Question

How do pH and K_f affect the end point of an EDTA titration?

Answer 1

Effect of pH on the composition of EDTA
The strength and stability of EDTA complexes is pH dependent.
The total concentration of EDTA will be the sum of all equilibrium species
CT = [H4Y] + [H3Y-] + [H2Y2-] + [HY3-] + [Y4-]
where CT is the total molar concentration of uncomplexed EDTA.
The fraction of any species can be found if desired where:
ao = [H4Y]/CT
a1 = [H3Y-]/CT
a2 = [H2Y2-]/C
T
a3 = [HY3-]/CT
a4 = [Y4-]/CT
The values may be estimated from a graph or calculated.
The value of aY4- depends only on the concentration of H3O+ and the acid
dissociation constants for EDTA.
Now, assume a divalent metal reacts with EDTA, Y4-, the following equilibrium
will be observed in alkaline medium:
M2+ + Y4- MY2-
Kf = [MY2-]
[M2+] [Y4-]
Y4- concentration is dependent on pH and, therefore, should be calculated as
follows:
a4 = K1K2K3K4
[H3O+]4+K1[H3O+]3+K1K2[H3O+]2+k1K2K3[H3O+]+K1K2K3K4
[Y4-] = a4CT
APCH231: EDTA
Notes complied by Dr. C. Southway
33
eing a weak acid, EDTA dissociates in solution and it is the unprotonated form –
abbreviated as Y4- – that is the species found in the complex. The fraction (aY)
of the total EDTA that is present as Y4- depends on the pH of the solution; as shown
in the table below.
pH aY pH aY
1 1.9x10-18 7 5.0x10-4
2 3.3x10-14 8 5.6x10-3
3 2.6x10-11 9 5.4x10-2
4 3.8x10-9 10 0.36
5 3.7x10-7 11 0.85
6 2.3x10-5 12 0.98
Note that Y4- is present at very low concentrations in acidic solution and does not
become the predominant species until the pH exceeds a value of 10.

Kf

Conditional Formation Constant For a titration reaction to be effective, it must go “to completion” (say, 99.9%),

which means that the equilibrium constant is large—the analyte and titrant are essentially

completely reacted at the equivalence poi