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

Re: Eliminating CaCO3 in buffer in "achival" watercolor papers


Acetic acid is rather a poor choice to dissolve calcium
carbonate, despite the high solubility of calcium acetate.
This is because carbonate is still present in the solution and
the dissolution is partly a competition between acetate adn
carbonate. The question is which one binds calcium to make a
more stable form at the given condition. In other words,
acetic acid may break down calcium carbonate momentarily, but
then the free calcium ion is abundantly available to react
with something else, such as carbonate. This is because
calcium acetate is not very stable in water. On the other
hand, if you used EDTA, Ca-EDTA complex is so stable that you
won't see many free calcium ions in the bath and the removed
calcium is unavailable to react with carbonate or other things
that may be present in the bath.

In order to dissolve calcium carbonate effectively, you will
need a chelating agent with a high stability constant with
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)? 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.

I guess a simple way to phrase this is, would chelators in a neutral to alkaline solution de-scale a kettle more or less as quickly as the relatively weak acetic, citric, or phosphoric acid solutions comonly used for this purpose?

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

I seem to recall that the solubility of CaCO3 is strongly dependent on the concentration of dissolved CO2. Perhaps salt-free club soda plus a chelator would work (assuming the chelator doesn't cause all of the CO2 to outgas)?

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

Best regards,