In black and white photography, the developed film is fixed by washing with

Answer

Developer is usually a weak reducing agent like potassium ferrous oxalate, an alkaline solution of pyrogallol or an alkaline solution of quinol.
In the process of development of the photographic film, the exposed//activated `AgBr` grain are preferentially reduced by one of the reducing agent described above.
`(##RES_INO_CHM_V01_XII_V02_C01_S01_262_A01##)`
`AgBr+Ag(s)+Br^(-)`
The photographic film is then by washing with hypo solution to remove the unreduced `AgBr` grain from film.
`AgBr+2Na_(2)S_(2)O_(3) to Na_(3)[Ag(S_(2)O_(3))_(2)]+NaBr`.
`S_(2)O_(3)^(2-)+H_(2)O to 2SO_(2)+2H^(+)+4e^(-)`
`S_(2)O_(3)^(2-)+6H^(+) to 2Sdownarrow("white milky")+3H_(2)O`.