Which chemical is used for fixing in radiographic film processing?

• A. Ammonium thiosulphate
• B. Sodium sulfite
• C. Sodium thiosulphate
• D. Hydroquinone

Answer: (C)

Fixing clears the image by removing unexposed silver halide from the emulsion. This is achieved with a thiosulfate-based fixer, which dissolves unexposed silver halide crystals to form a soluble silver–thiosulfate complex that is washed away. Among common fixer formulations, sodium thiosulfate serves as the active fixing agent, making it the best answer. Ammonium thiosulfate would also work as a fixative in some formulations, but the key action is the thiosulfate dissolving unexposed crystals. Hydroquinone is a developing agent, not a fixer, and sodium sulfite mainly acts as a preservative/antioxidant rather than the fixing component.