Help with some basic chemistry (theory) on developers

[elerion]

In this post I'd like to ask for advice regarding two quite basic photo-chemistry matters, on which I'm pondering.
Sorry if any of these are duplicated or previously answered questions. I tried to look for answers for several days prior posting, but still not sure.

1)
Knowing that two moles of silver can be developed by a single mole of ascorbic acid, we can estimate that 1g of the later can develop 1.225g of the former.
Just to illustrate, suppose that a developed 35mm film has 50mg of metallic silver. This would require 40.8mg of ascorbic acid.
On the other hand, it is said that 0.016g of phenidone is enough for one film. As phenidone's molar mass is close to ascorbic's (162.192 g/mol vs 176.12 g/mol), it seems like if a single mole of phenidone is able to develop six moles of silver.

Now, sure I'm making some false assumptions here.

2)
Hydroquinone is usualy present at higher mass than metol, and sometimes, in the same ratio to phenidone (in PQ developers) as ascorbic acid (in ascorbate developers).

But, HQ's molar mass is just 110g/mol, so it should be able to develop more silver than ascorbic acid (for the same mass, of course).
Even more relevant is: metol's molar mass is much higher.
Also, it is said that HQ is more "energetic" developer than metol.
When mixed, HQ is the source of electrons (due to superadditivity), while metol or phenidone are just the molecules that are more easily "bond" to silver halides.
So why so much HQ is used in most formulas?

Well, it just doesn't make much sense, or, better expressed, I'm not able to use my own logic to explain these facts.

Any help is much appreciated.

 

[dE fENDER]

1. One mole of phenidone able to develop 2 moles of silver

During developing HQ restores phenidone activity, so you can develop more than 2 mole.

2. The main role Hq is the same - restoring metol or phenidone.
Developer agent should be in excess in developing solution for 1) buffering purposes and 2) developing speed (which is stopping to increase after some quantities, specified to agent).

 

[Rudeofus]

While in theory 50 or whatever milligrams of ascorbate or HQ can develop the whole roll of film, the developer would be much weaker at the end of development than at the onset. This effect is sometimes intentionally used (c.f. two-bath development), but in most cases it is undesirable (concave characteristic curve, long dev times, large grain, somewhat unpredictable results).

The search term you would need for understanding the seemingly high capacity of Phenidone is "superaddtivity". It is extensively covered in just about any book about photo chemistry, including in the (now freely available) third edition of Kenneth Mees's book "The Theory of the Photographic Process".

 

[elerion]

So, basically, the phenidone's 0.016g figure per roll supposes there'll be HQ or ascorbic acid/sodium ascorbate too, and so, without superadditivity, phenidone should be used in a much larger quantity. Right?
In this case, I suppose that the 0.016g figure was found empirically.

Developer agent should be in excess in developing solution

Does it apply to HQ only in MQ/PQ developers? Or you mean all developer agents?
Point 2) was about, why so much HQ in relation to metol, when it is more powerfull and has a lower molar mass. Found empirically too maybe?

While in theory 50 or whatever milligrams of ascorbate or HQ can develop the whole roll of film, the developer would be much weaker at the end of development

Yes, I know. I want to find the absolute minium requirements, not the ideal, nor either the required to get good results in all situations.

"superaddtivity". It is extensively covered in just about any book about photo chemistry

I've read many times about it, and I know the basics (the two main developer groups, argentophilic end,...). But most books just don't get to actual numbers, just give a qualitative description. For example, "Developing: the negative".
Kenneth Mees's book is a little bit too complex for me right now, but I've read some parts, and I found it very valuable, just can't (or don't know how to) use it for the kind of questions I was asking.

 

[Rudeofus]

So, basically, the phenidone's 0.016g figure per roll supposes there'll be HQ or ascorbic acid/sodium ascorbate too, and so, without superadditivity, phenidone should be used in a much larger quantity. Right?

There are very few special purpose developers, which use Phenidone only, and these use 1-2 g/l Phenidone. Phenidone has some very peculiar properties as developer, which make it uniquely suited for superadditive combos but not very useful as sole development agent.

In this case, I suppose that the 0.016g figure was found empirically.

That's most likely the case.

Yes, I know. I want to find the absolute minium requirements, not the ideal, nor either the required to get good results in all situations.
Kenneth Mees's book is a little bit too complex for me right now, but I've read some parts, and I found it very valuable, just can't (or don't know how to) use it for the kind of questions I was asking.

Photochemistry is a complex topic, and you will have to invest some learning effort before you start experimenting. There are many variables in a developer (reduction potential, concentration, buffering, "induction time", balance with restrainers, ...), together with many performance indicators for the development result (film speed, sharpness, granularity, shape of characteristic curve, ...). Experimenting without prior knowledge will waste hundreds of liters of developer and countless kilometers of test film strips, yet yield few new insights and even fewer improvements to developers.

 

[dE fENDER]

Does it apply to HQ only in MQ/PQ developers? Or you mean all developer agents?
Point 2) was about, why so much HQ in relation to metol, when it is more powerfull and has a lower molar mass. Found empirically too maybe?

