that will last for years.

I think a few pages with short examples, as shown in the links above, would be good. Afterall, a Ksp calculation with AgCrO4 is nice, but some with AgBr vs. AgCl is much better, and would help those that are initially having difficulty with the concepts make the jump into inderstanding.

You don't need to lay it out so that they grok it, but merely enough for a normal understanding of the subject.

Photo Engineer
03-14-2011, 11:11 AM
Well, I can't include the entire model even if I had it. The code, as I left it, without the graphical user interface took about 3 feet (1 M) of shelf space printed in 8 point type. I could do some simple examples, but remember that this is HARD-CORE CHEMISTRY with math and logarithms and exponentiation. :D

Adding an edit.

Actually, we had no precise model for Cl/Br. The reason was that Chloride is a Solvent for AgCl and AgBr, as we know from its use in Microdol and other places in photography. This complex interaction further confounded the conditions when you had a mixed Cl/Br making prediction difficult. I can show single relationships and I can "guess" by weighted averages based on pKsp values, but the overall result is difficult to explain to non-chemists.

PE

JOSarff
03-14-2011, 06:06 PM
Did any of you look at the references I gave? That is essentially what I would have to wrestle with to incorporate into the book.

PE

Ron:

I looked. I read. I even understood (some). Now I understand why you do not want to include Br/Cl emulsions in this volume, and I agree with you.

It seems that Br/Cl emulsions are a catch 22 situation. The more research you do and the more you think you understand about them, the less you know about them.

Close?

Joe

Photo Engineer
03-14-2011, 06:11 PM
Joe;

Just about spot on! Thanks.

PE