Problems with ammonium iron citrate in cyanotype

The last batch of green ammonium iron citrate. I have purchased for cyanotype looks fine but behaves oddly. When dissolved the solution looks dark greenish. When mixed with the potassium ferricyanide solution the resulting solution turns to dark greenish with a hint of blue and the sensitized papers also look too dark. When this happens usually it’s said that the chemical is too old and deteriorated . This batch can’t be too old, according to manufacturers records it’s produced six months ago and has been stored properly. When sensitized paper is exposed and processed the prints look almost ok, but in highlights there is a blue fog which can not be removed by washing.

As much as I understand the chemistry behind, light causes an internal redox reaction in ammonium ferric citrate leading to reduction of the iron (ferric to ferro ion). When the reduced iron comes to contact with ferricyanide it forms the Prussian blue, the blue color of cyanotypes. Is this what happens in my case? To test this I added few drops of hypochlorite ( normal laundry whitener ) in the ammonium ferric citrate solution. The color of the solution got light yellow green, the combined sensitizer solution looked more like normal and the highlight fog was clearly reduced.

Does anyone have experience on this kind of problems and what do you think should be done to deal with it?

Thanks in advance,

Olli

Use hydrogen peroxide instead of hypochlorite in order to turn Fe(II) into Fe(III); just mix a slightly stronger than usual batch of AFC, then add OTC hydrogen peroxide solution (3%) until it stops fizzing - I mean if it does so, otherwise add some, test and add some more until it works as you like it. (BTW, the stronger concentration is to counteract the dilution that will be caused by the hydrogen peroxide solution...) Not that I've done that before, but if I were in your place that's how I would do it... (Hydrogen peroxide will not harm the paper as hypochlorite; to my knowing, that will act much harsher on the paper's fibers.)

Hope this helps,
Loris.