Page 171 - Modern Analytical Chemistry

1400-CH06 9/9/99 7:40 AM Page 154

154 Modern Analytical Chemistry

Moles Mg 2+ = 0.060 mol

2–
Moles Mg(EDTA) = 0 mol
Ca 2+

log K f,Ca(EDTA) 2– = 10.69

p EDTA

Ca(EDTA) 2–

Mg 2+

log K f,Mg(EDTA) 2– = 8.79

Mg(EDTA) 2–
Figure 6.7
Ladder diagram for metal–ligand complexes of ethylenediaminetetraacetic acid (EDTA)
2+
with Ca 2+ and Mg .

We can also construct ladder diagrams using cumulative formation constants
in place of stepwise formation constants. The first three stepwise formation con-
2+
stants for the reaction of Zn with NH 3
2+
2+
Zn (aq)+NH 3 (aq) t Zn(NH 3 ) (aq) K 1 = 1.6 ´10 2
2+
2+
Zn(NH 3) (aq)+NH 3(aq) t Zn(NH 3) 2 (aq) K 2 = 1.95 ´10 2
2+
2+
Zn(NH 3) 2 (aq)+NH 3(aq) t Zn(NH 3 ) 3 (aq) K 3 = 2.3 ´10 2
2+
show that the formation of Zn(NH 3 ) 3 is more favorable than the formation of
2+
Zn(NH 3 ) 2+ or Zn(NH 3 ) 2 . The equilibrium, therefore, is best represented by the
cumulative formation reaction
2+
2+
Zn (aq) + 3NH 3 (aq) t Zn(NH 3 ) 3 (aq)
for which
[Zn (NH 3 3 2 + ]
)
72 ´
b 3 = 2 + 3 = . 10 6
[Zn ][NH 3 ]
Taking the log of each side gives
)
[Zn (NH 3 3 2 + ]
3
log b 3 = log 2 + – log [NH 3 ]
[Zn ]