Page 69 - Pharmaceutical Analytical Chemistry II - Pharm D- 02-06-07102

1-3 Bi3+; Fe3+

4-6 Pb2+; Cu2+; Zn2+; Co2+; Ni2+, Mn2+; Fe2+; Al3+; Cd2+; Sn2+

8-10 Ca2+; Sr2+; Ba2+; Mg2+

(b) The effect of other complexing agents

Buffer is usually added during titration with EDTA in order to

maintain constant pH. If one of the buffer components is a complexing

agent (e.g. NH3) it will affect the stability of the EDTA complex because
the metal ion uncombined with EDTA may not be present as simple ion.

EDTA titration curves

In the EDTA titrations, if pM (negative logarithm of the 'free' metal
ion concentration: - log [Mn+]) is plotted against the volume of EDTA

solution added, a point of inflexion occurs at the equivalence point; in

some instances this sudden increase may exceed 10 pM units. The general

shape of titration curves obtained by titrating 10.0 mL of a 0.01 M

solution of a metal ion M with a 0.01 M EDTA solution is shown in the

following figure. The apparent stability constants of various metal-EDTA

complexes are indicated at the extreme right of the curve. It is evident

that the greater the stability constant, the sharper is the end point provided

the pH pM1is6 1016
maintained
14 constant.

12

10 1010

8 106
104
6
4
12 68

2 4 6 8 10 12

0.01M EDTA, mL