The chemical used as a 'fixer' in photography is

1. In black and white photography, after developing the film (which converts exposed silver halide crystals to metallic silver), the image needs to be made permanent and insensitive to further light exposure. 2. This process is called fixing, and the chemical used is called a 'fixer'. 3. The fixer's role is to dissolve the unexposed and undeveloped silver halide crystals from the film emulsion, leaving behind the stable metallic silver image. 4. The most common chemical used as a fixer in traditional photography is sodium thiosulphate (Na₂S₂O₃), often called 'hypo'. 5. Sodium sulphate, ammonium persulphate, and borax have other uses but are not the primary fixing agent in standard photographic processes.