Post on 13-Dec-2015
transcript
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
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
EDTA is a Hexaprotic acid (H6Y2+ )
The amount of ethylenediaminetetraacetate anion, EDTA-4 => Y-4 in solution is very important.
WHY????
pK1 = 0.0 ,pK2 = 1.5 ,pK3 = 2.0pK4 = 2.66 ,pK5 = 6.16 ,pK6 = 10.24
EDTA
[MY(n-4)+]KMY = --------------
[M][Y-4]
[Y-4]4 = ---------
CT
where
CT = [Y-4] + [HY-3] + [H2Y-2] + [H3Y-1] + [H4Y]
M+n + Y-4 ¾ MY (n-4)+