Ferrocyanide acid options

[Paul Verizzo]

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]

Only one for reversal processing.

 

[Gerald C Koch]

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]

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]

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]

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]

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]

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

 

[Gerald C Koch]

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]

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.