Cyano Complexes Hexacyanoferrates

Bleaches containing hexacyanoferrates (very stable complexes of iron and cyanide) are used in only a few photographic processes. These bleaches contain both ferri- and ferrocyanide, but the action of hypo and other chemicals in the effluent reduces most of the ferricyanide to ferrocyanide. A "total cyanide" analysis measures both of these ions, but they should not be confused with simple (free) cyanides. Unlike free cyanide, hexacyanoferrates have a low level of toxicity and are used in many common human applications, such as cosmetics (blue eyeshadow), paints, fabric pigments, and laundry bluing [5,6]. Cyano complexes are not typically found in black and white processing effluents (see subsequent section, "Ferrocyanide Precipitation").

It is possible that, during the chemical analysis for total cyanide, low levels of cyanide might be detected, due to a side reaction between any hydroxylamine and formaldehyde that might be present [13]. This reaction does not occur in the natural environment.

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