Which chemical stops the darkening of an image after the development time?

[Zenab]

Which chemical stops the darkening of an image after the development time?

[Tony Rowlett]

Stop bath, which is just strong vinegar, or simply a water rinse and then fixer (hypo) which is also acidic.

[Alan Marcus]

The chief working ingredient in the developer is the developing agent. There are many different chemicals for this task however most are associated with benzene. The developing agent seeks out exposure salts of silver. This is a timed rection based on chemical strength and fluid temperature. During this few minute’s soak in the developer, those silver salts exposed during the camera exposure are reduced. Reduced in this context means, the exposed silver salt crystal is fractured into its two component parts. The silver part is liberated from the salt compound, it forms a tiny tuft of metallic silver. This silver fluff balls is opaque and thus appears black.

The developing agent is naturally not selective. In other words, it will attract both exposed and unexposed silver. The developer solution formula contains a retardant which is bromine. This addition coaches the developing agent to operate mainly on exposed silver salts. The developing action is somewhat slow. To speedup this reaction, a chemical called an accelerator is added to the developer solution formula. The accelerator is an alkaline that sets the pH of the fluid, it is made basic (alkaline). Various accelerators are used. The stronger the base faster the developers will do its job. Should the pH become acid, developing is slowed or halted.

The next step after the developer solution is the fix solution. This solution renders the film or photo paper permanent. It does this by dissolving silver salts that the developer has ignored. The fixer is a sodium or ammonium thiosulfate. This solution tends to liberate sulfur as it ages. Making this solution acid plus adding a preservative adds longevity. Now the developer is an alkaline solution, and the fixer is an acid solution. Should the two fluids intermix the resulting pH moves towards neutral which is 7. This mixing together of the developer and the fix forms carbon dioxide gas. It forms in the mixed solutions, and it forms inside the film creating gas bubbles that spoil the film. The countermeasure is to use a stop-bath solution between the developer step and the fix step.

The stop bath is simply a mild acid. Mild to prevent gas generation. Also, since development takes place in an alkaline solution, the stop bath arrests all developing action. Further, since the life of the fix is prolonged by making it acetic, developer that otherwise would ride piggyback on film or paper is neutralized. Thus, using a stop bath is a win-win. It stops developing dead in its tracks and the acid in the stop bath carries over into the fix prolonging its life. We use acetic acid diluted with water for the stop bath. This is the stuff of table vinegar.

Many dark-room workers use plain water as a stop bath. This works but not as fast plus the life of the fixer is not well-preserved using water.  

Should sulfur be liberated in the fixer solution, it attacks silver and tarnishes it. The tarnish is blown or sepia. Sometimes we like this look, so we treat photo papers in a mild sulfur solution. The result is a brown toned print. A second reason, the brown toned print is archrival. Silver sulfide is inert so the image likely will outlast the paper.  

Edited November 10, 2022 by alan_marcus2

[glen_h]

My always favorite developer, Diafine, recommends only water.

For one, more time doesn't affect development much, and also part B has a lot of carbonate.

School kids learn early what happens if you mix carbonate and acid, and it isn't what you want inside your film.

[bgelfand]

As Alan wrote above, acetic acid is the usual stop bath, at least for Black and White film and paper. 

Kodak makes an Indicator Stop Bath, using acetic acid with an indicator that turns from yellow, when the bath is active, to purple when the bath is exhausted. A working solution of Indicator Stop Bath may be reused until it turns purple.