View Full Version : Solvent Precipitation?
12-13-2008, 01:59 PM
In Duffin's book on emulsion,a paragraph is included which mentions a early practice of percipitating the gelatin/silver-halide with alcohol. After the initial percipitation,how much water washing was done? Was the washing done with pure water or with a mixture of alcohol and water?
12-13-2008, 02:43 PM
I have no idea what this was for other than as an antifoamant. Alchohols are in common use (or were) to reduce foam. There are better ones than ethyl alcohol such as t-butyl alcohol.
Modern emulsions are often run in the presence of "oddball" solvents to modify grain type, but alcohols are not among them. I am not prepared to discuss these other solvents nor their characteristics as I don't know enough about them for my comments to be meaningful. This work has mainly been pursued in China and Russia IIRC.
12-14-2008, 12:00 AM
"SOLVENT PRECIPITATION The addition of a water-miscible organic solvent, ile [sic] which gelatin is not soluble, will precipitate the solid phase. The commonest example of this is the use of industrial ethanol, or methylated spirit, which was probably the earliest method of coagulation commonly used in production but has now been largely displaced by newer processes. The emulsion is precipitated in rather a slimy form which is difficult to handle and a clean separation from the aqueous solution is difficult. In production, the higher cost and inconvenience of recovering
the large volumes of spirit, frequently around 1 litre per gram molecule of silver, must also be taken into account."
PE - Duffin's not talking about antifoaming, he's talking about emulsion washing and that's what Bill is asking about.
Sounds like a mess to me.
Bill, have you tried the acid coagulation using phthalated gelatin like we did in the class and like I tried to demonstrate this summer at my house this summer? If you're not using phthalated gelatin, or don't have some, the Formulary has it now. Or you could use the next method shown in Duffin - salt precipitation with epsom salts and at a pH of a bit less than 4. And epsom salts are cheaper than alcohol.
12-14-2008, 12:27 AM
In Duffin's book on emulsion,a paragraph is included which mentions a early practice of percipitating the gelatin/silver-halide with alcohol.
In the context of Lippmann emulsions adding alcohol during precipitation is said to lower grain size.
12-14-2008, 12:28 AM
Bill - I forgot to mention - another book I've got, Zeligman and Levi, the Soviet version of Duffin, mentions this can also be done with acetone as well as alcohol. They say:
"In the alcohol-washing process the emulsion after *st ripening is cooled to 2-3O above its setting temperature (30-32OC), and 2'5 times its own volume of ethyl alcohol is added with rapid stirring during 10-20 seconds. The liquid is allowed to settle for 10-15 minutes and the spongy precipitate is separated by decantation and pressed out. The decanted solution contains soluble first-ripening components which may be recovered if necessary. The spongy precipitate is cut into pieces 8-10 cm in diameter, mixed with an equal volume of water, melted at 40-45OC (after neutralization with HC1 in the case of ammoniacal emulsions) and coagulated again as before (1-0 - 1-5 volumes of alcohol are sufficient at this stage). After the second coagulation the spongy mass is cut into pieces 2-3 cm in diameter and washed with water by decantation (three or four changes at 15-minute intervals). The washed concentrated emulsion is then diluted with water to the required volume before finishing. The main changes in the physico-chemical and photographic characteristics resulting from alcohol washing are increase of viscosity and melting point of the emulsion, and some increase of sensitivity."
It reminds me of when I was once trying to reverse engineer a paint-on fire retardant. It turns out it was based on acrylic latex paint. I was able to separate the latex from the mix by adding acetone to it and the latex coagulated. I the stuff into a Soxhlet extractor and let it redistill acetone into it, and when it was done, I had a gooey, stickly mess. it reminded my of snot. The cool thing was that as I was holding it, I could squeeze out the acetone with my fingers, and as I did it, it turned into what looked just like an old fashioned latex gum eraser. it even did a good job of erasing pencil marks from paper.
So I'm sure Duffin and the Soviets are right, but phthalated gelatin still seems easier to me. And faster.
12-14-2008, 09:32 AM
Yes, alcohol can be used, but it cannot be denatured. Acetone can be used as well but remember that ketones can react with certain organics used as doctors in emulsions. So, with that in mind yes, you can wash with alcohol. Now, the catch is that improper use can denature the gelatin and you end up with a gummy mess that cannot be redispersed.
This method was so cumbersome and messy, it was never (AFAIK) used in production at Kodak. It may have been in the 19th century, but not later.
As for the ISO wash, the overall reaction is the same, and the coagulum can denature there as well. I use more PA gelatin than we did at EK due to the rapidity of the wash as you go up, and due to the fact that it is easier to learn.
Do not neutralize ammonia makes with HCl. Use Sulfuric Acid or Citric Acid.
12-14-2008, 02:38 PM
Thanks Kirk and P.E.
Kirk,your U.S.S.R. quote answered my questuin, which realy was about how to wash a solvent percipitated emulsion. I do use the phthalinated gelatin with the H2SO4 percipitation,sometimes. Alcohol percipitation is something that I have been doing for a long time with polymers other than gelatin(I guess gelatin is a[mixed] polymer of sorts). I have been doing some side experements which are not directly relavent to the main topic of this forum. I don't want to be secrative. I just don't want to muddy the waters.
I was quite puzzled by that idea of precipitation (or percipitation as you put it) by means of alcohol. Never heard of that.
But I guess you are talking about flocculation where of course one has given alcohols a thought. To my knowledge though this has not been seen practical for industrial use. There high-molecular substances seem to be favoured.
12-17-2008, 02:17 PM
Yes, I did transpose and meant precipitation.
Actualy Duffin describes what I was refering to as "precipitation".
What I was asking about was what,according to Duffin, was done at an industrial leval in the past.. Instead of using phthalinated gelatin and precipitating with sulfuric acid, "regular"photograde gelatin was used and percipitated with alcohol.
Kirk, in this thread, has identified exactly what I was asking about and answered my question.
I must admit I did not come across the term precipitation in that meaning so far (or rather can't remember...)
As some of us are referring to german literature: there it is typically differenciated between
`Fällung´ and `Flockung´
Organic-Solvent-Flocculation is stated as useful for the lab, but due to volumes needed, their costs and the neccessary recovery from the effluent as too expensive for industrial use.
12-17-2008, 02:59 PM
The original patents also refer to "salting out" using Calcium Sulfate or Magnesium Sulfate. I did this in early precipitations, but when I had to develop a method to teach, I decided on ISO washing with Phthaled gelatin or noodle washing.
You can add a huge dose of these inert salts and the gelatin will curdle. Then you remove the curd and rinse with ice cold distilled water. Afterwards, you add the desired amount of water and then warm until the entire thing melts.
This leaves an inevitable amount of sulfate salts in the coating but does work. To this day, Kodak Park has piped into labs a "Sweet Water" which is Calcium Sulfate for washing. You see, they could combine the salt treatment with washing repeatedly and then a final water rinse.
The problem is that measuring the final pAg becomes complicated due to the inert (but not really inert) salts present. It was used though until ISO washing was invented.