Instant Notes: Analytical Chemistry

M +nL =MLn

bn =c (MLn)/(c (M). c (L) n)

The overall formation constant is the product of the stepwise formation

constants:

bn =Kf1. Kf2....Kfn or log bn =log Kf1+log Kf2+...+log Kfn

When a ligand is used that can bond to several sites, it is referred to as a multi-

dentate ligand. One of the most important examples is ethylenediamine

tetracetic acid (EDTA):

(HOOCCH 2 ) 2 -N-CH 2 -CH 2 -N- (CH 2 COOH) 2

This tetrabasic acid, abbreviated to H 4 Y, has four acetate group and two nitro-

gens, which may complex to the central metal ion, as shown in Figure 1.It is

important to remember that for a satisfactory titration, the equilibrium constant

K of the reaction must be greater than 1000.

Since the concentrations of the various species containing Y (H 4 Y, H 3 Y-, H 2 Y^2 -

, HY^3 - , Y^4 - ) will vary with the pH, a formation constant KMYmay be written:

Mn++Y^4 - =MY(n-4)+

KMY=c(MY(n-4)+)/(c(Mn+).c(Y^4 - ))

and c(Y^4 - ) =cL¥ a 4

where a 4 depends on the pH and the acid dissociation constants (K 1 , K 2 , K 3 , K 4 )

of EDTA, as shown in Table 1, and cLis the total concentration of all the ligand

species.

86 Section C – Analytical reactions in solution

OC

C

CH 2

CH 2

CH 2

H 2 CH 2

C

C

N

C

N

C

H 2

O

O

OO

O

O

O

M

2 –

Fig. 1. The structure of a metal-EDTA chelate showing its octahedral geometry.

Table 1. Values of a 4 as a function of pH

pH a 4 pH a 4

2 3.7¥ 10 -^148 5.4¥ 10 -^3

3 2.5¥ 10 -^119 5.2¥ 10 -^2

4 3.6¥ 10 -^910 3.5¥ 10 -^1

5 3.5¥ 10 -^711 8.5¥ 10 -^1

6 2.2¥ 10 -^512 9.8¥ 10 -^1

7 4.8¥ 10 -^4