Cyanotype process

Prussian blue is a mixed Fe(II)/Fe(III) complex polymeric species in which Fe(II) is octahedrally coordinated by C, and Fe(lll) is octahedrally coordinated by N, to give a structure containing Fe(II)-C-N-Fe(lll)-N-C-Fe(II)-linkages, in which the colour originates from electron transfer between the two metal oxidation states. It was discovered in 1704, and used in blueprints and also in the cyanotype photographic process developed by Herschel (see Chap. 11). The cyanotype process is made possible by the photochemical reduction of Fe(III) citrate (or oxalate) to Fe(II), which reacts with ferricyanide present in the coating formulation to give Prussian blue. A similar photoreduction of Fe(lII) oxalate to Fe(ll) is used in the ferrioxalate actinometer (see Chap. 14). 

The Cyanotype process is the most successful of a family of imaging methods based on the photochemistry of iron compounds. The basic system involves the photochemical formation of the pigment Prussian Blue or iron(III) hexacyanoferrate. 

Blueprint Photography by the Cyanotype Process, G. D. Lawrence and S. Fishelson,... 

Enhanced material on blueprinting or cyanotype process with two new problems... 

Lucas AD. 1992. Health hazards associated with the cyanotype Printing process. J Environ Pathol Toxicol Oncol 11(1) 18-20. 

This is not to say that silver printing will disappear. Platinum/palladium printing is still with us, even if Weston did jump ship. As is gum dichromate, cyanotype, albumen, and printing-out paper, among other alternative processes. But is the digital print better than silver Is it as good as platinum Gum In every case, it s not a matter of one being better than the other. It is simply a matter of difference. And as our French counterparts would say, vive la difference ... 

Coordination compounds have a wide variety of applications. Complexes of monodentate ligands are used in qualitative analysis, identification of copper(II) and iron(II), dyes [Prussian (TurnbuU s) blue], the cyanotype printing process, the separation of gold and silver from their ores, the purification of nickel, and as fixers in black-and-white photography. 

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