The chemical used as a fixer/developer in photography is

Sodium thiosulphate (Na₂S₂O–₃), commonly known as 'hypo', is the primary chemical used as a fixer in photography. Its critical role was discovered by John Herschel in 1819, who found that it could dissolve unexposed silver halides from photographic film or paper. During the fixing process, sodium thiosulphate reacts with the residual silver salts to form a soluble thiosulphate complex, which can then be washed away with water. This prevents the remaining light-sensitive silver compounds from reacting further to light, thereby making the photographic image permanent and stable. While modern rapid fixers often use ammonium thiosulphate, sodium thiosulphate remains a standard fixing agent in traditional black-and-white processing. Sodium sulphite is sometimes added to fixing baths, but primarily as a preservative to prevent the decomposition of the thiosulphate.

Sources

1. [1] https://nvlpubs.nist.gov/nistpubs/jres/64C/jresv64Cn1p65_A1b.pdf