Why are developing agents complex compunds ?

Ken Lee · Mar 17, 2006

Note: I am not asking why the overall forumulas are complex. I know about the need for a restrainer, etc. I am asking about the developing agents themselves.

If all that is required is a reducing agent, IE an Alkaline environment, why do most developers use more complex compunds for their active developing agent ? "Metol" is really p-methylaminophenolsulfate, and "Hydroquinone" is really p-dihydroxybenzene.

Why not just a simple solution of Hyrdogen Peroxide at the appropriate dilution ?

Gerald Koch · Mar 17, 2006

The problem with the use of just any reducing agent is that it would not be selective in its action. That is all silver grains would get reduced whether or not they were activated by exposure to light. Read the section on development and specifically on reduction potential in a book on photochemistry. The subject is too extensive to be covered here.

Metol was the Agfa tradename and Elon the Kodak name for p-methylaminophenol hemisulfate.

Kirk Keyes · Mar 17, 2006

Ken Lee said:
Why not just a simple solution of Hyrdogen Peroxide at the appropriate dilution ?

Also, hydrogen peroxide is an oxidizer, not a reducer.

avandesande · Mar 17, 2006

ferrous oxalate was the first developer, it's a pretty simple compound. If you took a course in organic chemistry you wouldn't think the compounds in common developers are very complex.

Photo Engineer · Mar 17, 2006

Ken;

The key is that the developing agent must discriminate between exposed and unexposed silver. So, many reducing agents exist, but most of them yield totally black developed silver with no image. The few that yield good images go into our tool box.

PE

Wayne · Mar 17, 2006

avandesande said:
If you took a course in organic chemistry you wouldn't think the compounds in common developers are very complex.

I took a course in organic chemistry. About all I remember is the elation I felt when it was finally over...

Wayne

Donald Qualls · Mar 18, 2006

One of the old tintype/ambrotype/wet plate developers contained only ferrous sulfate, acetic acid, ethanol, and water. Nothing very complex in that stuff. As suggested, ferrous oxalate isn't very complex, either.

The organic chemicals we use now are pretty simple for organics; the p-aminophenol in Rodinal is about as simple as an organic developer can get, I think (perhaps glycin is simpler, but not much simpler). These compounds have only a dozen to two dozen atoms in a molecule, comparable to aspirin or vitamin C (which latter is itself a developing agent). We use them because they have much better selectivity than the inorganic developers of the 19th century -- we get more silver, and less fog, from a given emulsion formula than we would have with ferrous sulfate or ferrous oxalate.

avandesande · Mar 18, 2006

if you want to scramble your brains look at the sensitizing dyes.

Ole · Mar 18, 2006

Quick summary:

With a very few exceptions, all developing agents consist of a benzene ring with addons. The benzene ring is stable enough to give compounds that don't disintegrate immediately, yet have a large reduction potential. The add-ons control how and how fast the reduction happens.

A benzene ring is a C6H6-thingy, six carbon atoms in a nice ring. Any H (Hydrogen atom) is an add-on, and can be replaced by just about anything. The simplest developing agents are Hydroquinone and Pyrocatechol - both are C6H4(OH)2, two of the "H"'s being replaced by "OH". But the spacing between the OH's also make a difference...

It's complicated, but not really complex compared to what organic chemistry can be.

Photo Engineer · Mar 18, 2006

avandesande said:
if you want to scramble your brains look at the sensitizing dyes.

I agree!

What do you think I've been doing lately. Look at my post on ortho emulsions.

Not easy chemistry and not inexpensive either.

For those of you complaining about the cost of some of your chemicals, some of the ones I buy are $200 / gram. But, at the rate they are used, that is a lifetime supply for 10 people.

So the average price of a group purchase is quite reasonable. The average price of pre-prepared usable quantites, by a good supplier, would aso be reasonable.

PE

Ryuji · Mar 18, 2006

Ole said:
With a very few exceptions, all developing agents consist of a benzene ring with addons. The benzene ring is stable enough to give compounds that don't disintegrate immediately, yet have a large reduction potential. The add-ons control how and how fast the reduction happens.

To be precise, benzene itself is not reducing. But it accepts functional groups that push in an electron onto the ring, and this makes a reducing agent as a whole.

