Chemical sensitization

[John Sager]

Hello everyone,


I have a question that I am sure has been discussed here but I can’t find an answer. Reading PE's book I understand that sulfur and gold are used as sensitizers but I can’t find a good explanation of exactly what their role is? How do they affect the chemistry or the physics to cause an increase in speed?


Thanks.

 

[Photo Engineer]

John, I tried to keep from being too technical, but Sulfur forms AgS specks which increase sensitivity. More is beyond the scope of a simple answer, but if you wish, I will do more tomorrow. It is late here.

PE

 

[pdeeh]

I've just acquired Basic Photo Science: How Photography Works (Attridge & Walls, Focal Press 1977) and have found it really clear and helpful in understanding what goes on in an emulsion.

It's been extremely useful for me in understanding some very basic concepts of chemistry, too.

It is a few years old now, so if PE knows it and thinks it is too simplistic or misleading or plain wrong, I'm sure he'll say so, of course.

 

[Athiril]

I've wanted to try selenium when I get into it, since selenium (and tellurium) are supposed to exhibit similar properties to sulphur.

 

[Photo Engineer]

Selenium and Tellurium are both quite toxic Dan. Be careful.

Kodak and Fuji apparently never got Se to work, but Fuji is using Tl in some of its emulsions. It is less toxic.

Your book is probably ok, depending on author. I'm not familiar with it.

PE

 

[dwross]

Selenium is/was commonly used for negative intensification. Intensification essentially bumps up the "speed" of a processed negative by increasing density. (I know you know that, but newbies to film might not and it's a fun, geeky fact.) I've never found any info in the texts that indicate selenium is a good raw emulsion sensitizer. Platinum pops up, but no one considers it as good as gold. A very effective and affordable gold sensitizer is Steigmann's formula. Two ingredients. 10 ml of gold chloride and 100 g of ammonium thiocyanate will last you for a long, long time.

http://stores.photoformulary.com/gold-chloride-1-solution/
http://stores.photoformulary.com/ammonium-thiocyanate/

I can post the recipe here if anyone is interested. It's also on The Light Farm, but I suspect I've posted my limit of links to there for awhile!

An addition of sodium thiosulfate (to the emulsion, not to the Steigmann's) may or may not increase the effectiveness. The effect is not identical for all emulsion recipes. Base fog can creep in, but a little is not a problem (for negatives) and may even be helpful when it comes to printing the negative.

 

[Photo Engineer]

There are numerous patents on using Se in place of S for chemical sensitization. One favorite was Dimethyl seleno urea. This is a urea molecule in which the Se replaces the O on the center carbon. But, it really never worked out. Tellurium is now in use at Fuji.

PE

 

[John Sager]

Thank you PE and everyone. PE, a little more on the chemistry and physics of sensitization would be facinating I think. Thanks!

 

[Photo Engineer]

John, as it turns out, I am writing that chapter for V2 of the book. Here is an example.

Chemical sensitization is a chemical reaction based on surface area, and spectral sensitization is an adsorption reaction, again based on surface area.

In a polydisperse emulsion, both of these effects go after the grains present with the highest surface area, and thus the amount of sensitization is hard to control, but in a monodisperse emulsion, the effect is rather uniform.

The amount of any of these addenda varies with grain size, with finer grains taking more than coarse grains. They are measured in mg of reactant / mole of Silver and thus if you say you are using 100 mg of Hypo (5H2O) per mole, then you are really only using about 25% of that amount of Sulfur as it represents only about 25% molar wise. I left this out of V I as it was "too much chemistry".

I'll post more later if you are interested in this. It can get pretty "heavy".

PS: This area was very confidential and not my forte!

PE

 

[John Sager]

Thank you PE. That helps. I look forward to V2.