EDTA is a hexaprotic system with the pKa values:
pkal = 0.00, pKa2 = 1.50, pKa3 = 2.00, pKa4 = 2.69,
pKas 6.13, and pKa6 = 10.37.
The distribution of the various protonated forms of EDTA
will therefore vary with pH. For equilibrium calculations.
involving metal complexes with EDTA, it is convenient to
calculate the fraction of EDTA that is in the completely
unprotonated form, Y4-. This fraction is designated ay.
at two pH values.
Calculate
ays-
OOC-CH₂
N-CH2-CH2-N:
OOC-CH₂
H₂C-COO
H₂C-COO
EDTA (or Y4)

pH = 3.60
ay4- =

PH
= 10.35
ay+- =