RE: Pt./Pd. question acid pre-soak

It is really a question of," Why does oxalic acid promote a higher Dmax for
PT/PD prints?" In what conditions? For both FO and AFO, RH factor, Paper
type?

If one is getting a higher dmax, and all other things being equal, than is
it that the oxalic acid is making the ferric oxalate somehow more accessible
to the process of conversion to ferrous and able to create more image?

Sandy, Citric acid is not used in the production of ferric oxalate, at least
not by me nor have I seen it listed in any text. I have however seen oxalic
acid used in production and was recommended in excess to help "preserve"
ferric oxalate and prevent in decay into ferrous. I'd look for an
interaction between the light converting ferric to ferrous and any role that
oxalic acid may have in those changes with the presence of UV light. Does it
hold true for both PT and PD?

I ran test when this was talked about years ago and found little to support
it. I did not run test on a large sample of papers. I also queried those
that made the claim to see if they could also just add some oxalic acid to
their ferric and achieve the same increase in dmax. I got no response.

Or is it simply a pH issue? Years ago Martin Axxon had suggested to me that
he felt a lower pH paper gave a better print with PT solution.

Eric Neilsen Photography
4101 Commerce Street
Suite 9
Dallas, TX 75226
http://e.neilsen.home.att.net
http://ericneilsenphotography.com
 
> -----Original Message-----
> From: Ryuji Suzuki [mailto:rs@silvergrain.org]
> Sent: Monday, August 08, 2005 9:59 PM
> To: alt-photo-process-l@sask.usask.ca
> Subject: Re: Pt./Pd. question acid pre-soak
>
> I don't do platinum/palladium printing so I can't answer the question,
> but if anyone is to do the test, I suggest for the comparison part to
> use unbuffered mineral acid in a controlled amount to maintain the
> target pH, if you want the results to be interpretable. Oxalic acid is
> a dicarboxylic acid, which can be involved in reactions with metal
> ions. In particular, oxalic acid makes a soluble compound with ferric
> ion. Citric acid is more active in similar reactions with a wider
> range of metals, and it's not a good comparison target for this
> purpose. Citric acid having more carboxyl groups as well as having one
> carboxyl group attached to the same carbon as the hydroxyl group makes
> it very reactive with metal ions. Though not ideal, acetic acid is ok
> for the comparison in spite of its carboxyl group.
>
>
> From: Sandy King <sanking@clemson.edu>
> Subject: Pt./Pd. question acid pre-soak
> Date: Mon, 08 Aug 2005 20:37:03 -0400
>
> > As we know a pre-soak in a 1-2% solution of oxalic acid can add quite
> > a bit of Dmax in Pt./Pd. printing with some papers. Does anyone know
> > the mechanism of how this works? Is it merely a question of
> > acidifying the paper? Has anyone ever compared the results between
> > papers pre-soaked in oxalic acid and in some other dilute acid
> > solution, say a 1-2% solution of citric acid?
> >
> >
> >
> >
> > Sandy
> >
Received on Mon Aug 8 23:24:55 2005