What chemical is used to develop film?

Jim Mowreader
Since I spent many happy years in total darkness as a professional developing and printing all my own black and white, I can address the question from a monochrome standpoint.

Most black and white (monochrome) developing solutions are what old timers call "M-Q" developers.The M stands for Metol (monomethyl-p-aminophenol hemisulfate), and the Q is short for hydroquinone, or just Quinone, a type of phenol. These two are often used in various combinations to formulate many common monochrome developers for either film or paper. Metol and hydroquinone in aqueous solution have the property of reducing sensitized silver halides (salts of silver) found in films and papers to metallic silver, which is what the developed image is made of. By varying the proportions of each, many types of developers may be made for longer scale or higher contrast. There are ready to mix powders or liquid concentrates, or the worker with a laboratory scale and a sense of daring can formulate his/her own. Eastman Kodak used to publish a fat book of all their formulas, but I don't know if it's still available.

Many developers also include Phenidone (1-phenyl-3-pyrazolidinone), which is much more active than Metol but harder to control because of its reactivity.

Together with the necessary developing (reducing) agents are usually mixed alkalis because an aqueous alkaline solution works much better and faster than one with a neutral pH. Sodium carbonate is one typical alkali added to developers. Many others may be used alone or in combination depending on the contrast wanted and the speed of working.

Developers are perishable and liable to combine with atmospheric oxygen, since all developer is, is an oxidizer. To prevent accidental underdevelopment due to aerial oxidation of an older solution, many workers use "one shot" developers, mixing and using them fresh. Sodium sulfite is usually added to developers as a preservative. It is also alkaline.
 A little more about b&w developersAs the above answer stated, there are three basic chemicals in use: metol, phenidone and hydroquinone. There are a LOT of agents that have been used in the past, like paraminophenol, amidol and pyrocatechol, but the trinity of metol, phenidone and hydroquinone has served us well for decades and will into the future.

Metol and phenidone are "detail" agents. In general a formula will contain either metol or phenidone; very few formulas use both. It's possible to replace metol with phenidone in most formulas, and it's better if you do--metol is a weird, weird ingredient. For one thing, it won't dissolve if there's too much sodium sulfite in solution. If you buy packaged D-76 from Kodak, you will pour it into water just a little at a time while you stir and stir and stir until your arm tries to fall off, and in the end you'll have all these little metol crystals floating on top. Come back the next day, and the metol basically just gives up. Ilford made a version of D-76 they called ID-11. The difference was it came in two little pouches. One had the developing agents plus just a little sodium sulfite, the other had all the rest of the ingredients. You mixed Packet A into water, then Packet B and added more water to equal one litre. That dissolved really easily and gave the same result on film. The other problem with metol is, it's a sensitizing agent--a lot of people are allergic to metol, and the more you work with metol the more likely you are to be allergic to it. Phenidone is hypoallergenic and dissolves in any sulfite concentration.

One thing you need to know if you're trying to substitute phenidone for metol: phenidone is so active it's capable of producing chemical fog in the highlights. You need to add a restraining agent to a phenidone developer; the classic restrainer is benzotriazole aka "Kodak Anti-Fog Number 1." The other restrainer is 6NBN (trust me, you do NOT want to know what that means!) aka "Kodak Anti-Fog Number 2." Number 1 is available wherever fine photochemicals are sold. 6NBN isn't so easy to come by.

Hydroquinone is a "contrast" developer. If you're looking for contrast quick, this is the stuff. Metol doesn't NEED this agent to work right, if you're willing to develop film for 23 minutes. (There are also "stand" developers that produce really spectacular events. They're pure metol, are very dilute, and they're used by pouring them into the tank and letting it stand undisturbed for a couple of hours.) Phenidone requires hydroquinone; you won't get any contrast without it--hence, no phenidone-based substitute for the hydroquinone-free D-23 or D-25 developers, which contain only metol, can be made. If you need a very contrasty developer, like a lith developer (lith negs have black, white and nothing in between), use only hydroquinone in your developer.  And from the color side...Color images are formed of dyes--cyan (negative red), magenta (negative green) and yellow (negative blue). The film contains layers that are sensitive to red, green or blue light, and each contains light-sensitive silver halide grains plus color couplers--partial dyes.
When a silver halide grain is developed into a silver grain, it oxidizes. The color coupler grabs the product of that oxidation, and converts into a visible amount of dye.
There are two chemicals used to develop color today: Kodak CD-3 (used in the processes for color slides and prints from slides) and Kodak CD-4 (used in the processes for negatives and prints from negatives). No one uses the proper chemical names for these two because they are very long.
Please click the Related Question below for an even more detailed answer on photo chemistry.