Is film development “exothermic” or “endothermic”?

[Heroique]

Does traditional film development – converting silver halide into metallic silver – produce heat, or rob heat?

I know, funny question – and the answer, one way or the other, probably has no practical significance to my darkroom work, or anyone else’s. (And my apologies in advance if anyone just suffered an unpleasant flashback to high school chemistry class – like I just did. ;) )

But as a frequent tray developer, I’m simply curious…

I think the question occurred to me because I’m always thinking about better ways to sustain a desired temperature during the development step – such as taking into account air temperature, contact with the fingers, temperature of the surface under the tray, etc.

Any chemists out there w/ a quick answer in layman’s terms why “exothermic” or “endothermic” – or “neither” – is the answer? Does the emulsion type or choice of developer make a difference? Might this actually become a “measurable” issue if one tray-develops several film sheets in a relatively small amount of solution?

[Steve Gledhill]

Does traditional film development – converting silver halide into metallic silver – produce heat, or rob heat?

I know, funny question – and the answer, one way or the other, probably has no practical significance to my darkroom work, or anyone else’s. (And my apologies in advance if anyone just suffered an unpleasant flashback to high school chemistry class – like I just did. ;) )

But as a frequent tray developer, I’m simply curious…

I think the question occurred to me because I’m always thinking about better ways to sustain a desired temperature during the development step – such as taking into account air temperature, contact with the fingers, temperature of the surface under the tray, etc.

Any chemists out there w/ a quick answer in layman’s terms why “exothermic” or “endothermic” – or “neither” – is the answer? Does the emulsion type or choice of developer make a difference? Might this actually become a “measurable” issue if one tray-develops several film sheets in a relatively small amount of solution?

I can't help directly here other than to possibly coin a new word for your "neither". How about 'neutrothermic' or 'nonothermic'. Or you could use athermic which I suspect already means no temperature change.

But seriously, and on a related matter, years ago I carried out a series of tests to document what I called "The Warm Hand Effect". I plotted the rise in temperature of my developer during the course of tray development arising from the heat of my hand. And I did it at varying room temperatures (where the developer started at the same temperature as the room - just to introduce a bit of control). The one conclusion I reached was that the effect of my hand was very marked with a change of several degrees (Celsius). Some of my tests were at rather low room temperatures but this effect was marked even at 20 Celsius. Soon after that I moved to a Jobo drum from tray development so the issues weren't the same.

[Steve Gledhill]

...

But seriously, and on a related matter, years ago I carried out a series of tests to document what I called "The Warm Hand Effect". I plotted the rise in temperature of my developer during the course of tray development arising from the heat of my hand. And I did it at varying room temperatures (where the developer started at the same temperature as the room - just to introduce a bit of control). The one conclusion I reached was that the effect of my hand was very marked with a change of several degrees (Celsius). Some of my tests were at rather low room temperatures but this effect was marked even at 20 Celsius. Soon after that I moved to a Jobo drum from tray development so the issues weren't the same.
I've just dug back through my old files and found this "Warm Hand Effect" chart dated 1996. Obviously it's a totally personal chart as it depends on just how hot your hands are and on your agitation technique!!!

I think it shows that the warm hand effect must surely outstrip any endo or exothermic effect. Or does someone else know different?

Amazing what we keep!

[Heroique]

Steve, that’s quite revealing – and very helpful information for tray developers.

A 2.5° C jump over 7 minutes – or 4º C jump over 15 minutes – is enough to throw anyone’s temperature/development time combination “out of whack.” The implication is that your developer’s average temperature is significantly higher than the temperature you read at the start of the step, and which you might assume stays the same, or near the same.

I usually soak my fingers in ice water between agitation cycles – per AA in The Negative – and as much as I think this helps sustain a consistent temperature during development, I can’t help but notice that when I move the film into the stop bath tray, the stop solution “feels” cooler than the developer solution – even though the temperature of each was the same at the start. “Exothermic” indeed! Maybe rubber gloves are also in order.

Thanks for the hard evidence.

;)

[Ron Marshall]

Your answer is here:

http://books.google.com/books?id=xx_uIP5LgO8C&pg=PA453&lpg=PA453&dq=reduction+of+silver+latent+image+exothermic&source=bl&ots=gkeAgjlEYh&sig=S0QYHbzmPYfa4kYZrcq-udM4HD4&hl=en&ei=-cNQTJS3MsWAlAeOtPi7CQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q=reduction%20of%20silver%20latent%20image%20exothermic&f=false

[Heroique]

Your answer is here:

http://books.google.com/books?id=xx_uIP5LgO8C&pg=PA453&lpg=PA453&dq=reduction+of+silver+latent+image+exothermic&source=bl&ots=gkeAgjlEYh&sig=S0QYHbzmPYfa4kYZrcq-udM4HD4&hl=en&ei=-cNQTJS3MsWAlAeOtPi7CQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBIQ6AEwAA#v=onepage&q=reduction%20of%20silver%20latent%20image%20exothermic&f=false

That’s fantastic, Ron. “Tiny rockets” and all…

“This oxidation-reduction reaction [i.e., the chemical reaction at development] is quite exothermic, and the heat released propagates the reduction throughout the crystal and propels the crystal like a tiny rocket, leaving behind a trail of silver metal. That is the image seen on a photographic print.”

Now I’m curious how much of Steve’s upward-trending line is explained by this “quite exothermic” reaction, and how much by Steve’s hot little hands! Thanks for the link. (BTW, that’s a very-well written science explanation.)

[Darin Boville]

Why worry about a slight change in temperature....As long as it changes in the same way each time?

--Darin

[Heroique]

I should add that I’m also curious whether Steve’s unique chart would look the same for different emulsion/developer combinations. If the same, then the “warm-hand effect” might be the principal cause for the noteworthy temperature change. If different, that would be quite interesting – it might suggest that the so-called “oxidation-reduction reaction” produces a variable (and measurable) influence on temperature, depending on the emulsion/developer combo responsible for it – an influence that might be stronger than I had thought. (Darin: I agree with you, but this is less about worry, more about the “simple curiosity” I mentioned above. ;) )

[Brian Stein]

The total energy released by the reaction will depend on how exothermic the reaction is, and how much mass is involved in the reaction. That question thus turns on the mass of silver reacting. The temperature change will depend on energy, mass to be heated, and thermal properties of the material to be heated.

So how much silver is in an emulsion, and of this how much has been reduced by light? The second is easier to answer: about 20%. I dont know how much silver is in a sheet of film but given the price of film and silver a reasonable guess will be milligram amounts. Thus the amount of energy released by the reaction is going to be limited (I cant be damned digging out chem texts to do the math :-) )

Considering the larger the film sheet or the more sheets you put in the tray, the greater the volume of water in the developer solution. So you are going to have a relatively constant ratio of film (heat) to developer, with lots of developer to the mass of silver. Developer at first approximation is just water, and water takes a lot of joules to heat.

Order of magnitude analysis says that you are going to get no *significant* heating of your developer from the film. You can always test this empirically: I would predict the temperature change will be below that which a standard thermometer will read.

[DanK]

But as a frequent tray developer, I’m simply curious…

I think the question occurred to me because I’m always thinking about better ways to sustain a desired temperature during the development step – such as taking into account air temperature, contact with the fingers, temperature of the surface under the tray, etc.




Whether a temperature increase is a result of 'hot hands'...or anything else...increasing the volume of developer, or other chemicals used will help stabilize the temps...

my .02

Thanks,
Dan