To get back to your original question, here's one answer. If you expose a cyanotype and don't wash it, but dip it first in a solution of tannic acid, the whole sheet will turn black. I've tried this. Judging by this experiment, tannic acid does not discriminate between Fe3+ and Fe2+. It shouldn't matter whether you use citrate or oxalate.
Originally Posted by htmlguru4242
I imagine gallic acid would work the same way, since the distinction between the two acids seems quite vague. Also, judging by the use of tannic acid in conservation as an antirust agent, it would make sense for it to bond with all forms of iron.