Ammonium Dichromate vs Potasssium Dichromate in Reversal Processing

[artonpaper]

Does any know if ammonium dichromate can be substituted for potassium dichromate when compounding the bleach used in reversal processing of black and white films, such as Tri-X? The formula calls for 12ml 5N Sulfuric Acid and 9.5 grams potassium dichromate and water to make 1 Liter.

 

[jochen]

Hello,
yes, I think this is no problem, the active part ist the chromium(VI) which is the oxidizing agent. Instead of 9.5 g take 8,14 g ammoniumdichromate. In other formulas they take only 5 g potassiumdichromate. and 5 ml conc. sulfuric acid.
I'm sure you know that all chromium(VI) compounds are toxic and carcinogenic. Before you discard the used bleach you can mix it with as much used developer until it turns green (do this in open air as sulfur dioxide is formed by reaction of the sulfuric acid with the sulfite from the developer). The orange chomium(VI) is reduced to the green chromium(III) which is much less toxic and harmful to the environment and is no longer carcinogenic.

 

[artonpaper]

Thanks, this saves me from trying to get the potassium dichromate in a hurry. I'm trying to help out a grad student who is processing Tri-X 16 mm cine film.

 

[Rudeofus]

The trick of b&w reversal bleach is that it only dissolves Silver, but not the Silver Halide. Be aware that Ammonium ion is a weak Silver Halide solvent, so Ammonium Dichromate might yield different results. Since only small amounts of Dichromate are needed it might be a non issue.