RE: Solubility Issues with AFO - from Mike Ware

[Eric Neilsen <ejnphoto@sbcglobal.net>]

Loris, That might explain the problems I had with early testing of AFO with
my platinum printing. I did get some ammonium based platinum which worked
great, but was only available from Engelhard on an occasional basis. 

Sandy, I have been using ammonium based palladium for many years. On some
prints it is common to see three or four as color blends. 

Eric


Eric Neilsen Photography
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> -----Original Message-----
> From: Loris Medici [mailto:mail@loris.medici.name]
> Sent: Thursday, November 09, 2006 12:10 AM
> To: alt-photo-process-l@usask.ca
> Subject: 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
> shelf-life.
> 
> Now, both these salts ionize completely in aqueous solution, providing
> the ferrioxalate anion [Fe(C2O4)3]3-  and the cations K+ or (NH4)+
> respectively.
> 
> 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
> ammonium.)
> 
> 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:
> 
> http://www.mikeware.co.uk/mikeware/Ferric_Print-out.html
> 
> (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
> do."