Stop-Baths

The main reasons for using an acidic rinse or stop bath between development and fixation are to better control development by neutralizing the transferred developer and to maintain the acidity and capacity of the fixer. Stop baths are inexpensive and extend the life of the fixer solution.

An acidic stop bath also minimizes the formation of dichroic haze, removes calcium foam that may have formed in the developer, and helps prevent the formation of alum foam and sludge in the fixer.
The use of a hardening stop bath is really only desirable when developing films and plates in very hot weather, since such a bath prevents excessive swelling of the gelatin and protects the softened emulsion.

The use of a stop bath is especially recommended for paper prints, which often tend to stain when transferred directly from developer to fixer without thorough agitation in the fixer bath.