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.
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Chapter 13 EDTA Titrations
Lewis acid-base concept
Lewis acid :electron pair acceptor
Lewis base : electron pair donor
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.
At Low pH? Which Functional Groups will be protonated?
EDTA is a Hexaprotic acid (H6Y2+ )
pK1 = 0.0 ,pK2 = 1.5 ,pK3 = 2.0
pK4 = 2.66 ,pK5 = 6.16 ,pK6 = 10.24
The amount of ethylenediaminetetraacetate anion, EDTA-4 => Y-4 in solution is very important.
M+n + Y-4¾ MY (n-4)+
KMY = --------------
a4 = ---------
CT = [Y-4] + [HY-3] + [H2Y-2] + [H3Y-1] + [H4Y]
Minimum pH for Effective EDTA Titrations
Erichrome Black T
MgIn + EDTA MgEDTA + In
(red) (colorless) (blue)