Handout9A-ComplexEquilibria

Handout9A-ComplexEquilibria - CEM 834 II. COMPLEXATION...

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CEM 834 COMPLEXATION EQUILIBRIA FALL 2005 II. Complexation with Secondary Equilibria The complexation equilibria for a metal ion (M) with a ligand (L) is given by 2 n-1 n M L ML ML L ML ML L ML M M M However, many common ligands are the conjugate bases of weak acids. They can undergo acid–base equilibria as follows: + + 2 + n-1 n H L HL H HL H L H H L H L M M M In addition, many of the buffers used to control pH are able to form complexes with the metal ion. The buffer species (A) can undergo complexation equilibria as follows: 2 n-1 n M A MA MA A MA MA A MA M M M These secondary equilibria can have a significant effect on the metal–ligand complexa-tion. As an example of these effects, we shall consider binding of a metal ion with the ligand ethylenediaminetetraacetic acid 2 2 2 2 2 2 ((HOOCCH ) NCH CH N(CH COOH) ). With four carboxyl groups and two amine groups, ethylenediaminetetraacetic acid (abbreviated as H 4 Y) can act as a tetra- to hexa-dentate ligand that forms almost exclusively 1:1 complexes with metal ions z+ 4 z M Y MY  (1) The structure of such complexes is shown in the figure below
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The formation constants f MY (K ) for selected metal ions with ethylenediaminetetraacetic acid are given in the table below Metal Ion log f MY K * Metal Ion log f MY K * Fe 3+ 25.1 Al 3+ 16.1 Th 4+ 23.2 La 3+ 15.4 Hg 2+ 21.8 Fe 2+ 14.3 Ti 3+ 21.3 Mn 2+ 14.0 Cu 2+ 18.8 Ca 2+ 10.7 Ni 2+ 18.6 Mg 2+ 8.7 Pb 2+ 18.0 Sr 2+ 8.6 TiO 2+ 17.3 Ba 2+ 7.8 Zn 2+ 16.5 Ag + 7.3 Cd 2+ 16.5 Na + 1.7 Co 2+ 16.3 *Formation constants in aqueous solution of 0.1 M ionic strength at 20 ºC. 2
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equilibria 4 3 H Y HY f 10 a1 K 1.82 x 10 (2) 2 H HY H Y   f 6 a2 K 1.45 x 10 (3) 2 3 H H Y H Y  f 2 a3 K 4.68 x 10 (4) 3 4 H H Y H Y f 2 a4 K 1.00 x 10 (5) The fraction of ligand Y in the unprotonated form is 4 0 Y 4 2 3 4 Y α Y HY H Y H Y H Y   (6) 0 2 3 4 Y f f f f a1 a2 a3 a4 1 α H β H β H β H (7) and the fractions of ligand Y in the protonated forms are f 1 a1 0 Y Y 2 f 2 a2 0 Y Y 3 f 3 a3 0 Y Y 4 f 4 a4 0 Y Y α β H α α β H α α β H α α β H α The fractions of ethylenediaminetetraacetic acid present in the various forms are shown as
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Handout9A-ComplexEquilibria - CEM 834 II. COMPLEXATION...

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