Re: acid pre-soak

Before pissing off anyone with little background in this kind of
things, I should say you don't have to worry about it. Just go ahead
with whatever acid that works and doesn't upset your process. What's
discussed here is just a framework for systematic ways to narrow down
to the best candidates from many possible ways to deal with the
problem.

From: Etienne Garbaux <photographeur@softhome.net>
Subject: RE: acid pre-soak
Date: Mon, 15 Aug 2005 21:23:30 -0400

> I do not subscribe to the cult of EDTA (thinking that it is good for
> whatever ails your process).

I didn't know there was a cult for EDTA---I would be more often
anti-EDTA than pro-EDTA, if there's any politics for it. The reason is
simple; there often is a better agent than EDTA for the particular
task being dealt with. It's just that EDTA is famous, cheap, and easy
to try, so many industrial chemists use it when it's good enough. (Of
course, I have nothing against it when it works cheaply.)

> EDTA has a specific function -- to chelate (i.e., complex or bind)
> di- and trivalent metal ions in aqueous solution so that they are
> not available to react with anions. Iron happens to be the most
> strongly chelated metal by EDTA.

Actually valence itsels has little to do with it. This kind of things
are described in a branch of chemistry called "coordination chemistry"
becauyse the underlying mechanism has to do with the electron
configurations of the metal and the chelator, rather than their ionic
states. Ferric ion and EDTA make stable complex because both of them
exhibit strongly ionic nature and they are hexacoordinate. However, so
do alkaline earth metals (Ca, Mg, etc) and they make very stable (and
soluble) complex with EDTA. (That is, disodium EDTA would work
similarly well for the removal of calcium carbonate, _as long as_ the
residual EDTA doesn't upset the process being used.)

On the other hand, ions like ferrous and silver exhibit more
covalent-like (less strongly ionic) coordination with ligands, so they
have poorer binding with strongly ionic ligands like EDTA.

From: Etienne Garbaux <photographeur@softhome.net>
Subject: RE: acid pre-soak
Date: Mon, 15 Aug 2005 21:22:25 -0400

> For maximum solubility of the neutralized calcium (so that it can be
> rinsed away readily), nitric (1 or 2%) and hydrochloric (2%) acids are
> indicated. I have found that citric acid is good enough in this regard,
> although the solubility of calcium citrate is little more than
> "slightly soluble," so it presumably requires a more thorough rinse.

I see where you got that idea. But, the solubility of 1:1 salt of
calcium and citrate is not really relevant here. When immersing the
paper in citric acid bath, CA is available in excess, and calcium is
going to make more stable and more soluble 1:2 or 1:3 complex with the
citrate. And at this point, the solubility is no longer a part of the
equation, but the diffusion rates of the complex ions are...

After all, combined, what you (Etienne) and others described suggests
that the removal of alkalinity is the primary mode of effects, and
removal of iron and/or calcium is unimportant, though, as you said,
they can be removed pretty easily.
Received on Mon Aug 15 21:07:45 2005