Question

Calculate Fe³⁺ in 0.10M FeY^- at pH 2.0 and 8.0

Answer 1

EDTA complexes

Fe+3 + Y-4   -----> FeY-

Kf = [FeY-] / [Y-4] [Fe+3]

Kf = Foramtion constant for FeY- = 1.3*10^25

At pH = 2 ; aY-4 = 2.6*10^-14 ( aY-4 = fraction of EDTA in the form of Y-4 )

Kf1 = Kf * aY-4   = 1.3*10^25 * 2.6*10^-14 = 3.38*10^11

3.38*10^11 = [FeY-] / [Fe+3] [Y-4]

Let the concentration of Fe+3 and Y-4 is x

3.38*10^11 = [0.1] / x*x

x = 5.44*10^-7 M

[Fe+3] = 5.44*10^-7 M

At pH = 8 ; aY-4 = 4.2*10^-3

Kf1 = Kf * aY-4 = 1.3*10^25 *4.2*10^-3 = 5.46*10^22

5.46*10^22 = [FeY-] / [Fe+3] [Y-4]

Let the concentration of Fe+3 and Y-4 is x

5.46*10^22 = [0.1] / x*x

x = 1.35*10^-12 M

[Fe+3] = 1.35*10^-12 M