WATER RINSE BATH To  slow  down  the  action  of  development,  you  must immerse the film in a water rinse bath. A plain water rinse  bath  is  commonly  used  between  development  and fixation  to  slow down  the  development  by  removing  all the developer that is clinging to the film (or paper) surface.  A  rinse  bath  does  not  completely  stop development but retards it. A rinse bath has little affect on the developer that is actually in the swollen emulsion. Rinsing is accomplished by quickly immersing the film in plain, clean water. A water rinse bath should be changed often to ensure it does not become loaded with developer. It is better to use running water. The rinse bath, then, serves two purposes: first, it slows down development and second, it reduces the work that has to be done by the acid in the fixer. Rinsing, therefore, protects or prolongs the useful life of the fixer. Following rinsing in plain water, the material (that is still light sensitive) must be treated in an acid fixing bath  to  stop  the  development. ACID STOP BATH Although a plain water rinse bath is commonly used between development and fixation, a better procedure is to use an acid stop bath. The function of a stop bath is not only to remove the developer that is clinging to the surface  of  the  material  but  to  also  neutralize  the developer  in  the  swollen  emulsion  to  stop  development completely. The acid stop bath stops the action of the developer because developing agents cannot work in an acid solution. An acid stop bath also helps protect or prolong the life of the fixer by neutralizing developer carry-over. An  acid  stop  bath  should  meet  the  following requirements: lThe pH must be low enough to neutralize the action  of  the  developer  carried  over. The acidity should be limited so the small amount carried over into the fixing bath does not increase the free-acid   content   of   the   fixing   bath   and   cause sulfurization. It must not contain enough acid to produce blister formations  in  an  emulsion. You should use only a weak acid stop bath between development and fixation. Strong acid and the acid in the fixing bath have a tendency to form carbon dioxide gas bubbles in the emulsion. When the film is taken from the developer and placed directly into a strong acid or fixing bath, these bubbles may break and cause small, round  holes  in  the  emulsion.  These  bubbles  are sometimes mistaken for pinholes like those caused by dust particles settling on the emulsion before camera exposure. When you are using an acid stop bath, remember that some of the stop bath is carried into the fixer when materials pass through it. Therefore, you cannot use a strong acid (such as sulfuric acid) because it can cause precipitation of sulphur in the fixer. Acetic acid is the type of acid used for stop baths. In its pure form as glacial  acetic  acid  (99.5  percent),  it  freezes  at  a temperature of about 61°F. Its freezing tendency gives it the name “glacial.”  For use as a stop bath, 99.5 percent glacial acetic acid is diluted with water to make a 28 percent working solution. Approximately 1/2 ounce of 28 percent acetic (not glacial) acid is added to 32 ounces of water. The process of determining the concentration of a liquid is discussed in chapter 8. FIXING When a light-sensitive material is removed from the developing solution, the emulsion contains a large amount of silver salts (halides) that has not been affected (developed) by the developing agents. This silver salt is still sensitive to light, and if it remains in the emulsion, light ultimately darkens and discolors the salt which obscures the image. Obviously, when this action occurs, the  negative  (or  print)  is  useless. The fixing bath is used to prevent this discoloration and  to  make  the  developed  image  permanent.  It accomplishes this by removing the undeveloped silver halides by making them water soluble. Therefore, to make  an  image  permanent,  you  must  “fix”  the light-sensitive   material   by   removing   all   of   the unaffected  silver  salt  from  the  emulsion. The  fixing  bath  contains  five  basic  ingredients:  the fixing agent, preservative, neutralizer or acidifier, hardening  agent,  and  an  antisludge  agent. Fixing  Agent All fixing baths must contain a silver halide (salt) solvent. This solvent is known as a fixer or fixing agent. The two most commonly used in photography are sodium and ammonium thiosulfate, commonly termed hypo   (taken   from   their   other   chemical   name hyposulfite). Ammonium thiosulfate is used in rapid fixers that are stronger and require less fixing time. 10-5