# fixer

In analog photography, a fixing agent is a chemical that makes the exposed film material insensitive to light. A bath in this chemical changes the chemical structure of the film in such a way that the light-sensitive substances in the film material are converted into light-insensitive substances. This then enables the further processing and subsequent storage of the negatives created from the film material .

The fixative for a film nowadays is a thiosulfate solution. This takes advantage of the fact that the thiosulfate ion (S 2 O 3 2− ) forms a more stable complex with the silver halides. During the fixation, the silver bromide present as a solid is converted into water-soluble compounds via several intermediate steps:

(1) ${\ displaystyle \ mathrm {2 \, AgBr _ {\ left (s \ right)} \, + \, Na_ {2} S_ {2} O_ {3 \ left (aq. \ right)}} \, \ rightleftharpoons \ , \ mathrm {Ag_ {2} S_ {2} O_ {3} \ downarrow \, + \, 2 \ NaBr _ {\ left (aq \ right)}}}$

(2) ${\ displaystyle \ mathrm {Ag_ {2} S_ {2} O_ {3} \, + \, Na_ {2} S_ {2} O_ {3} \,} \ rightleftharpoons \ mathrm {\, 2 \, NaAgS_ { 2} O_ {3} \ downarrow}}$

(3) ${\ displaystyle \ mathrm {NaAgS_ {2} O_ {3} + Na_ {2} S_ {2} O_ {3}} \ rightleftharpoons \ mathrm {Na_ {3} \ left [Ag \ left (S_ {2} O_ { 3} \ right) _ {2} \ right]}}$

(4) ${\ displaystyle \ mathrm {Na_ {3} \ left [Ag \ left (S_ {2} O_ {3} \ right) _ {2} \ right] \, + \, Na_ {2} S_ {2} O_ { 3}} \ rightleftharpoons \ mathrm {Na_ {5} \ left [Ag \ left (S_ {2} O_ {3} \ right) _ {3} \ right]}}$

The dithiosulfatoargentate (I) complex that is now in the solution is simply washed out and the film is thus fixed, since no more silver halides can be converted to silver when exposed to light, since there are no longer any silver halides.

In the past, ammonia (NH 3 ) was used as a fixing agent , the effect of which was discovered by Carl Wilhelm Scheele in 1777 . Ammonia forms a similarly stable complex: [Ag (NH 3 ) 2 ] X - the diammine silver (I) halide. The X stands for the halide anions (Cl - , Br - , I - ).