Question

Diprotonated EDTA (ethylenediaminetetraacetic acid) is hexaprotic (H₆Y²⁺) with the following step-wise acid dissociation constants.

K₁  = 1.0

K₂  = 3.2×10^-2

K₃  = 1.0×10^-2

K₄  = 2.0×10^-3

K₅  = 7.4×10^-7

K₆  = 4.3×10^-11

a) Use these values to compute α_Y^4- in a solution at pH 9.26.

b) Now, consider the complex formation reaction:

Sr²⁺ + Y⁴⁻ ⇌ SrY²⁻  logK_f = 8.72

What is the conditional formation constant for SrY²⁻ at pH 9.26?

c) What is the concentration of free Sr²⁺ in 0.050 M Na₂[Sr(EDTA)] at pH 9.26?

Answer 1

ANSWER:

Question a

• The fraction of EDTA tat is founf as Y^-4 is calculated as:

where

• A = K₁ x K₂ x K₃ x K₄ x K₅ x K₆
• B = K₁ x K₂ x K₃ x K₄ x K₅
• C = K₁ x K₂ x K₃ x K₄
• D = K₁ x K₂ x K₃
• E = K₁ x K₂
• Then, we need to determine the values of [H+], A, B, C, D and E

[H+] =	10-pH = 10-9.26 = 5.5x10-10
A =	(1.0) x (3.2x10-2) x (1.0x10-2) x (2.0x10-3​​​​​​​) x (7.4x10-7​​​​​​​) x (4.3x10-11​​​​​​​) = 2.04x10-23
B =	(1.0) x (3.2x10-2) x (1.0x10-2​​​​​​​) x (2.0x10-3​​​​​​​) x (7.4x10-7​​​​​​​) = 4.74x10-13
C =	(1.0) x (3.2x10-2) x (1.0x10-2​​​​​​​) x (2.0x10-3​​​​​​​) = 6.40x10-7
D =	(1.0) x (3.2x10-2) x (1.0x10-2​​​​​​​) = 3.20x10-4
E =	(1.0) x (3.2x10-2) = 3.20x10-2

• Finally, the fraction of Y^-4 at pH 9.26 is

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Question b

• The conditional formation constant (K'_f) is defined as

• For, this reaction the value of K_f is

• And the conditional formation constant at pH 9.26 is

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Question C

• Solution = 0.050 M Na₂[Sr(EDTA)] at pH 9.26 => K'_f =3.81x10⁷
• We need to do an ICE table

	Sr+2	+	EDTA	<----->	SrY-2
Initial	0		0		0.050 M
Change	+ X		+ X		- X
Equilibrium	X		X		0.050 - X

• Using the expression of conditional formation constant

or

As K'f is very large, this means only a very small portion of SrY^-2 will be dissociated to Sr⁺². Then, the value of X will be very small compared to 0.050 M and we can approximate 0.050 M - X to 0.050 M. Then

• The concentration of free Sr²⁺ in 0.050 M Na₂[Sr(EDTA)] at pH 9.26 is

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