Film developer chemistry part 1

[alanrockwood]

Hi,

I have some questions about film developer chemistry. I have several questions, so I am naming this thread "film developer chemistry part 1", anticipating that there may be part 2, 3, etc. later.

This one relates to the acid/base chemistry of common developers, such as metol, 4-aminophenol, hydroquinone, ascorbate, etc. What I am looking for is a confirmation/refutation of the following hypothesis: The developer in its active form is a negatively ionized molecule. For example, if we symbolize the neutral 4-aminophenol molecule as HPAP then the active form would be PAP-, i.e. the negatively charged ion formed by the loss of a proton from the phenol group. Similarly, the active form of hydroquinone (which I will call H2HQ) would be HHQ-, i.e. the negatively charged ion formed by the loss of a proton from one of the phenol groups.

If this is true then it should be possible to correlate the activity of a developer with the pH of the solution. If the pH is high the developer will be active because most of the molecules will have become negatively charged ions through acid/base chemistry. If the pH is low then most of the developer will be inactive because the molecules are in their neutral form rather than their ionized form. The transition region would be in solutions with a pH around the pKa of the molecule. If the pH is at the pKa of the molecule then 50% of the molecules will be in their ionized form, so the developer activity will be somewhat reduced compared to the activity of a higher-pH solution but still with a lot of activity. If the pH is one unit below the pKa of the developer molecule then the activity is probably going to be only about 10% of maximum because only 10% of the developer molecules will be ionized. If the pH is one unit above the pKa of the developer molecule then the activity is probably going to be nearly at its maximum because 90% of the developer molecules will be ionized.

If this hypothesis is true (or even roughly true) then it should be useful for designing developer mixtures.

I should also add that I am hypothesizing that this does not take into account super-additive pairs. My thought is that one of the molecules in the super-additive pair might be un-ionized but still make an indirect contribution to activity of the mixture by teaming with the other compound that is ionized.
.
I should also add that this hypothesis would not apply to all developers since some are known to be active in acidic solutions.

For all of you photochemical gurus out there, does this hypothesis make sense?

 

[Photo Engineer]

If this were true, then how do you explain acidic amidol developers? How can they be active?

 

[alanrockwood]

Thanks for the reply Photo Engineer. As I mentioned in my email, the hypothesis would not apply to all developers. Wikipedia mentions that amidol is unusual in being active in slightly acidic solutions. I believe there are some other developers that are active in acidic solutions, mainly some that rely on metal salt redox reactions.

How about some of the others? Why does pH have anything to do with developer activity for these developers?

 

[alanrockwood]

Here is a link that may be relevant.

http://www.chemgapedia.de/vsengine/...toff/oxidation_phenole/mechanismus.vscml.html

It presents a generic mechanism for the oxidation of hydroquinone.

This scheme is basically the mechanism I proposed. First the molecule loses a proton from a phenol group. Then the ionized molecule loses an electron. Whether this is what happens when hydroquinone reduces silver in a silver halide is a question, but it seems that at least my proposed mechanism seems plausible in the light of the basic organic chemistry for the oxidation of these compounds.

 

[Rudeofus]

Alan, your theory sounds mostly legit to me, and it is confirmed by photographic literature. I suggest you grab a free copy of Kenneth Mees's The Theory of the Photographic Process, it covers this in excruciating detail in part III. For further reading I highly recommend T. H. James's "The Theory of the Photographic Process", ideally 4th edition. It is available as used book from the usual suspects.

 

[Europan]

You’re not thinking wrongly but you don’t see the entire picture. Ions discharging or redox reactions are only part of what happens in a photographic gelatin under water. The reducing agents as they’re often called not only give or take electrons but also disintegrate, recompose as new compounds to the exploding silver, so to speak. The metal aggregates into a wool as was said once with quinone taking part in the structure as some kind of a scaffold. I’m trying to describe the most common film-developer combination.

+1 for Rudeofus’ literaric suggestions.

 

[Photo Engineer]

Here is a link that may be relevant.

http://www.chemgapedia.de/vsengine/...toff/oxidation_phenole/mechanismus.vscml.html

It presents a generic mechanism for the oxidation of hydroquinone.

View attachment 189016

This scheme is basically the mechanism I proposed. First the molecule loses a proton from a phenol group. Then the ionized molecule loses an electron. Whether this is what happens when hydroquinone reduces silver in a silver halide is a question, but it seems that at least my proposed mechanism seems plausible in the light of the basic organic chemistry for the oxidation of these compounds.

This is a correct scenario for HQ in water with an oxidant, but in a developer it is quite different. For example, the Q reacts with Sulfite ion to form HQMS anion and this is a developer. In addition, the Q can be regenerated by electron transfer with another reducing agent. This cycling of reactants can increase the life or the activity (or both) of any developer. Also, alkali produces more phenoxide ion and thus the entire sequence is dependent on pH.

Just a few comments.

PE

 

[Rudeofus]

In addition, the Q can be regenerated by electron transfer with another reducing agent.

Can you give a suitable example for such a reducer? Allegedly Ascorbate does this, but it is a developer by itself.

 

[Photo Engineer]

Rudi, any reducing agent may be an ETA. It may even have no discriminatory power to tell exposed silver from unexposed, and thus it can cause fog when used alone. Finding them is sometimes T&E. The key signature is a developer with a large amount of something like HQ, and a small amount of an ETA, say 500 mg/l. The ETA does the developing and is regenerated by the huge pool of HQ.

And, lets not forget superadditivity such as we find with Metol and HQ. This changes the picture entirely.

PE

 

[Rudeofus]

Ron, I read your previous posting as "there are compounds which are not developers by themselves, but which will happily reduce Q back to HQ in moderately alkaline environment". Did I misread this? Are there such compounds anyway?

 

[Photo Engineer]

Yes there are. But, some are just fogging agents. Remember, T&E. BTDT.

PE

 

[Kawaiithulhu]

This changes the picture entirely

Your pun is well appreciated 'round these parts