Question

EDTA is a hexaprotic system with the p K a values: p K a1 = 0.00 , p K a2 = 1.50 , p K a3 = 2.00 , p K a4 = 2.69 , p K a5 = 6.13 , and p K a6 = 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, Y 4 − . This fraction is designated α Y 4 − . Calculate α Y 4 − at two pH values.

1. pH= 3.20; αY4−=?

2. pH=10.35 αY4−=?

Answer 1

(1.) pH = 3.20, _\Y^4- = 5.97 x 10^-11

(2.) pH = 10.35, _Y^4- = 0.488

Explanation

(1.) pH = 3.20

[H⁺] = 10^-pH

[H⁺] = 10^-3.20

[H⁺] = 6.31 x 10^-4 M

pK_a1 = 0.00

K_a1 = 10^-pK_a1

K_a1 = 10^-0.00

K_a1 = 1

Similarly,

K_a2 = 3.16 x 10^-2

K_a3 = 1.00 x 10^-2

K_a4 = 2.04 x 10^-3

K_a5 = 7.41 x 10^-7

K_a6 = 4.27 x 10^-11