developer theory part I

[alanrockwood]

I have lots of questions about developer theory. Let me start with one, presenting a hypothesis for discussion.

I suggest that the purpose of alkali is mainly to pull of a proton from the developer, leaving a negative charge on certain groups. For example, p-aminophenol would lose a proton from the phenol group, leaving a negatively charged oxygen atom attached to the ring, and this would then reduce the silver halide. Of course, not all negative ions would act as reducers. For example, caboxylic acids would not act as reducers. However, some would.

This theory makes sense if one considers tylenol vs. p-aminophenol. Tylenol, being esterified at the phenol position, is not able to ionize and can't act as a developer, whereas if the ester is hydrolized you get p-aminophenol, which can ionize by losing a proton and then (presumably) act as a developer.

If this theory is right then one should see little development at pH much below the pKa of the relevant ionizable group, a rapid change with pH when pH is near the pKa of the compound, and a plateau as pH climbs much higher than the PKa. In other words, developer activity would correlate strongly with pKa when examined over a series of compounds.

Am I on the right track with this thinking?

Thanks.

 

[walter23]

There's got to be literature on how developers work out there. I don't think the phenolate anion's oxygen would be the primary electron donor though as oxygen is pretty electonegative - it's quite happy to have excess electrons, in fact it kind of prefers it. It gets kind of complex though, you really have to consider these things quantum mechanically (or at least approximated in terms of resonance structures or molecular orbitals), so saying the electrons belong to the oxygen, or don't, is kind of meaningless. There's probably a redox partner for p-aminophenol similar to this one for hydroquinone though, which I guess suggests a basic solution could facilitate the silver reduction by sucking up protons from the developer. It might go through a free radical though, I guess.

Guess I didn't answer your question. If you find any good literature let me know; I'm curious. It's hard to wade through all the irrelevant material to find descriptions of the basic chemistry involved.

 

[Photo Engineer]

If it were based on pH, then the acidic Azo developer formulas would not work. Amidol, a phenol derivative works in acidic solution.

Also, p-phenylenediamines are bases themselves and do not lose a proton and yet are very good developers.

PE

 

[alanrockwood]

Does anyone know the pKa of amidol? If it is low enough then some fraction of the amidol might be ionized (as the phenolate ion), even in acidic solution. This might allow development via the negative ion mechanism outlined above, even in acidic solution.

Looking at the organic chemistry, amicol is a phenol with amino (NH2) groups subsituted at the ortho and para positions. I believe that ortho and para substituted amino groups would both act as electron withdrawing groups, which would stabilize the phenolate anion, making amidol more acidic than singly substituted phenols. This would lower the pKa and allow significant concentrations of the anion to exist at lower concentrations than something like p-aminophenol. This might be able to account for the ability of amidol to act as a developer in acidic solutions.

On the other hand, there might be more than one mechanism for development, so the anion mechanism I suggested might not apply in all cases.

Concerning p-phenylenediamines, yes they are bases, but the aniline group is also weakly acidic. This acidity might be enhanced in this compound via the electron withdrawing effect of the other NH2 substituted at the para position on the ring, so in a strongly basic solution it might be possible to have some concentration of the anion, which might allow development to occur via the anion mechanism I suggested. Even in this case ionization would likely be weak, which could be consistent with the fact that p-phenylenediamine is not a strong developer.

By the way, my suggestion that amino groups are electron withdrawing when substituted at the ortho and para positions might be wrong. It has been about three and a half decades since I took organic chemistry, and I am doing this by memory, so someone please correct me if my organic chemistry concepts are wrong on this point.

 

[Jordan]

The chemistry of developers is pretty complicated and goes beyond pKa's, though they're important. I'll let PE chime in with the more important points here, but ultimately what matters is the reducing power of the active species. In the case of phenolic compounds this reducing power is greater (i.e. the compounds are easier to oxidize) when they are deprotonated. (Right, PE?)

The tylenol / p-aminophenol case is different because tylenol is p-aminophenol acetylated on the aniline nitrogen -- the phenolic -OH is free. That tylenol on its own is not a developer shows the importance of push-pull-type resonance effects where the nitrogen can stabilize some of the incipient charge.

Anilines (aromatic -NH2) are actually considered activators / electron-donating groups in organic chemistry. Nitro groups (-NO2) are electron-withdrawing.

 

[Photo Engineer]

Jordan;

You are right. In my post I only wanted to point out that if the potential is right development can take place in acidic solutions. Mees and James lists developing agents by structure, rate and redox potential. There is a direct relationship between reducing power and rate, but if the potential gets too high, there is little discrimination and you get fog.

PE

 

[Michel Hardy-Vallée]

Can't help you with that one, but did you read Modern Photographic Processing by Grant Haist? It has substantial chapters on the chemistry of developers which could be helpful.

 

[Photo Engineer]

Yes, Grant pretty much covers it all just as well as Mees and James.

PE

 

[Michel Hardy-Vallée]

I do remember reading something about proton exchange in Haist, methinks.

 

[Photo Engineer]

Well, the latest thing was electron transport. I have discussed it here a bit.

PE

 

[alanrockwood]

Thanks to all for responses so far. Please continue to supply information.

Alan

 

[Ole]

Jumping to (very) basic developing, one of the first developers listed in pre-WWI books is the iron sulfate acidic developer. While it is only of historical interest now, it shows that any compound with a reduction potential in the right range can work as a developer.