Composition of fixer solution:

Fixing solution also contains four components, all dissolved in water:  (1) clearing agent, (2) acidifier,  (3) preservative, and (4) hardener.   Clearing Agent An aqueous solution of ammonium thiosulfate (“hypo”) dissolves the unexposed silver halide grains.   Acidifier The fixing solution contains an acetic acid buffer system (pH 4 to 4.5) to keep the fixer pH constant.   Preservative Ammonium sulfite is the preservative in the fixing solution, as it is in the developer.   Hardener A hardening agent, usually aluminum sulfate, complexes with the gelatin during fixing and prevents damage to the gelatin during subsequent handling.        Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 70