Stop bath -- interruption of development process
As long as a print is in a developer, the darkening of the print continues, due to reduction of silver halide to silver metal. So, in order to stop the development of the print, it must be taken out of the developer solution. This, however, does not stop the development process at once. Quite some developer is present in the emulsion layer and in the paper base. The print is totally soaked with it.
Getting the developer out of the print in sufficiently short time is not possible, but what can be done easily is reducing the pH of the emulsion and paper base to such a value that the development process comes to a halt. As explained in the page about developers, a high pH is required for the development process.
So, when the developing process must be stopped, the print is taken out of the development solution, and it is immersed in an acidic solution, called the stop bath. The acidic solution quickly diffuses into the emulsion layer and the paper base, and within a few tens of seconds, the development process is stopped completely. A typical stop bath consists of a solution of acetic acid at a concentration of a few percent (plain white vinegar is suitable, diluted with an equal volume of water). Some people really don't like the smell of vinegar. For such persons, there is the somewhat more expensive option of using a 2% solution of citric acid in water.
If large numbers of pictures are processed, then one can add a few drops of a pH indicator to the stop bath, which changes color when the pH of the solution goes above 5. One then can use the stop bath, as long as the indicator shows that its pH is well below 5.
After the print has been in the stop bath for a few tens of seconds the light may be switched on. The development of the print must be done in almost complete darkness, but once it has been in the stop bath for a while, the developer does not work on it anymore and any new latent image, which is formed due to the renewed exposure does not result in additional visible blackening of the image. The small amount of silver atoms, formed directly by the light is so small that this definitely does not result in any visible change of the image.
The final step in processing the image is removal of any remaining silver halide, which is not reduced, and rinsing away of all chemical residues. This final step, called fixing, is described in the next page.