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EDTA is a hexaprotic system with the following pKa values: pKa1 = 0.00 pKa4 = 2.69...

EDTA is a hexaprotic system with the following pKa values: pKa1 = 0.00 pKa4 = 2.69 pKa2 = 1.50 pKa5 = 6.13 pKa3 = 2.00 pKa6 = 10.37 The distribution of 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, Y4– . This fraction is designated αY4–. Calculate αY4– at the following two pH values:

pH=3.45

pH=10.85

Expert Solution

[alphaY4-] = K1K2K3K4K5K6/[H+]^6+K1[H+]^5+K1K2[H+]^4+K1K2K3[H+]^3+K1K2K3K4[H+]^2+K1K2K3K4K5[H+]+K1K2K3K4K5K6

(a) pH = 3.45

[H+] = 3.55 x 10^-4 M

K1 = 1 ; K2 = 0.0316 ; K3 = 0.01 ; K4 = 2.042 x 10^-3 M ; K5 = 7.413 x 10^-7

K6 = 4.266 x 10^-11

Feed values we get,

[alphaY4-] = 2.0406 x 10^-23/2.002 x 10^-21+5.64 x 10^-18+5.02 x 10^-16+1.414 x 10^-14+8.132 x 10^-14+1.70 x 10^-16+2.0406 x 10^-23

= 2.12 x 10^-10

(b) pH = 10.85

[H+] = 1.41 x 10^-11 M

K1 = 1 ; K2 = 0.0316 ; K3 = 0.01 ; K4 = 2.042 x 10^-3 M ; K5 = 7.413 x 10^-7

K6 = 4.266 x 10^-11

Feed values we get,

[alphaY4-] = 7.87 x 10^-66+5.58 x 10^-55+1.25 x 10^-45+8.87 x 10^-37+1.28 x 10^-28+6.76 x 10^-24+2.044 x 10^-23

= 0.7514

queen_honey_blossom answered 4 months ago

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