Chemists' Question: Reaction of Silver Halide

<p>I was thinking about how a photographic image is formed. It's based on silver, of course. But what about the actual reaction that happens? It's a reduction reaction, something like (for silver chloride) Ag(+)Cl(-) + light --> Ag(+) + Cl(2-) + (e) --> Ag + Cl(2-) .. is that right? In that an electron from the other element is released by impact with light, attracting the silver and making it neutral. Where does the element go? Does it become a gas or part of the gelatine..? <br>

My main reason for asking is because, if the reaction can happen, can it be reversed? Like if you accidentally exposed a film to light, could you carry out a chemical reaction that would rehalogenise the silver, rendering the film reusable? Would C41 bleach do that?<br>

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I read that 'printing out' is when so much light hits the film that the silver visibly darkens. Could you in theory take a picture for a looong time and only need to fix the subsequent image?</p>

<p>Yes, you can re-halegonate the silver. Typical bleaches do that. But, you wind up with a VERY SLOW emulsion, nothing like the original. You would also lose the sensitizing dyes, so it would only be blue-sensitive.<br>

I don't think C-41 bleach would be suitable, as it would also have some hypo in it. But it does work with C-22 bleach.<br>

Yes, printing out papers, like Albumen papers, required only exposure (hours) and fixing. Well, they were often toned as well.</p>

 

<p>Woops, I meant for silver-only film! Ahh, interesting. So there is a major loss in ISO..?</p>

<p>hypo is fixer/ammonium thiosulphate, right? can you tell me why that would be? it just seems counterintuitive. Part of the chemical develops the silver while the other dissolves it. (I also read about hypo in developer in the Ilford B&W transparency PDF)</p>

<p>The blackening of silver nitrate by the action of light was first observed by Johann Heinrich Schulze (German 1687 – 1744) in 1717. He make a chalk laced with silver nitrate and placed this mixture in a glass bottle. Covering the bottle with a stencil he was able to make images but all quickly faded. His work was followed up by <a title="Thomas Wedgwood (photographer)" href="https://en.wikipedia.org/wiki/Thomas_Wedgwood_(photographer)">Thomas Wedgwood</a> and <a title="Humphry Davy" href="https://en.wikipedia.org/wiki/Humphry_Davy">Humphry Davy</a> who produced shadow images on pottery and treated leather and paper. In 1816 <a title="Nicéphore Niépce" href="https://en.wikipedia.org/wiki/Nic%C3%A9phore_Ni%C3%A9pce">Nicéphore Niépce</a> succeeded to make a camera-produced image using paper coated with <a title="Silver chloride" href="https://en.wikipedia.org/wiki/Silver_chloride">silver chloride</a>. He made the first permanent image in 1827. The action is: The bonds holding the silver salts are weakened and eventually broken by photon hits. <br>

In normal photography we only allow the crystals to receive a tiny exposure. This will generate a bud of metallic silver.<br>

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Next we complete the reduction of the crystal with a developer which is a reducing agent that has an affinity for oxygen and is able to liberate metals from their salts. Black & white film consists of the metal silver combined with one of three halogen elements. These are iodine, chlorine, and bromine. When one of the halogens (Swedish for salt makers) combines with silver a crystal is formed. The crystals resemble table salt, however they are much smaller and slightly yellowish. To make a film we glue them onto a base using a binder of animal gelatin. The gelatin silver salt mixture is called an emulsion. This is a misnomer; the mixture is actually a colloidal suspension. Too late to rename this stuff now.<br>

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After exposure in the camera the silver salt undergoes a chemical change. When immersed in a developer solution, the developer has the ability to identify those crystals that have been light-struck. This is because the exposed crystal self-reduces, and a bud of metallic silver forms on the surface of the crystal. <br>

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Having identified a crystal as exposed by the bud, the developer reduces the crystal to its two component parts. The metallic silver component being not soluble remains imbedded in the gelatin. The halogen component being soluble is dissolved away by the developer which is mainly water.<br>

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The key ingredients, the developing agents, were all discovered between 1880 ~ 1900. These are derivatives of benzene extracted from coal tar. <br>

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In a printing out paper, the exposure is prolonged and the crystals self-reduce. The silver and the halogen remain side by side. The silver is unpolished and appears black. The halogen, likely bromine, is ruddy and the image appears maroon. <br>

