Final processing: fixing and rinsing
After the print or film has been in the stop bath for some time, the developing process is stopped and the visible image is stable, at least for a short time. In the long run, however, the image is not stable. Exposure to light still produces new free silver metal atoms and although this is not a problem for a few minutes, or even hours, it is a problem when this process continues for days, weeks or even longer. In order to make a film or print really permanent, one has to remove all left over silver halide. This process of making the print or film permanent is called fixing. The state of the image is fixed in this process and no further blackening can occur after this step.
Basic fixing bath: solution of sodium thiosulfate
Fixing is done, by immersing the print or film in a so-called fixing bath, after it has been in the stop bath. Some people use a brief rinsing stage between the stop bath and the fixing bath in order to remove adhering acid solution, but this is not really necessary. It, however, improves the useful life time of the fixing solution somewhat.
The simplest possible fixing solution is a solution of sodium thiosulfate in water. This thiosulfate ions form a soluble coordination complex with silver ions.
Ag+(aq) + 2S2O32-(aq) → [Ag(S2O3)2]2-(aq)
Silver ions are in equilibrium with the silver halide in the grains.
AgX(s) ↔ Ag+(aq) + X–(aq),
but the silver ions are taken away from this equlibrium by means of complex formation with the thiosulfate, and the net result is that the equilibrium goes to the right and the grains of left over silver halide completely dissolve. After the print or film has been in the fixing bath for some time, it can be taken out and it must be rinsed thoroughly in order to remove all soluble chemical remains. After rinsing the print must be dried.
More durable fixing bath
The basic fixing bath, being a solution of sodium thiosulfate in water, is perfectly useful for a single session of processing images or films, which is not longer than a few hours.
Such a solution, however, cannot be stored for a long time. It slowly decomposes, while depositing sulphur, producing small quantities of sulphur dioxide and all kinds of other complicated oxo-species of sulphur. This decomposition is enhanced by the acid remains from the stop bath:
2H+ + S2O32- ↔ H2S2O3
H2S2O3 ↔ H2O + SO2 + S
The sulphur very slowly forms larger particles, which at a certain point can become macroscopically large and then the sulphur precipitates and is taken away from the equilibrium, resulting in the reaction being driven to the right.
If one expects to use the fixing bath again within reasonable time, then it must be stabilized. This is done by dissolving some sodium metabisulfite in the solution, besides the sodium thiosulfate. Such a solution contains some free sulphur dioxide, and this suppresses decomposition of thiosulphuric acid. The last equilibrium then is forced more to the left, because of the sulphur dioxide, already present in solution. Sometimes, also some sodium sulfite is added as well. This makes the solution a little bit less acidic, while keeping the concentration of free sulphur dioxide still at an acceptable level. Such a stabilized fixing solution can be stored for a few months, but in the long run, this also will deposit solid sulphur and will become less active in due time.
A good allround practical fixer solution is the following solution, which can be used for all general work: http://www.jackspcs.com/f24.htm.
After the fixing and thorough rinsing, the print or film can be considered stable, also for long term storage. It must be dried in a dust-free area (this is particularly important for film) and then it can be stored in a dry and clean place.
Further processing can be done, which is known under the name of toning. This is covered in another part of this website.