For example, hydroxyl group and amino group have strong tendency to push in electron to the ring. Thus hydroquinone, p-aminophenol, pyrogallol, amidol, etc.. are developing agents.

On the other hand, carboxyl group tends to pull away an electron from the backbone it is attached. Salicylic acid is not a reducing agent for this reason. Also, glycin is carboxylated at the nitrogen of p-aminophenol, so glycin is a weaker reducing agent than p-aminophenol. And so on... But at this point it's a bit more than your quick summary.

To be more precise, the backbone doesn't have to be benzene, as you said. It can be many things that contain -C=C- in between. For example, ascorbic acid has HO-C=C-OH and this is the moiety acting as a reducing agent. This is, in fact, analogous to the reducing moiety in catechol.

joeyk49 · Mar 18, 2006

phew...this thread got my head spinning, and there were no dissertations, either....

Although I have an vested interest in the principles of photochemistry and ignorance of these may prove to my detriment (or my photograph's), I prefer my chemistry stirred, not shaken...

FilmIs4Ever · Mar 18, 2006

I agree, developing agents are rather simple. I did my high school chemistry project on them. Most consist of benzene rings with various chemical groups attached, phenols and such. Another thing to consider is that most of the commonly-used B&W reducing agents in developers today were discovered in the 1800s. There is quite a long history behind them, and they really aren't that complex. If you'd like complexity, look to color. The actual names for some of the color developers are more than ten syllables long.

Regards.

~Karl Borowski

Ryuji · Mar 19, 2006

FilmIs4Ever said:
I agree, developing agents are rather simple. I did my high school chemistry project on them. Most consist of benzene rings with various chemical groups attached, phenols and such. Another thing to consider is that most of the commonly-used B&W reducing agents in developers today were discovered in the 1800s. There is quite a long history behind them, and they really aren't that complex. If you'd like complexity, look to color. The actual names for some of the color developers are more than ten syllables long.

Please allow me to expand on some of those.

Most b&w developing agents in use today were discoved in last 20 years of 1800s, to be more specific. I have several pages long of list of agents, discoverers, patent numbers and date, BUT you should realize that the number of agents that were actually used is far fewer, and the number of agents commercially manufactured and offered for sale is even fewer. People like Andersen, Hauff, et al were crazy in naming potentially useful compounds and getting patents. Many of them are agents that are complex (and expensive) to manufacture, compared to Metol and hydroquinone. Many of those agents, I never see the name anywhere else. I didn't count the number of syllables, but they are longer than some color developers. Color developers are very stereotypical because they are aromatic diamine derivatives, and they don't have the kind of diversity b&w developing agents have. Formulation of color developing solutions is a completely different matters, and it is very much more tricky. Preservatives, for example, is already a very difficult problem, particularly in RA-4 type print developers.

Anyway, anything can be very deep if you try to understand it to the fundamental principle. If you ignore the principle and just look at things at surface, anything can be simplified or oversimplified.

avandesande · Mar 19, 2006

You could write a thick book about molecular hydrogen. We have to generalize/simplify or it would take days to talk about simple subjects.

Thanks for the heads up that looking deeply at chemistry is complicated.

Ryuji said:
Anyway, anything can be very deep if you try to understand it to the fundamental principle. If you ignore the principle and just look at things at surface, anything can be simplified or oversimplified.

Donald Qualls · Mar 22, 2006

FilmIs4Ever said:
The actual names for some of the color developers are more than ten syllables long.

As opposed to, say, metol? N-methyl para-aminophenol sulfate? And then there's Dimezone S -- I don't recall that one offhand, but it's about 50% longer than metol. There's a good reason most developing agents are known by a popular or commercial name...

Jordan · Mar 22, 2006

FilmIs4Ever said:
If you'd like complexity, look to color. The actual names for some of the color developers are more than ten syllables long.

Names in chemistry are funny. People attach too much weight to them. We use complicated names because they provide structural information -- theoretically, they should be enough to describe the positional and bonding relationships of all the atoms. But even molecules with beautiful internal symmetry can have horrendous systematic names. "Natural" discrete molecules can be equally horrendous. I'd be surprised if the systematic name for Vitamin B12 was less than half a page long.

Why are developing agents complex compunds ?

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