Editing Acid dissociation constant (section)

=== Isoelectric point ===
{{Main|isoelectric point}}
For substances in solution, the isoelectric point (p''I'') is defined as the pH at which the sum, weighted by charge value, of concentrations of positively charged species is equal to the weighted sum of concentrations of negatively charged species. In the case that there is one species of each type, the isoelectric point can be obtained directly from the p''K'' values. Take the example of [[glycine]], defined as AH. There are two dissociation equilibria to consider.
: <chem>AH2+ <=> AH~+ H+ \qquad [AH][H+] = \mathit{K}_1 [AH2+]</chem>
: <chem>AH <=> A^-~+H+ \qquad [A^- ][H+] = \mathit{K}_2 [AH]</chem>

Substitute the expression for [AH] from the second equation into the first equation
: <chem>[A^- ][H+]^2 = \mathit{K}_1 \mathit{K}_2 [AH2+]</chem>

At the isoelectric point the concentration of the positively charged species, {{chem2|AH2+}}, is equal to the concentration of the negatively charged species, {{chem2|A-}}, so
: <math chem>[\ce{H+}]^2 = K_1 K_2</math>

Therefore, taking [[cologarithm]]s, the pH is given by
: <math>\mathrm{p}I = \frac{\mathrm{p}K_1 + \mathrm{p}K_2}{2}</math>

p''I'' values for amino acids are listed at [[proteinogenic amino acid#Chemical properties|proteinogenic amino acid]]. When more than two charged species are in equilibrium with each other a full speciation calculation may be needed.