Different temperature coefficients for different developing agents
Since I begun developing my own films I've used the tables and graphs published by Kodak, Agfa or Ilford, to find the correct developing time if my temperature differed from the standard 20 deg C. Last week I stumbled upon a very comprehensive, but old book (1956) on photo chemistry. (A swedish book called "Fotografisk kemi" by Artur Boström.)
To the interesting part: The author doesn't give a lot of explanation on this, but there seem to be a difference in how various developing agents react to a change in temperature. This was quite a revelation to me. Not that it doesn't make sense, because it does. It's just that noone have really pointed this out before (to me that is). I havn't really had any problem with this either, but on the other hand, I didn't use Pyro and Pyrocat developers until the last 3-4 years.
The facts from the book are (roughly translated from swedish):
"In general you state the temperature dependency by giving the temperature coefficent, i.e. that number which tells how much quicker the process is at 10 deg (C) rise of temperature. The more temperature dependent a developing agent is the larger the temperature coefficent. This is for:
(A table feature in the editor would be nice. )
Anyhow, it's the last line in the table which makes me react. Even though e.g. Pyrocat HD uses a mix of Hydroquinone and Catechin, the table above gives me a hint that Pyrocat developers are more sensitive to a change in temperature than e.g. D-76. Also, Pyrocat-HD should more sensitive than Pyrocat-MC.
Do I have any question? Yes, as I first don't really get how to use the coefficent in practice. D-76 should be a standard Metol-hydroquinone developer with a coefficent of 1.9, but looking at e.g. the Ilford table they seem to use a higher coefficent (like 2.3 or so). Now you might say that I got my facts wrong, which is possible, but given the quality of the rest of the book, I recon that the author (Mr. Boström) does have the facts right.
It would be nice if some of our knowledgeable experts on the subject of developing agents would like to cast some more light on this issue.
Perhaps PE will see this and chime in, but I believe that just having a 'factor' for the developer is only one-half way toward a time-temperature table. You would need a corresponding factor for the individual film. We had a long thread on this a while back and I concluded that empirical test of each developer/film combo are needed, however some generalizations can be made for groups of developers and films.
Well, firstoff there was a long thread about this very subject a short while ago.
Second, I really don't think that the simplistic approach will do, as rate also depends on pH and buffer capacity as well as sulfite content. And, these are also affected by temperature.
I personally think that the tables are better than factors. In addition, where are ascorbic acid developers, phenidone developers, dimezone developers and etc.... So simple data such as you post are too incomplete to be truly useful.
Just my $0.02! (for you non US readers - my two cents worth)
I've been around the 'net long enough to know the value of two cents.
First, I've been reading this and other sub-forums of Apug for a few years now, but I must have missed out on the threads you mention. ( ... or I wasn't really ready for them ...) A quick search did indeed give me the threads you mention. (It should be this one: http://www.apug.org/forums/forum37/5...n-formula.html back in July -08. I must have been on a heavy work schedule back then...)
To give PE and ic-racer some kind of answer, I recon that back in 1956 the table was rather complete. (In the thread linked above DF Cardwell refers to a similar source and table from a book written in 1955. Now, I like 1956 better, as I was born then. ) Besides, while the book is comprehensive from an amateur point of view, there is also an indication of the intended audience, i.e. the amateurs of 1956. (Since when general knowledge of these matters have deterioated very rapidly.)
Anyhow, before the translated quote from the book I said that the author didn't give a lot of explanation on the subject matter. It seems like I have to 1) thorughly read the rest of the book, and 2) make my own testing...
... as I realize that both the recommended times from e.g. Ilford are "starting times" and that the compensation charts (tables or graphs) which are issued by again e.g. Ilford are recommendations.
At least I now do know where to look for more information on the matter.
Finally, this posting was more a matter of curiosity than an actual problem. I bought a Jobo a few years ago to take care of any inconsistensy problems as such.
Well, to add to my previous post, many developing agent combinations are superadditive and also are influenced by reaction with sulfite, so just assigning a figure to Metol-Hydroquinone of 1.9 is misleading as the ratio of the two is important as is the ratio of HQ to Sulfite as is the ratio and absolute values vs pH in the range from 9.5 to about 11 which covers low to high contrast developers.
This, I guess, led me to the conclusion that a single number is simply insufficient. The tables or graphs can be seen to change slope for different developers, and I posted a sample table in another thread. Given a "starting point" can waste a lot of film, but given the table can just about lead you to the right value immediately.
In addition, please note that on the time (X) axis, the values are logarithmic which means that the data is non-linear and this is certainly not shown in the use of single values, nor can it be shown.
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I should point out that Pyrocat HD does not contain hydroquinone. The two agents it uses are pyrocatechin, AKA catechol, and Phenidone. Hydroquinone is 1,4 dihydroxybenzene and catechol is 1,2 dihydroxybenzene. They are both capable of being used as staining developers, each with its own color.
One would expect a Phenidone-Quinol developer and a Phenidone-Catechol developer, all else being the same, to have similar temperature coefficients, though expectations in such matters are not always met.
A quick look at Kodak tech pub J-78 for D-76 shows the slopes of temp vs development time varying by a factor of 1.7 across 6 films at 1:1 dilution. With that much variation using the same developer and dilution, I don't see how you can generalize usefully using the factors you quote across different developer formulations or different films.
Better to go with the charts as PE and others have said.
Hmm, to clarify a bit more...
I didn't open this issue up to bang a few numbers into your heads, rather to start a discussion. As PE have pointed out in this thread and certainly many others in the (now found) older thread on the subject, there are many factors which affect the dev.time when changing the temperature. All of that info seems to be covered in the thread linked in my post above.
Understanding the subject a (very tiny) bit better, it certainly adds to the concept of keeping it down to as few variables as possible. I.e. try to standarize on one developer and one film, until you really know what the combination is capable of. Then maybe one more film with the same developer. The only good thing about the "recession" of analogue photography is that there arn't as many temptations out there as there used to be.
A rule of thumb that I learned in Physical Chemistry is that the rate of a reaction doubles with 10 C degrees increase of temperature. This curve plots as a straight line on semilog graph paper. Over short temperature range, this rule is good enough for most of the developers we use.
i.e. a 10% increase in reaction speed for every 1 deg C increase. i use this rule of thumb for dev times on the plus and minus side of 20 degs C. However, I have generally found that when i have large changes, such as developing at 17 degs or at 25degs C, I reduce the amount of change somewhat. I generally shoot rollfilm shot in a wide variety of conditions, so add or subtract a little depending on my perception of the contrast of the scene. Never had real problems. In the summer my water temperature is about 23-25 degs and in winter 16-17, so I have to try to stabilise the room temperature for a little bit before i develop to bring water containers as close to 20 degs C as I can. A few degs C either way and the 10% rule gets me nice and close.