Developing Agents - Chemistry

Hi guys,

I'm curious about something. How come all developing agents are organic? Is this a mere coincidence, or is there a specific reason why no inorganic compound can be used as a developer?
We need a compound that can react with silver ions which have been exposed to light and reduce them to silver atoms, but without reacting with the other ions, which have not been exposed to light. Does this job actually require some specific property of organic compounds, which inorganic compounds lack?
Or maybe it's not so much about being organic or inorganic, but rather about containing a specific component (like benzene, for instance, which seems to exist in all developing agents)?

Thanks.

I'm not up on the math, but I suspect that to only develop the silver ions with a latent image supplied tiny eV jump from a few electrons needs a fairly gentle oxidising agent.

Inorganics oxidizers may supply too much oxidation and in turn reduce all silver ions, light struck or not, to elemental silver.

In a bleaching process of a bleach and redevelop style toner this is what happens. The organic developer has reduced the latant image silver ions only to elemental silver. The fixer dissolves away the still ionic silver. Then after a rinse we bleach, usually with a K ferri to K ferric, although K permanganates are also used and it turns all the elemental silver back to ionic, which, in the presence of the (usually) bromide ion forms silver bromide. The the toner takes the ionic or elemental silver and bonds it with the toning element.

I am no chemist but, if I recall correctly, ferrous oxalate, ferrous lactate and ferrous citrate are inorganic. All are listed as having historic utility as developing agents, however outmoded.

I'm not up on the math, but I suspect that to only develop the silver ions with a latent image supplied tiny eV jump from a few electrons needs a fairly gentle oxidising agent.

Inorganics oxidizers may supply too much oxidation and in turn reduce all silver ions, light struck or not, to elemental silver.

Click to expand...

Thanks Mike, that's what I suspect, too. But I'm very curious why this happens. What exactly makes inorganic reducers so much more powerful than organic ones? Is it maybe because most inorganic reducers are ionic, while organic ones are covalent? Or maybe some other characteristic? :confused:

In a bleaching process of a bleach and redevelop style toner this is what happens.
[........]
which, in the presence of the (usually) bromide ion forms silver bromide. The the toner takes the ionic or elemental silver and bonds it with the toning element.

Click to expand...

Yes, but that's not the same. The toner reduces all silver, regardless of whether it has been exposed to light or not. So it wouldn't make a good developer.

if I recall correctly, ferrous oxalate, ferrous lactate and ferrous citrate are inorganic.

Click to expand...

Well, the iron may be, but the anions are all organic (that is, covalent compounds based on carbon).

There certainly are inorganic developers.
Many metals, H2O2, hydroxlyamine, etc. will develope latent images...
I am not seeing any reason why one would necessarily always need carbon bonds somewhere, but the silver has to come from somewhere... either the AgX in situ, (chemical development) or from an external source, (physical development)... for it to come from the AgX, the Ag has to be "liberated" from the X and having something in the solution to solublize the silver helps... so perhaps many of the non-organic chemicals you will find workng with inorganic chemical developers are there to either solublize the silver or to tie up reverse reactions. In anycase, they are best thought of as enablers since they probably won't do much alone.

Perhaps as you hinted at, more "fine tuning" can be done with an organic pallette?

I thought the conceptual boundry between organics and inorganics had nearly been erased....

LFA Mason,Photographic Processing Chemistry, p177 notes that development by metallic ions is confined to metals showing variable valency,the lower valency ion being the developing agent.In 1877 C Lea discovered the ferrous/ferric oxalate system.Mason gives formulae for Titanium and Vanadium based developers.
Karlheinz Keller, Science and Technology of Photography, 1993, notes to the effect that the loss of importance of developers based on iron,chromium,molybdenum,copper and titanium is due to their low activity,rapid exhaustion,poor stability (I don't think he meant all at once).A vanadium developer for motion picture film that can be electrochemically regenerated develops very rapidly but did not come into widespread use because it works in hydrogen bromide solution which is highly corrosive.