Answer:
Fixation involves these chemical reactions (X = halide, typically Br−): AgX + 2 S2O32− → [Ag(S2O3)2]3− + X. AgX + 3 S2O32− → [Ag(S2O3)3]5− + X. In addition to thiosulphate the fixer typically contains mildly acidic compounds to adjust the pH and suppress trace amounts of the developer.