To any normal developer recipe with some exceptions.

Why so much HQ in HQ only developers compare to metol only developers? There is equilibrium between dissotiated developing agent and undissotiated form in solution. Undissotiated is inactive. The more you add developing agent, the more equilibrium shifts towards to dissotiated form. This equilibrium is different to different agents, but It's just only one reason for the excess.

 

[Alan Johnson]

By a popular theory one developing agent (phenidone, metol) in a binary mixture is adsorbed on the grain surface and not used up, it just passes electron from the secondary developer (hydroquinone, ascorbate) which is used up.
The developer operates at its highest rate according to a ratio that is found experimentally, it has nothing to do with stoichiometry.

 

[Photo Engineer]

Alan, this is the basic theory behind the ETA or Electron Transfer developers.

And, developers act unexpectedly sometimes due to other factors such as halide release, alkali exhaustion, sulfite interactions and other factors too complex to comprehend for most unless you understand some physical chemistry and phsical organic chemistry. I am no great expert in either, but I can understand and hold my own in discussions.

PE

 

[Alan Johnson]

There is a good account of this theory in UK book "Basic Photo Science" by HJ Walls and GG Attridge 1977 p170-204, IDK if it is easily obtainable in N America and elsewhere.
Kodak and Ilford never published details of the photochemistry of acorbates but there is some work by Gainer and others on Photrio internet.
After Kodak labs closed, work on photochemistry has been continued by Ilford (Ilfosol 3) and by Drs Schain of Spur and Raffay and others but afaik they never published anything and it has not been discussed on Photrio.

 

[Photo Engineer]

Both Haist and Mees do a bit of this.

PE

 

[elerion]

Photochemistry is a complex topic, and you will have to invest some learning effort before you start experimenting.

Sure. I'm now reading "photographic chemistry" by P. Glafkides. Very interesting old book. Somehow easier for me than Kenneth Mees's, also it's a little bit "newer", and talks about phenidone.

developers act unexpectedly sometimes due to other factors

Indeed. This time I was just making some numbers and theorizing. It helps me understand and put pieces in place.

Is there anywhere I can read or learn about why, for example, phenidone is much more efficient (per mole) than metol?
Metol doubles g/mol, but it is commonly accepted than just about 1/8 or 1/12 (mass) phenidone is required respect to metol, for a equivalent ascorbate developer.
Even if a single mole of metol develops a single mole of silver (not two; I consulted several books and articles, but haven't found this information), still there's approximately two-fold advantage to phenidone.
Maybe just another empiric rule difficult enough to probe quantitatively?

 

[Rudeofus]

Is there anywhere I can read or learn about why, for example, phenidone is much more efficient (per mole) than metol?

There are several reasons for this:

1. Phenidone forms a very stable radical ion after giving up an electron. This gives the secondary development agent ample time to bring it back into its original state.
2. Oxidized Phenidone does not react with Sulfite, therefore the only action which would permanently lose Phenidone during development would be Phenidone giving up another electron - something unlikely to happen as per #1
3. Phenidone binds strongly to silver and silver halide, therefore you don't need much Phenidone to begin with to accelerate development by secondary developer.

I don't have Glafkides, but I do have T. H. James' Theory of the Photographic Process 4th edition, which is one of the more recent (as in: published in 1977) books on photo chemistry - it's quite a bit harder to read than Mees's Theory of the Photographic Process, and there is no legal&free PDF out there. Some information is only available through scientific articles from publications such as "The Journal of Photographic Science" and "Photograpic Science&Engineering", typically available through university libraries, and there are lots and lots of patents which can reveal interesting tidbits.

Note: a lot of information has never been published at all, as it was considered a trade secret back then, and may now well be lost forever. A lot of these scientific articles contain graphs and data, where you ask yourself "why didn't they look at this, too, this would be so much more relevant?", until you conclude "they definitely did, but didn't get green light to publish it".

 

[Alan Johnson]

Phenidone vs metol is discussed in Photographic Prrocessing Chemistry by LFA Mason p135-8 and by Walls and Attridge p195-198.who note:
"....development by metol-hydroquinone, unlike that by phenidone-hydroquinone, is almost certainly not entirely regenerative,(This is the probable explanation of the fact that, for mixtures of equal developing activity, more metol than phenidone is needed).The superadditivity may then be partly due merely to the more easily oxidised adsorbed metol ions starting development and "breaking a trail" for the hydroquinone. In that case , part of the metol is oxidised irreversibly to its sulphonic acids."

 

[elerion]

Phenidone forms a very stable radical ion after giving up an electron. This gives the secondary development agent ample time to bring it back into its original state.

Let me see if I got it.
As dEfENDER illustrated, one mole of phenidone can develop two moles of silver.
I suppose then, that each electron develops one silver bromide molecule.

So, phenidone never really looses both electrons (ending up like in the state shown on the right side of the illustration), as per #1,
If this is the case, it develops one mole of silver, then gets restored, then develops another mole of silver, gets restored, and so on,...

