It is different from diazo technology, but it seems the general principle of positive-acting chemistry is similar.
In the former, diazonium salt disassociates when exposed. The unexposed molecule then can complex with the coupling agent to give the blue azo dye. In this case, unexposed thionin ferricyanide is insoluble. I could be wrong, but this is the actual molecule that is photosensitive. Most likely (again a conjecture on my part without thoroughly researching, photochemists please weigh in) the EDTA/DEA forms the "sensitizing" dye - to make the formulation sensitive to visible zone of light. On exposure, the thionin ferricyanide decouples to revert to the soluble potassium ferricyanide which is then washed out. Now you are left with the insoluble thionin ferricyanide which then can react with the "developer" ferrous sulfate to make the all-familiar prussian blue.
This is how I understand so far.
:Niranjan.
Well at the bottom of the word document the mechanism is spelled out in quite detail, superseding my simplistic (if not erroneous) early take:
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In the imaging process light causes the dyed photosheet to bleach and momentarily to
produce a colourless compound known as leuco-thionin. This reduced form of the dye
then seeks to return to its non-reduced coloured form by reducing its ionically attached
ferricyanide anion to form thionin ferrocyanide. The proportion of ferricyanide ion
converted to ferrocyanide ion is dependent on the light intensity on that area and also of
course, the exposure time. In the highlight areas of the image, the conversion to
ferrocyanide can approach 100% and in the darkest areas the conversion can be almost
zero. When the exposed sheet is immersed in the acidic ferrous sulphate solution, the
famous Prussian Blue (PB) pigment forms only where thionin ferricyanide still exists. It
does not form when it meets thionine ferrocyanide. Therefore, in the highlight areas
very little Prussian Blue is formed. That therefore makes this a “positive-working”
system. However the ferrous sulphate solution. will also inevitably contain some
oxidised iron in the form of ferric sulphate, from molecular oxygen absorption.
Consequently, when ferric sulphate in solution meets any thionine ferrocyanide,
especially in the highlight areas of the image, it will also form PB. This can lessen the
whiteness of the highlights. But by keeping the solution fresh, the proportion of ferric to
ferrous sulphate will be low and its effect will be very slight. We think that most of the
thionin gets freed to be washed away as its soluble sulphate salt by a combination of PB
getting formed from the ferricyanide ions, and the very insoluble white ferrous
ferrocyanide being formed from the ferrocyanide ions.
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