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The bleaches used in color films and papers do re-halogenate. In the color process we need to remove all the metallic silver as its presence veils the colors. We need also to remove all the exposed but not developed silver salts as these will eventually self-reduce and darken. We use a fixer as this is a solvent for silver salts. The bleach converts metallic silver back to a silver salt (silver halogen). Now a single bath in the fixer removes almost all traces of silver from color films and papers. </p>

<p>So (forgive my crappy non chemist approach) the part of the silver halide that's been reduced by reaction to light serves as something of a nucleation site for a reaction with the developer which proceeds to liberate even more silver from the crystal as long as it's in contact. The result is you go from a molecule-scale reaction to something the products of which are visible. This would also be part of how pushing the development of a film enlarges the grain, without necessarily improving the shadows.</p>

<p>When the developer reduces the silver salt, it is totally reduced. More aggressive and/or lengthy developing reduces more crystals. For example, aggressive developing reduces those crystals that normally would not develop as they have yet to receive sufficient exposure. If developing is allowed to continue, most all crystals will reduce, exposed or not. The tuft of silver that results from any one crystal is microscopic and thus not visible. Grain is a clumping of tufts of metallic silver. I use the word tuft because under the electron microscope the reduced silver has a filamentary structure like a cotton ball. My point is, I don’t think there is such thing as a partially reduced silver salt crystal. </p>

<p>The solid state physics explanation has the photon generate an electron hole pair, similar to what light does in a silicon photodiode.</p>

<p>In AgBr, the electron and hole move through the crystal until they find a nice place to settle down. This place has lower energy, which will either be a crystal defect or surface state. On the surface, the electron will convert Ag+ to Ag, and somewhere else, hopefully not too close, the hole converts Br- to Br. Additional electrons and holes tend to find the same place, neutralizing additional Ag+ and Br- in similar places, creating larger nucleating sites for the developer.</p>

<p>When the bromide is removed, the result isn't a nice small crystal of silver, but one with lots of holes, very rough looking. </p>

<p>C-41 bleach has hypo in it since the goal in color film processing is to remove all the silver, leaving only dye. But hypo only removes halogenated silver, so that's why the bleaches re-halogenate the silver, to prepare for the fixer to remove it.</p>

<p>John, I misread what you said. Oops! I mistakenly thought of developer; I was reading the Ilford guide to B&W reversal and they said to add hypo to the first developer. I really don't see why to do that, can someone clarify??<br>

Great explanation Alan Marcus! I really appreciate the time all of you put into responding to me. I'm learning a lot thanks to all of you :)<br>

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TetC41 combines the bleach and fix. Can I use the chemicals separately by making two solutions?</p>

<p>The goal is good clear highlight areas. This is a function of developing. Adding a silver solvent to the developer accomplishes this objective. Adding a small amount hypo to the developer does the trick. Normally hypo will contaminate, this is because a normal fix formula contains acetic acid. Here we add hype without the acid. Let me add that a developer/hypo mix has been used a mono-bath. The idea is to simplify processing by reducing the normal thee step process (develop – stop – fix) to just one solution. </p>

<p>highlight as in what was bright when you took the photo (and black after first development)?<br />I was considering adding a little Rapid Fixer to the first developer. It's going to be acidic, but only a little, right? <br>

How does it increase the clarity? is it because it slows down development of the highlights? How would pictures from a negative developed with developer only compare to ones using developer + hypo? </p>

<p>There are good discussions of photographic chemistry in this thread. I'm going to pick one historical nit. The first permanent photograph by Joseph Nicéphore Niépce didn't use silver. He coated bitumen of Judea (a natural asphalt) on a pewter plate. This material polymerized when struck by light. The unexposed (and unpolymerized) material was washed off. The exposure took many hours. In 1999 I visited the Musee de Niépce in Chalon, France. It was in the home once occupied by Niépce. Some of the cameras and other original equipment were on display. I was disappointed that only a poor quality xerographic copy of the original image was on display. </p>

<p>Buffering but destroying.</p>

<p>There is silver sulfide on the image specks, I think it comes from gelatin. There was a story about using the wrong kind of gelatin and tracing it to the grass the cows eat, IIRC.<br>

https://books.google.co.uk/books?id=g8zyCAAAQBAJ&pg=PA77&lpg=PA77&dq=silver+sulfide+image+specks&source=bl&ots=fLhr-BJ7E0&sig=bTpAVRclnAlEmT0V43epPc4XyNQ&hl=en&sa=X&ved=0ahUKEwi88_zzw7vKAhXMfRoKHVAIC-kQ6AEINzAE#v=onepage&q=silver%20sulfide%20image%20specks&f=false</p>