Or, is developing the two moles of silver done with just the first electron? In this case, phenidone moves back and forth between both sides of the illustration while developing and being restored by HQ or ascorbate.

I didn't know "Theory of the Photographic Process". Let's see if I'm able to find it second hand.

Phenidone vs metol is discussed in Photographic Prrocessing Chemistry by LFA Mason p135-8 and by Walls and Attridge p195-198.who note:

Thank you, very interesting.

 

[Rudeofus]

Let me see if I got it.
So, phenidone never really looses both electrons (ending up like in the state shown on the right side of the illustration), as per #1,
If this is the case, it develops one mole of silver, then gets restored, then develops another mole of silver, gets restored, and so on,...

Correct, that's precisely what typically happens in a developer with Phenidone and a secondary development agent such as HQ, Ascorbate, ... once Phenidone is forced to give up another electron, it turns into a compound which can no longer be reduced back to Phenidone and is therefore lost.

Or, is developing the two moles of silver done with just the first electron? In this case, phenidone moves back and forth between both sides of the illustration while developing and being restored by HQ or ascorbate.

One electron is needed to convert each silver ion into metallic silver. As long as there are developable silver ions, Phenidone will keep effectively passing on electrons from HQ to these silver ions, one at a time.

 

[Photo Engineer]

But regardless, adsorption of the developer molecule onto the grain is also a governing factor in this all. Redox potential, adsorption, regeneration and etc. All of these contribute.

PE

 

[elerion]

In "Theory of the photographic process" (Mees) there's a table showing how many moles of AgBr can be reduced by some developing agents.
"In a solution not containing sulfite, one mol of p-aminophenol reduced only 1.2 mols of silver bromide."
But, in a sulphite added developer, one mole of p-aminophenol reduced 3.9, while HQ did 4.4. How can this be? Who is restoring their oxidation state? It seems I can't find the explanation for this along the chapter.

But, in another place (attached) there's a diagram, from which I understand that HQ can be restored (from quinone back to HQ), so it is able to further reduce more halides.
Is it so? Does it apply to p-aminophenol, metol, ascorbate,... too?
Phenidone is quite a "strange" molecule compared to the formers.

 

[Alan Johnson]

I cannot give a reference for p-aminphenol but methyl aminophenol (Metol) is one of those developing agents whose oxide strongly inhibits development but is removed by sulfite.
Look in the index of Mees for "Metol effects of added sulfite" Refers only to the first part of your question.

 

[Photo Engineer]

This is a complex problem in Organic Chemistry. Many of us at EK have puzzled over these problems for years and yet we have those advanced degrees. I suggest that you just study the references for now and try to understand and then get back to us.

I know, this is a non-answer, but it is the best I can give you. Many here try to work out problems without understanding the basic science behind the problem. Sorry, I can't do better. It is even hard for me.

PE

 

[elerion]

I know, this is a non-answer, but it is the best I can give you.

On the contrary, it's very helpful.
I'm currently learning lots of things almost everyday, and I fully agree with you and rudeofus about the necessity of a solid background.

By the way, I'm able to get "Theory of the Photographic Process" third edition (1966). Do any of you know if the fourth edition (1977) is worth it?
It is much more expensive and I'd have to buy it overseas, which adds and extra cost (customs, and also a fixed fee of about $25).

 

[dE fENDER]

By the way, I'm able to get "Theory of the Photographic Process" third edition (1966). Do any of you know if the fourth edition (1977) is worth it?
It is much more expensive and I'd have to buy it overseas, which adds and extra cost (customs, and also a fixed fee of about $25).

I think it worth. It has been quite rewritten in this edition and both 3rd and 4th should be used together (as it stated at preface and some chapters).

 

[elerion]

3rd and 4th should be used together

Together? You mean that even if someone owns the 4th edition, there're things on the 3rd edition that are not included?
Sounds weird. Most of the times a new edition kind of overrides previous ones (or I think it should).

 

[Photo Engineer]

Either version of Mees, as revised and added to by James is the best of all of them, but I still use my original Mees.

PE

 

[elerion]

I'm reading Mees's 1942 book and also the bit newer P. Glafkides's. Both are good.
Particulary about Mess's, I 'm getting the feeling that there're so many things (regarding film and develoment) that are just suppositions and hypothesis, based on the work of different people. Many times along several chapters the author is exposing some fact, then asks himself "where does this come from?", and then presents several posibilities. This of course, is of great interest, but from today's perspective, I'd rather know what things remain a "mistery", and what things are already well known.
That's the only reason I consider getting the last edition.

 

[Alan Johnson]

If you want to get another textbook I suggest Photographic Processing Chemistry by LFA Mason. He was head of research at Ilford at about the time Mees & James gave the Kodak view. IMO there have not been any later books that give such comprehensive coverage as these two.Unfortunately both predate the introduction of ascorbates on a commercial scale and have very little coverage of this topic which is commercially important today, nor have their been any other books dealing with ascorbate chemistry.