Fixing Baths

In addition to the hypo (sodium or ammonium thiosulfate) that dissolves the undeveloped silver halves, most fixing baths contain an acid (such as acetic acid), a preservative ( sodium sulfite), and sometimes a hardening agent (potassium alum). The alum is added to harden the gelatin of the emulsion, preventing excessive swelling in the wash water, especially under warm conditions.
The acid is used to provide the best conditions for efficient curing action, and the sodium sulfite is necessary to prevent the hypo from being decomposed by the acid. Addition of boric acid to such a bath increases the curing power of the alum and also helps to prevent the formation of aluminum sulfite sludge that could form if the stop bath did not neutralize the developer carried by the emulsion.
Time for fixation
The fixing bath should not only dissolve the unused silver halide, but also remove the dissolved silver salts from the emulsion.

• When fixing films and plates, a good rule of thumb is to let the bath soak twice as long as it takes for the milky appearance to disappear, allowing the silver salts to diffuse out of the emulsion.
• For paper prints, the timing is not visible, so the fix time recommended for the paper should be used.
  Excessive fixation times should be avoided as they increase the wash time required to remove the hypo.

During fixation, the fixer must be in contact with the entire surface of the emulsion. Therefore, several films or prints should not be placed in one tray at the same time.

When using the fixer, silver compounds accumulate in the solution:
The bath slows down over time and has greater difficulty removing the last traces of silver salt from the emulsion. If the bath is in use too long, it may lose its ability to remove all unused silver salts.
Although the silver salts are not visible, they remain in the emulsion even after washing and decompose with age, discoloring the negative or print.