Sodium   and   ammonium   thiosulfate   changes undeveloped silver halide to soluble silver sodium thiosulfate.  It  removes  this  compound  from  the emulsion  and  refills  the  space  it  occupied  with nonexhausted  fixing  solution.  Therefore,  the  function  of the fixing agent is to convert the silver salts remaining in the emulsion after development to soluble compounds and to remove these soluble compounds by constantly diluting and replacing them in the emulsion. The number of substances capable of functioning as fixing agents is small because a good fixer must meet the following requirements: It must dissolve silver salts without affecting the metallic  silver  image. The compounds it forms must be soluble so they can be removed from the emulsion. The fixer should neither swell excessively nor soften  the  gelatin. Preservative A preservative prevents oxidation of the developing agents that are carried over into the fixing bath by the film.  It  also  prevents  decomposition  of  the  fixer. Oxidized  developer  in  a  fixing  bath  produces  stains. Strong acids may cause a fixing agent to decompose (sulfurize).  You  must  add  preservative  (sodium  sulfite) in the fixer to prevent sulfurization. The preservative prevents  the  acid  from  decomposing  hypo  into  free sulfur,  prevents  discoloration  of  the  solution  because  of oxidation,  and  aids  in  preventing  stains. Neutralizer After  development,  the  pores  of  the  swollen emulsion retain a portion of the developer. If allowed to remain,  the  developer  continues  its  activity.  Even  when the emulsion is thoroughly rinsed in a water bath before being placed in the fixer, some developer activity remains.  This  causes  uneven  stains  in  the  gelatin  of  the emulsion and makes the negative unusable. To stop development and prevent stains, you must add an acid neutralizer to the fixer. The most frequently used neutralizer  is  acetic  acid. Hardening  Agent During development, the gelatin becomes softened and  swells.  Frilling,  reticulation,  scratches,  and  other undesirable effects may result when processing is continued without hardening the emulsion. A hardening agent is included in the fixer to harden the emulsion in the fixing bath. The most common hardening agent is potassium alum. The hardening and toughening of the gelatin by the alum stops the tendency of the emulsion to swell but leaves it expanded and rigid enough for the washing  process. Antisludge  Agent The pH range of the fixer is limited. It must be low enough to neutralize the activity of the developer and also be high enough to prevent sulfurization. The reduced acidity of the bath is gradually neutralized by the alkali of the developer carried into the fixer by the film. When the active acidity is neutralized too far, a sludge of aluminum sulfite forms that can make the fixer useless. An antisludge agent, such as boric acid, is added to the fixer. This agent is capable of absorbing a large quantity of the developer before sludge occurs, thus lengthening the useful life of the fixer. Time  Required  for  Fixing The time required for film to fix depends on several factors: The type of emulsion and its thickness. All else being  equal.  fine-grain  emulsions  fix  faster  than coarse-grained ones. Thin emulsions require less time to fix than thick emulsions. The type of fixing bath and degree of exhaustion. When  sodium  thiosulfate  is  the  fixing  agent,  a concentration of about 75 percent gives the fastest rate of fixation. However, because of the tendency of hypo to  bleach  out  the  image,  most  fixers  for  negatives  have a concentration between 20 and 40 percent. The fixing bath temperature. An increase in the temperature increases the rate of fixation. (Do not interpret this to mean that you can raise the temperature of the fixer above the temperature called for by the particular process being used.) The temperature of the fixer is not as critical as the temperature of the developer. However, you should keep all processing solution temperatures constant to avoid an increase of graininess. The amount of agitation. The rate of fixation is affected  by  diffusion  of  the  chemicals,  so  agitation reduces  fixation  time. The  amount  of  exposure.  The  more  exposure  the film has to light, the less unused silver halide to be removed by the fixer, and hence the faster the rate of fixation. 10-6