Amino Acids and EDTA
Zwitterionic Acids
Common N-protic acids are abbreviated by HNA. At the next higher level of generalization, we then come to zwitterionic acids:
| (1a) | N-protic acid: | HNA |
| (1b) | zwitterionic acid: | HNA+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 math description of buffer capacities and intensities is the same as for common acids (except the non-zero offset Z).1 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) CT = 500 mM and (ii) infinite CT. The latter case mimics high-concentrated acids.
The buffer capacity (in blue) represent the so-called titration curves. The buffer intensity β (in green) is the pH-derivative 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 particularly interesting because it represents the acid with the highest number of dissociation steps: N = 6 (defined by the six acidity constants in our table).
There are N+1 = 7 acid species, where the highest protonated species is H6A+2 and the fully deprotonated species is A-4. The two left diagrams below display the pH dependence of EDTA’s seven ionization fractions a0, a1 to a6 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 CT. The blue dots represent numerical calculations with aqion.2 [Note: In contrast to the diagrams above, which also contain titration curves of EDTA, the x and y axes are interchanged.]
Remarks & Footnotes