An interesting aspect of acutance developers

Yes, I remember that rule. It is not a mystery that either sulfite of ascorbate behave as they do. What is interesting is that the simpla observation of the speeding up in both cases might make one think that the same mechanism is involved in both cases. In one sense, it could be considered as the same. The products of reaction are removed. But I would expect the long term results to be different if they are removed by regeneration than if they are removed by creating a less inhibitive but also less active product.

There is of course the inhibiting effect of bromine and iodine to be considered. One of the first things I read about developer capacity was that developer dies from bromide long before it dies from exhaustion. This was in "Principles of Optics" by Hardy & Perrin, published in 1931. I read in a compilation of photographic research we had at NASA that Phenidone was not only least sensitive to bromide, but in fact its activity could in some cases be increased by it. I think that article was gotten from the Russians, so I don't know much about it. I do know that on the basis of that article for some years I used a homebrew of Phenidone, HQ and sulfite for at least 8 rolls per liter without replenishment and without loss of activity. I would mix a new batch when I could no longer stand the silver sludge in the bottle.

Gerald Koch said:

Some food for thought. There are two ways of compounding an acutance developer. The most common method is to limit the amount of developing agent. However, there is a second method and that is to limit the amount of preservative.

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Rodinal (if I remember right) only has one developing agent, and it has a pretty high level of sulfite in it which violates both of your stated criteria.

I've thought poor pH buffering to be important in Rodinal-type developers. Sodium hydroxide is an absolutely bad pH buffer (i.e. it does not buffer), and when diluted at high levels people like to use I always figured the pH was dipping too low to be active at a microscopic level in the emulsion from the development by-products.

Kirk

Kirk;

I have used a surface electrode to track the pH of emulsion during development. In some cases, with low buffer capacity, the emulsion actually becomes acidic.

PE

Sodium or potassium hydroxide in combination with other compounds can make a buffer. Combine either with boric acid, for example. A fair amount tof the hydroxide in the usual Rodinal recipe goes to convert the HCl on the p-aminophenol.HCl to either KCl or NaCl. I think the rest of it goes to make sodium or potassium aminophenolate. Rodinal at 1+50 dilution has on the order of 4 grams of sulfite per liter of working solution. So, if you put a gram or two of potassium p-aminophenolate, maybe 4 grams of sodium sulfite and a pinch of salt in a liter of water you would have about the same as a 1+50 working Rodinal solution. What did I miss?

Michael Gudzinowicz suggested "One could design a developer which operates near the threshold value (pH) and is weakly buffered":
http://www.groups.google.com/group/rec.photo.darkroom/msg/ad128c7dd785032b
For PC,PQ,metol it does not appear to have been tried yet.

gainer said:

Sodium or potassium hydroxide in combination with other compounds can make a buffer. Combine either with boric acid, for example. A fair amount tof the hydroxide in the usual Rodinal recipe goes to convert the HCl on the p-aminophenol.HCl to either KCl or NaCl. I think the rest of it goes to make sodium or potassium aminophenolate. Rodinal at 1+50 dilution has on the order of 4 grams of sulfite per liter of working solution. So, if you put a gram or two of potassium p-aminophenolate, maybe 4 grams of sodium sulfite and a pinch of salt in a liter of water you would have about the same as a 1+50 working Rodinal solution. What did I miss?

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You missed the fact that there is not a significant level of buffer, which means no buffer capacity. Only the pH is at the right level in the example above, but there is no capability to stay at that level during development.

PE

Alan Johnson said:

Michael Gudzinowicz suggested "One could design a developer which operates near the threshold value (pH) and is weakly buffered":
http://www.groups.google.com/group/rec.photo.darkroom/msg/ad128c7dd785032b
For PC,PQ,metol it does not appear to have been tried yet.

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Alan;

This is well known to chemists and professional developer designers.

PE

[QUOTES=Alan Johnson;529624]
"Crawley's article BJP Jan 6 1961 does say 'To prevent
regeneration of the developing agency ....the concentration
of sulfite must be kept below 6g/L' "

Was not that in the context of carbonated developers;
developers of somewhat high ph? Redox potentials do
vary with ph.

"There it is noted that the oxidation of metol (development)
is speeded up because sulfite removes one of the reaction
products."

Now what would that be? Surely not the bromide.

"Specifically, unlike Crawley, it does not say the metol
is regenerated. It's not clear if Crawley's writing could
have been worded better."

Or more worded. My bet, Crawley did not wish to be wordy.
More sulfite, more removal, more regeneration. All add up to
lowering the accutance. By regeneration and or by removal
of retardant by-products, same end result.

Mr. Crawley's objective is the elimination of regeneration
and removal. That is there be an issue of removal. Dan

Photo Engineer said:

You missed the fact that there is not a significant level of buffer, which means no buffer capacity. Only the pH is at the right level in the example above, but there is no capability to stay at that level during development.

PE

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I'm sorry. I didn't mean to imply that Rodinal is buffered, but that it does not necessarily have a high pH, a lot of hydroxide or even a lot of sulfite in the working solution. I do not know if the aminophenolate has any buffering capacity before or after it reduces the silver halide. It seems to be not too unrelated to a well diluted metol-sulfite-carbonate developer if the aminophenolate is alkaline enough. It would be interesting to measure pH of Rodinal working solution before and after developing its rated limit of film. I only have test strips, and they show pH of about 10, but I won't stake my life or even my reputation on their precision or accuracy.

Patrick;

As I've mentioned elsewhere, I have seen the pH of a coating become slightly acidic during development measured with a surface pH electrode. It is not unreasonable to expect this to happen with dilute developers even if the stock has a high level of buffer capacity.

PE