Potassium bromide in Farmer's Reducer

[David Lyga]

Why is potassium bromide 'required' in Farmer's reducer? I never use it in the reducer and it works fine. Am I missing something? Is it merely a question of slowing down the process? One can dilute for that; yes? - David Lyga

 

[jochen]

Hello David,
I don't know any formulation for Farmer's reducer which contains potassium bromide (KBr). Farmer has only two ingredients: Sodiumthiosulfate and potassiumhexacyanoferrate(III). The hexacyanoferrate oxidates the elementary silver to silver-ions which form a very stable water soluble silver-thiosulfato-complex which is washed out of the emulsion.
What you mean is probably a bleacher which is used before toning (sulfur or selenium). This bleacher consists of KBr and potassiumhexacyanoferrate(III) and converts silver into unsoluble silverbromide which remains in the emulsion layer and is converted by the toner into silver sulfide or selenide.

 

[Robert Hall]

Agreed. Bromide is for re-halogenation.

 

[Ian Grant]

Some people bleach in a dilute Fericyanide/Bromide solution before a Thiosulphate (Hypo) or fixer bath because until the latter stage it's reversible (after a short wash). It can also be easier to just have one Ferricyanide bleach on the shelf used for reducing and toning.

Ian