I'd expect that citric acid with calcium carbonate will just become calcium citrate and carbon dioxide. Calcium citrate is moderately soluble in water and I'd expect sufficiently so to mostly wash away. Even if it doesn't, the carbonate is still gone at this point and I don't expect that the lingering citrate will do much harm.
Not a chemist, but my understanding is that many complexes form using buffered papers. May not be as simple as you suggest. My memory is that soluble chemicals do not form since they remain ions in solution. The least soluble chemicals would remain, which is calcium carbonate in the many complex forms that it has. The amount in solution is a little under 0.002 g per liter.
From the cyanomicon II page 215
If buffered papers are unavoidable, such as Fabriano Artistico, or
Whatman Watercolour, the chalk should be destroyed by pre-treated in a
bath of dilute (5% v/v) hydrochloric acid, or 5% - 10% w/v sulphamic acid,
for about 10 minutes, then washed.
page 276
Most contemporary practitioners have been driven
to the last resort of soaking their chalk-buffered papers in dilute acid in
order to destroy the calcium carbonate before coating them with iron(III)
sensitizer. Unfortunately many practitioners think that oxalic acid is also
suitable for this purpose, but calcium oxalate is just as insoluble as
calcium carbonate. Hydrochloric acid can be used, but recently it has
been found very convenient and effective to employ 10% sulphamic acid
for this purpose.677 Nonetheless, it is a tedious and unnecessary
procedure that tends to degrade the paper surface and strength.
Citric acid is mentioned as additive for preventing fogging on p. 216
7.4.6 Citric acid prevents fogging
Chemical fogging (“greening”) of the coating due to paper impurities may
often be prevented by adding citric acid to the sensitizer, before coating,
to a final concentration of
ca. 2%. Add one drop (0.05 cc) of a 40% w/v
solution of citric acid per cc of sensitizer. Do not add citric acid to the
stock of sensitizer, because it will shorten the shelf-life.
The organic acids are not mentioned for use to remove the chalk, although one would think eventually that may work. The low solubility like me mentioned with oxalic is likely part of the problem.
From a more practical viewpoint I can only conform what
@Raghu Kuvempunagar also said, i.e. that citric acid does appear to work to remove or at least very dramatically reduce the alkaline buffer from a paper. I guess ultimately that's what counts.
The test of this is if his prints fade in the next fifty years or so. Ware is adamant about removing all the buffer, even though testing has shown only the buffer contacting the image is relevant.
Whether or not this only happens on the surface, I don't know, but I do understand the logic that we may only have to avoid the carbonate from getting close to the image layer esp. during the time the paper is wet. However, given the marginal solubility of calcium carbonate and the sensitivity of Prussian blue to any amount of carbonate, I would expect that also remaining carbonate deeper in the fiber mat would present problems with bleaching if they remained in the paper.
There would be almost zero carbonate ions in the paper, since as the paper dries they will form stable compounds. In dry paper, the calcium carbonate is locked in position.
And I honestly doubt citric acid couldn't get there while at the same time removing it at the superficial layers of the paper.
Not sure what you mean here. I really hate looking up references in the Cyanomicon. Bottom line si Ware says use muriatic acid of sulfamic acid to remove the chalk. The weaker acids would take much more time. How deeply in the paper do we need to remove or disable chalk is the experiment I will do in the near future.
