There appears to be a lot of confusion re EDTA.

EDTA is the acid itself and it is only slightly soluble in water.
It has four potential sites for attachment.
The salts that are formed e.g. sodium EDTA vary with the pH of the
solution e.g.
mono-sodium, disodium, trisodium, tetrasodium EDTA.
I don't have the exact figures in front of me but mono around 3.65,  and
tetra sodium EDTA around pH 11.
When you prepare a solution for demineralization, it makes sense to use
the disodium and then adjust the pH with HCl or NaOH to 7.2 or whatever.
This solution then contains a mixture of di and trisodium EDTA.
Concentrations cited vary from 5% through 20%. The concentration is not
as important as agitation and/ changing solutions. Considering the cost,
changing the solutions may be the cheapest route. If demineralizing
large blocks then maceration can occur after some months. If there is
danger of this, tissue can be "refixed" and then placed in new EDTA
solution. Using various resins or HCl to speed up the process have not
been shown to be effective. HCl is a poor demineralizing agent  for
blocks of tissue.
Using fixative (e.g. glutaraldehyde) in with the EDTA to prevent
maceration slows down the demineralizing process dramatically.
Can make the solution in phosphate buffer if you wish (this is routinely
used in tissue culture made in PBS).
There is probably no reason that di-potassium EDTA (versene) used some
years ago in blood work could not be used and might be cheaper than
di-sodium EDTA
Barry