[alt-photo] Re: dilution of pt/pd

[Alberto Novo]

> I had one student who bought some pure ferric oxalate from B & S, good stuff, nothing wrong with it, but it did not produce good platinum prints in the same way as the ferric oxalate as we had made in class without using molar solutions. The problem was solved by adding oxalic acid to the B & S solutions. unsurprisingly,we deduced that the ph of the dilution had to be more acid.  From other experience we also deduced that he it needed to be an organic acid. Why should that be ?

Because:
1) ferric oxalate was once defined as Fe2(Ox)3 (Ox means oxalate for short, 
otherwise it would have been C2O4 from my pointy hat) but it is also defined 
and sold as Fe2(Ox)3.5H2O. It easily reacts with oxalic acid forming 
H3Fe(Ox)3. Usually the formulae refer to Fe2(Ox)3, without the 
crystalization water.
Instead, there is only one iron(III) ammonium oxalate, Fe(NH4)3(Ox)3.5H2O.
If you prepare it from the solution made of iron tri-hydrate (precipitated 
from a iron(III) salt and ammonia) and oxalic acid you certainly need some 
extra oxalic for the complete dissolution of the hydrate, but you also 
exactly know how much iron you have in your solution. 

2) the acid must be an organic, and more precisely a di- or tri- caboxylic 
acid (like oxalate, citrate, tartrate, etc.) because this organic anion is 
needed in order to the reduction of iron(III) to iron(II) could take place. 
If something is reduced, something other has to be oxidized: in this case, 
the organic acid (oxalic becomes CO2, the other a more complex thing). Why 
di- or tri- carboxylic instead of mono (i.e. acetic, formic, etc.), it 
depends on the oxidation potentials involved in the reaction. 

Alberto
www.grupponamias.com
www.alternativephotography.com/articles/art102.html