Question

Calculate the value of the conditional formation constant, Ki', for EDTA with the following metal ions. State for each if a titration of that ion at the listed pH would be quantitative or not. (Consider the pH values as exact numbers.) a. K+ at pH 8 Ca2+ at pH 11 Co3+ at pH 1 Cu2+ at pH 4

Answer 1

EDTA titration results in formation of complexes with 1:1 to ration with metal ion:

Mⁿ⁺ + Y^4- → Zn Y^(n-4)

We have,

C_EDTA = [H₆Y²⁺] + [H₅Y⁺] + [H₄Y] + [H₃Y^–] + [H₂Y^2–] + [HY^3–] + [Y^4–]

At any pH a mass balance requires that the total concentration of unbound EDTA equal the combined concentrations of each of its forms.

$\alpha$_Y4– (it is measure of measure of side reaction ), of EDTA present as Y^4–

$\alpha$_Y4–  =   [Y^4–]/  C_EDTA

Kf' = Kf* $\alpha$_Y4–

1. K⁺

$\alpha$Y4– (at pH 8 ) = 5.4* 10–3 ; Kf (K-EDTA)^3- = 10^0.8

Kf' =  5.4* 10^–3 *10^0.8 = 0.034

EDTA titration is not quantitative , as conditional stability constant is very low.

2. Ca²⁺

$\alpha$Y4– (at pH 11 ) = 0.85 ; Kf (Ca-EDTA)^2- = 10^10.67

Kf' = 0.85 *10^10.67 = 3.97*10¹⁰

EDTA titration is quantitative , as conditional stability constant is having higher value.

3. Co³⁺

$\alpha$Y4– (at pH 1 ) = 1.9* 10^–18 ; Kf (Co-EDTA)^- = 10^41.4

Kf' = 1.9* 10^–18 *10^41.4 = 4.8* 10²³

despite very low pH , EDTA titration is quantitative , due to very high value of  conditional stability constant .

4. Cu²⁺

$\alpha$Y4– (at pH 4 ) =3.6 *10^–9 ; Kf (Cu-EDTA)^2- = 10^18.8

Kf' =  3.6 *10^–9 *10^18.8 = 2.27 *10¹⁰

EDTA titration is quantitative , as conditional stability constant is having higher value.

Note : As a general rule of thumb we use 10⁸ Kf 'as a cutoff. Thus Kf’ must be >=10⁸ for an EDTA titration to work.