EDTA is a hexaprotic system with the following pKa values: pKa1 = 0.00 pKa4 = 2.

Question

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– (see the figure). This fraction is designated Y4–. Calculate Y4– at the following two pH values: Calculate Y4– at the following two pH values PH=3.80 PH=10.65

Explanation / Answer

Y4– = [Y4-] / { [[H6Y2+] [H5Y+] [H4Y] [H3Y-] [H2Y2-] [HY3-] [Y4-] }

or Y4– = [Y4-] /[EDTA]

[EDTA ] is the total concentration of free ions ib the solution

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