Amino Acids and EDTA
Zwitterionic Acids
Common Nprotic acids are abbreviated by H_{N}A. In the next higher level of generalization we arrive at zwitterionic acids:
(1a)  Nprotic acid:  H_{N}A^{} 
(1b)  zwitterionic acid:  H_{N}A^{+Z} 
where N is the number of H^{+} in the highest protonated species, and Z is the positive charge of this highest protonated species.
The most simple zwitterions are characterized by N=2 and Z=1; a representative is glycine discussed here. Other examples (compared to carbonic acid) are:
Titration Curves and Buffer Intensities
The mathematical description of buffer capacities and intensities is the same as for common acids (except the nonzero offset Z). In the diagrams below, calculations are presented for four zwitterionic acids from the table above. This is done for two amounts of the acid: (i) C_{T} = 500 mM and (ii) infinite C_{T}. The latter case mimics highconcentrated acids.
The buffer capacity (in blue) represent the socalled titration curves. The buffer intensity β (in green) is the pHderivative of the buffer capacity. In other words, the green curves show the slope of the blue curves. The small circles are the zeros of dβ/dpH, which indicate the extrema of the buffer intensity β and mark the inflection points of the titration curves.
EDTA
Ethylenediaminetetraacetic acid (EDTA) is especially interesting because, in our table, it represents the acid with the highest number of dissociation steps: N = 6 (defined by the six acidity constants).
There are N+1 = 7 acid species, where the highest protonated species is H_{6}A^{+2} and the fully deprotonated species is A^{4}. The diagrams below displays the pH dependence of EDTA’s seven ionization fractions a_{0}, a_{1} to a_{6} based on the formulas given here.
Titration Curves. The analytical formulas are used to calculate the titration curves in the right diagram. This is done for three amounts of C_{T}. The red dots represent numerical calculations with aqion.^{1} [Note: In contrast to the diagrams above, which also contain titration curves of EDTA, the x and y axes are interchanged.]
Remarks & Footnotes

In these calculations complexformation of EDTA are ignored. ↩