Ferrocyanide acid options

Paul Verizzo · May 2, 2015

Is sulfuric acid the only way to make Potassium Ferrocyanide (spelling?) a film bleach?

What about citric acid or muriatic (pool) acid?

Thanks!

Photo Engineer · May 2, 2015

Only one for reversal processing.

Gerald C Koch · May 2, 2015

Ferricyanide bleaches are not made using an acid. A simple solution of potassium ferricyanide will act as a bleach. Even a moderately strong acid like citric acid is capable of releasing deadly hydrogen cyanide from ferricyanide salts. I believe you are thinking of dichromate bleaches which are made with sulfuric acid.

pdeeh · May 3, 2015

I believe you are thinking of dichromate bleaches which are made with sulfuric acid.

or permanganate bleaches, also made with Sufluric (or Sodium sulfate)

nworth · May 3, 2015

I checked the formulas I have for ferricyanide bleaches used for color processing and toning, and none use sulfuric acid. Most do not use acid at all, but a few use a mild acid like boric or acetic. Dichromate bleaches, used in intensifiers and in reversal processing, use sulfuric acid.

Athiril · May 3, 2015

2.5N Sulphuric Acid is used for adjusting ferricyanide bleach pH.
http://motion.kodak.com/motion/uplo...t_en_motion_support_processing_h247_h2407.pdf
Page 7-32

Photo Engineer · May 3, 2015

I have to correct my post. Gerry is right. Don't use acid with Ferricyanide. However the most effective bleach with Ferricyanide uses a Bromide salt.

However, if you are doing a reversal B&W process you must use either Dichromate or Permanganate along with Sulfuric Acid for the bleach to be effective.

PE

Paul Verizzo · May 3, 2015

OK, thanks all. I guess I WAS mixing up the bleach types in my poor, overburdened head.

Gerald C Koch · May 3, 2015

Athiril said:
2.5N Sulphuric Acid is used for adjusting ferricyanide bleach pH.
http://motion.kodak.com/motion/uplo...t_en_motion_support_processing_h247_h2407.pdf
Page 7-32

Technically correct but the pH is maintained at 6.5 which is too high for the ferricyanide ion to be destroyed.

jochen · May 6, 2015

Hello,
the ferricyanide (hexacyanoferrate(III)) acts as an oxidizing agent for metallic silver(0) to silver(I). It is reduced itself to ferrocyanide (hexacyanoferrate(II)). By the potassium bromide KBr in a rather high concentration this is transformed into silverbromide AgBr again (re-halogenation). This can either be redevelopped or toned, e.g. by a thiocarbamide toner or another sulfur toner. During this toning silverbromide is transformed into brown silversulfide AgS. The final effect of this procedure is an oxidation of metallic silver to silversulfide.

Ferrocyanide acid options

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