Anyone with an in-depth knowledge of the chemical processes in film development?

[Compaq]

Yes, I meant silver ion.

where the light quanta aren't energetic enough to dislodge an electron

Ah, yes. Black and white film isn't much affected by red light - it's low in energy, with larger wavelengths. But I thought using UV lamps was normal to light up the darkroom? UV light is strong in energy, and if visible light can throw off electrons, surely UV-light can? Or, wait a minute, the development of the film is done in complete darkness inside these small bucket things? After the fixer, there won't be much un-reacted crystals to be affected by the UV-light? But that's much later in the process, though.


Would it be possible to draw a small section of the lattice where light hits? Just to visualize it? I think in images, especially in chemistry, but I can't visualize what's happening within the lattice. Halogen ion loses an electron, but does the halogen atom remain in the lattice? No, it migrates out to be "picked up" by a receptor. Does the silver ion travel toward this "hole" in the lattice? And the electron as well. If they arrive at the same time, the silver ion is reduced to elementary silver. The more light that hits these grains, the larger will these pure silver accumulations be => brighter image (though darker on the negative). It's this I'm having trouble visualizing... and I'm not even sure I recapped it correctly

Thanks for the help!

[TPF Sponsor]

[Helen B]

I'll sort out some images.

Infrared is used in darkrooms, not UV. Silver halide is very sensitive to UV naturally. Look at Kodak's spectral sensitivity curves for their B&W films but not at, for example, Ilford's - because they are wedge spectrograms which can be very misleading.

The halogen atom itself does not move, as I hoped I explained earlier. It swaps an electron with its neighbour and so the movement of electrons changes which halide ion becomes a halogen atom temporarily. It's the electrons that move, not the halogen atoms.

[Compaq]

Hmm.

"Meanwhile the 'positive hole' that is the halogen atom swaps an electron with its neighbour and the positive hole moves in that way to the edge of the microcrystal, where the halogen will be received by the gelatin and, if present, a secondary receptor."


Positive hole that is the halogen atom?
You mentioned that the halogen is very immobile in the lattice, and here you say the halogen will have traveled to the edge of the microcrystal. Sorry, but I'm not quite grasping it!

[Helen B]

It's not the halogen atom that moves. Imagine that you are in the middle of a very crowded pub, with no room to move. Everyone in the room has a pint of beer in their hand except for you, and you are too far from the bar to reach it. You are the sober hole in the room - all alone and feeling out of place, with no beer. Everyone else is very kind and generous. Your neighbour hands you her beer, her neighbour then hands her a beer and so on, until it's the guy beside the door who has no beer. Nobody has moved, but the sober hole is now beside the door and can leave to go somewhere where he will be made welcome.


'Positive hole' is just the name that chemists give to the halogen atom within the crystal lattice.

[Compaq]

Okay, I see. So does that mean that the electrons wander from halide to halogen until there's a halogen at the edge of the lattice that can leave/react with something else?

[maris]

Compaq, every question you have asked is answered in comprehensive rigorous detail in C.E. Kenneth Mees "The Theory of the Photographic Process". It is said that Mees is so much the ultimate authority that he answers just about every question you can imagine to ask! And it is a good read because the chemistry and physics are full-on; no talking down to amateurs.

[unpopular]

It's not the halogen atom that moves. Imagine that you are in the middle of a very crowded pub, with no room to move. Everyone in the room has a pint of beer in their hand except for you, and you are too far from the bar to reach it. You are the sober hole in the room - all alone and feeling out of place, with no beer. Everyone else is very kind and generous. Your neighbour hands you her beer, her neighbour then hands her a beer and so on, until it's the guy beside the door who has no beer. Nobody has moved, but the sober hole is now beside the door and can leave to go somewhere where he will be made welcome. 'Positive hole' is just the name that chemists give to the halogen atom within the crystal lattice.

Don't yell ferricyanide in a crowded lattice.

(carry on)