[alt-photo] Re: On working with hydrochloric acid

[Pgf]

While the oxalate salts of calcium and magnesium may not be very soluble in water, the solubility may increase under the acidic conditions of oxalic acid. Often this is the case that an insoluble salt will become quite soluble under excess acid. I can't say if this is the case here, but it is quite possible.

Peter Friedrichsen

On 2013-02-16, at 3:35 PM, etienne garbaux <photographeur at nerdshack.com> wrote:
Tent
> Marek wrote:
> 
>> I have tried oxalic acid with FA hot press and was never 100% happy with my workflow. The hydrochloric acid treated sot press was a wow! moment for me.
> 
> There is a simple explanation for your poor results with oxalic acid.
> 
> Oxalic acid is wonderful for many things, but it is not a good choice for removing the calcium or magnesium carbonate from buffered paper.  Calcium oxalate is even less soluble in water than the calcium carbonate you are trying to remove!  So, to the extent that it reacts with the carbonate, it leaves calcium oxalate in the paper rather than allowing it to wash out (and to the extent that it doesn't react, it leaves the original carbonate in the paper).  Magnesium oxalate is slightly more soluble than magnesium carbonate, but not enough to make any practical difference.  Oxalates are also more than moderately toxic.
> 
> On the other hand, calcium (and magnesium) acetate, chloride, formate, and nitrate are all hundreds to thousands of times more soluble in water than the corresponding carbonates, meaning that acetic, hydrochloric, formic, and nitric acids are all effective in solubilizing the buffering agents in paper, allowing them to be washed out and not leave insoluble products behind.  Citric acid will also work, but may require more time and multiple fresh baths if the buffering agent is calcium carbonate.
> 
> Best regards,
> 
> etienne
> 
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