RE: acid pre-soak

Sandy wrote:

> Why rinse in water after the acid pre-soak? Most people I know who
> use the oxalic acid pre-soak just pull the print from the acid bath
> and hang it to dry.

First, note that I have tried a number of acids, but not oxalic acid. In
processes that already employ oxalates, some extra oxalate ion may be
benign in a way that extra citrate, phosphate, chlorine, or sulfur ions may
not be. Presumably, it is approximately equivalent to adding a bit of
oxalic acid to the coating solution.

My use of the acid bath is exclusively to neutralize the alkali (generally
calcium carbonate), not to adjust the chemistry of the process itself.
Once that is done, I see no reason to leave the byproducts and excess acid
in the paper.

Speaking of byproducts: calcium oxalate is soluble only in miniscule
amounts (which, incidentally, is why the majority of kidney stones are
composed of it). Thus, when you neutralize calcium carbonate with oxalic
acid, you precipitate basically all of the calcium as calcium oxalate. I
don't know that this is problematic, but doubt that it provides any
benefit. Just one more contaminant (where contaminant is defined as
something other than paper fiber (cellulose) or platinum metal (in the case
of Pt prints)). Note that because of its low solubility, the calcium
oxalate probably wouldn't be removed by a water wash, which is why I've
never tried oxalic acid. Calcium citrate, calcium nitrate, and calcium
chloride are more soluble -- thus, using citric, nitric, or hydrochloric
acid avoids this problem.

Best regards,

etienne
Received on Mon Aug 15 17:34:40 2005