U of S | Mailing List Archive | alt-photo-process-l | Solubility Issues with AFO - from Mike Ware

Solubility Issues with AFO - from Mike Ware

Below you can find Mike Ware's explanation / clarification about the
subject recently discussed according K metal salts and AFO sensitizer.

Thank you very much Mike!

Quoting Mike Ware below:

"    Solubility Issues with AFO

The problem to avoid is caused by the low solubility of the salt
potassium ferrioxalate (PFO) (potassium iron(III) oxalate)
K3[Fe(C2O4)3].3H2O, which is given as 6.6 g per 100 cc of water at 15
degrees C (I don't know the value at 20 C). This means the saturated
solution of this salt has a concentration of ca. 0.13 Molar - which is
too dilute to be useful as a sensitizer.

By contrast, the solubility of ammonium ferrioxalate (AFO) is given as
48 g per 100 cc water at 15 C (and is ca. 60 g per 100 cc of solution at
20 C - giving a saturated solution of ca. 1.4 Molar) - which makes a
very good, reproducible 'single coat' sensitizer, with a very long

Now, both these salts ionize completely in aqueous solution, providing
the ferrioxalate anion [Fe(C2O4)3]3-  and the cations K+ or (NH4)+

So if sufficient potassium salt (of any anion) is added to a strongish
solution of AFO, the K+ cations will cause PFO to be precipitated or
tend to crystallise out. At high concentrations, this will happen when
the solutions are mixed, (this is the basis for preparing my new
cyanotype sensitizer, which makes use of this effect to eliminate most
of the potassium ions from potassium ferricyanide, replacing them with

Or, at lower concentrations, crystallization may happen as the mixed
sensitizer dries within the paper - when the crystals of PFO will cause
unwanted dendritic ('fern-like') patterns, which are bad for pictures!

Therefore, as a general rule, all salts containing potassium ions should
be avoided in sensitizer formulations using AFO. This is essential in my
POP Pt/Pd formulation, because the final concentration of AFO in the
mixed sensitizer is high (0.7 Molar). You cannot successfully use
K2PtCl4 or K2PdCl4 as the noble metal salt; the salts I prefer are
(NH4)2PtCl4 and (NH4)2PdCl4 which are highly soluble, and have other
advantages explained on my web pages at:

(NB. One cannot use the contrast control agent disodium
hexachloroplatinate(IV) with this 'ammonium system', because the salt
ammonium hexachloroplatinate(IV) (NH4)2PtCl6 is quite insoluble.)

In my new chrysotype sensitizer formulae, however, the final
concentration of AFO is lower, generally between 0.2 and 0.13 Molar. It
may be possible to "get away with" using the potassium gold salt KAuCl4
at these lower concentrations, especially since it only contains one
potassium ion per mole. I do not yet recommend this, but the advantage
of cost may be overwhelming... so it's worth testing, which I'm about to