Chapter 13 EDTA Titrations Lewis acidbase concept Lewis

Chapter 13 EDTA Titrations Lewis acid-base concept Lewis acid : electron pair acceptor metal Lewis base : electron pair donor ligand coordinate covalent bond ligand donates both electrons of the electron pair bond

coordinate covalent bond

Species with two or more nuclei that are good Lewis bases are potential chelate ligands. The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew, who stated: "The adjective chelate, derived from the great claw or chela (Greek: chely) of the lobster or other crustaceans, is suggested for the caliper-like groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings. " This term is generic for all ligands which have at least two atoms coordinated to the central metal ion.

Chelate Effect • the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands • results from the formation of 5 membered "ring" with metal and two atoms on the ligand

Metal ion Monodentate Stability

Metal ion Bidentate Stability

Metal ion Tridentate Stability

Metal ion Tetradentate Stability

What forms of EDTA exist? At Low p. H? Which Functional Groups will be protonated?

EDTA is a Hexaprotic acid (H 6 Y 2+ ) p. K 1 = 0. 0 , p. K 2 = 1. 5 , p. K 3 = 2. 0 p. K 4 = 2. 66 , p. K 5 = 6. 16 , p. K 6 = 10. 24 The amount of ethylenediaminetetraacetate anion, EDTA-4 => Y-4 in solution is very important. WHY? ?

EDTA-4

[Fe(EDTA)(H 2 O]

Let’s take a look at the a plot for EDTA

EDTA M+n + Y-4 ¾ MY (n-4)+ [MY(n-4)+] KMY = -------[M][Y-4] a 4 = ----CT where CT = [Y-4] + [HY-3] + [H 2 Y-2] + [H 3 Y-1] + [H 4 Y]

Minimum p. H for Effective EDTA Titrations

Metal Ion Indicators Erichrome Black T Mg. In + EDTA Mg. EDTA + In (red) (colorless) (blue)