# Amino Acids and EDTA

*Zwitterionic Acids*

Common N-protic acids are abbreviated by H_{N}A. At the next higher level of generalization, we then come to zwitterionic acids:

(1a) | N-protic 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 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) C_{T} = 500 mM and (ii) infinite C_{T}. 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 H_{6}A^{+2} and the fully deprotonated species is A^{-4}. The two left diagrams below display 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 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*