U of S | Mailing List Archive | alt-photo-process-l | RE: Eliminating CaCO3 in buffer in "archival" watercolor papers

RE: Eliminating CaCO3 in buffer in "archival" watercolor papers

	A late question:  What concentration of Na4 EDTA do you recommend to
remove the CaCO3?

-----Original Message-----
From: Ryuji Suzuki [mailto:rs@silvergrain.org] 
Sent: Wednesday, August 13, 2008 5:22 PM
To: alt-photo-process-l@usask.ca
Subject: Re: Eliminating CaCO3 in buffer in "achival" watercolor papers

From: etienne garbaux <photographeur@nerdshack.com>
Subject: Re: Eliminating CaCO3 in buffer in "achival" watercolor papers
Date: Wed, 13 Aug 2008 14:19:36 -0400

> Do chelating agents actually promote dissolution of solid
> CaCO3 to the point it could be considered "fast" (i.e., fast
> enough to remove substantially all of the CaCO3 from
> buffered paper in several minutes)?

It depends on the agent, concentration, temperature,
agitation, and pH. But I don't see why not.

> The sequestration would reduce the free CA2+ ion
> concentration in the bath, thereby presumably encouraging
> more CaCO3 to dissociate -- but the solubility of CaCO3 is
> so low that I would not expect this to be a very rapid
> process in neutral to alkaline solutions at room
> temperature, even with the help of a chelator removing the
> free Ca2+ ions.

Like I said before, the process is a competition between
chelating agent, carbonate, and water. I'm suggesting to
use a more effective chelating agent to take calcium away from
its competition. Acetic acid is a poor chelator for calcium
(and probably most other metals).

> If not, would acetic acid + an appropriate chelator work
> faster?  (The acid to dissociate the CaCO3, the chelator to
> sequester the Ca2+.)

Like I said before, the stability constant of metal-ligand
combination varies with pH, because the form of anion has a
strong influence on this and the concentration of the
effective spiecies varies with the pH. With most acidic
ligands, you want to use them in the pH range where the
carboxyl groups are dissociated (alkaline pH).

> I seem to recall that the solubility of CaCO3 is strongly
> dependent on the concentration of dissolved CO2.

Dissolved carbon dioxide gas forms carbonic acid. The rest of
the mechanism follows the textbook definition of solubility
constant (ksp).

> Perhaps salt-free club soda plus a chelator would work
> (assuming the chelator doesn't cause all of the CO2 to
> outgas)?

Actually, it works the opposite way. If you add more carbonic
acid, you are driving the equilibrium to form more CaCO3. This
is called Le Chatelier's principle. You want to remove
carbonic acid and/or free calcium ion to drive the equilibrium
to the other way. Chelating agents with high stability
constant can remove Ca++. An acid (other than carbonic acid)
can reduce carbonate from the solution as well, but I bet the
overall removal of CaCO3 can be most effectively carried out
using an effective chelator.

> Just curious about chelation chemistry -- HCl works
> beautifully for my purposes.

I don't doubt that HCl would work. It's just that HCl is not
that pleasant to handle when other safer agents would do.

I expect glycolic acid to be a bit better than acetic acid but
not as good as citric, gluconic, etc... and of course NTA,
EDTA, DTPA, polyphosphates. Agents like NTA, sodium
tripolyphosphate and sodium hexametaphosphate are so effective
in removing calcium scum in alkaline water and they are very
effective in laundry detergents. (However, polyphosphates are
uncommon in modern detergents, despite their very high
effectiveness as a laundry ingredient.)

In standard B&W processing, fixer solution works more or less
in the same mechanism as removal of calcium carbonate. Silver
bromide is a highly insoluble compound, but thiosulfate makes
very stable forms of complex ion with silver, and thereby
facilitates dissolution of silver bromide. There are other
silver chelating agents, such as thiocyanate, thiourea,
cysteine, and other organic thiol compounds, but some of them
damage gelatin, some smell bad, some toxic, some very
expensive, and thiosulfate is about the only practical choice
for the main fixing agent.

Ryuji Suzuki
"Strange how people who suffer together have stronger connections
than people who are most content." (Bob Dylan, Brownsville Girl, 1986)

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