Re: I have a question
From: BOB KISS <bobkiss@caribsurf.com> Subject: RE: I have a question Date: Sat, 20 Oct 2007 22:42:59 -0400 > You are the chemistry man so please suggest just one or two > excellent silver bleach alternatives to potassium > ferricyanide that would work and be more environmentally > friendly (as well as regulation free), given standard silver > gelatin processing procedures...yes, thiosulfate fix, > washing, etc. Like I said in the previous email, refer to US Patent 5149618 and look at the compound I. There is only one compound in question in this patent, and it is called nitrilomonopropionic diacetic acid in this patent, although this compound can be called by other names as well. German company BASF owns a patent to manufacture this compound from iminodiacetic acid and acrylic acid. Iron (III) complex of this compound is a good b&w bleach, although this compound is not likely used in practice of color processing as suggested in that patent. (If used in C-41 processing, it causes magenta fog.) Another possibility is iron (III) complex of iminodiacetic acid itself, or alkylated iminodiacetic acid (the latter is more preferred). This was a subject of a Kodak patent. However, these chelating compounds are more irritating to human than nitrilomonopropionic diacetic acid, the subject of the AGFA patent above. In reality, any rehalogenating bleaching agent that can be used in C-41 or RA-4, like iron (III) complex of EDTA or that of DTPA is better than ferricyanide. Most modern C-41 bleach bath uses iron (III) complex of 1,3-propylenediaminetetraacetic acid, and this is a very good b&w bleach agent (although it requires a bit of a trick to make it work cleanly). However, none of these compounds is biodegradable and wouldn't be allowed to dump to the sewer in some parts of Europe (Germany for sure) and some other parts of the world. These compounds are very much usable as b&w bleach in place of rehalogenating ferricyanide bleach. The bath should contain the bleaching agent, potassium bromide, and pH buffer. The amount of bleaching agent and KBr is comparable to the amounts of ferricyanide and KBr used in ferricyanide bleach. None of these compounds is suitable for direct bleach like dichromate or permanganate bleach. > It would be nice if they were readily available chemicals. The compounds named above are all common but not common enough for small photographic chemical retail business like Photographer's Formulary to carry, perhaps except for iron(III) EDTA, which has been used in color processing chemicals for some time. Ideally those companies should be aware of the issue and they ultimately phase out ferricyanide (except for where it is absolutely needed) and introduce better alternatives. -- Ryuji Suzuki http://silvergrain.org
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