Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry233pEDTACa 2+logK Ca(EDTA) 2- = 10.69Ca(EDTA) 2– Mg 2+logK Mg(EDTA) 2- = 8.79moles Ca(EDTA) 2– = 0.060 molmoles Mg 2+ = 0.060 molmoles Mg(EDTA) 2– = 0 molThe metal–ligand ladder diagram in Figure 6.7 uses stepwise formationconstants. We can also construct ladder diagrams using cumulative formationconstants. The first three stepwise formation constants for the reactionof Zn 2+ with NH 32+ 2+Zn ( aq) + NH ( aq) Zn(NH ) ( aq) K = 16 . × 103Mg(EDTA) 2–Figure 6.8 Metal–ligand ladder diagram for Ca(EDTA) 2– and for Mg(EDTA) 2– . The areas with blueshading shows the pEDTA range where the free metal ions are the predominate species; the metal–ligand complexes are the predominate species in the areas shown with pink shading.2+2+Zn(NH ) ( aq) + NH ( aq) Zn(NH ) ( aq) K = 195 . ×10 23 3 3 223122+2+Zn(NH ) ( aq) + NH ( aq) Zn(NH ) ( aq) K = . ×10 23 2233 3 33show that the formation of Zn(NH 3 ) 3 2+ is more favorable than the formationof Zn(NH 3 ) 2+ or Zn(NH 3 ) 2 2+ . For this reason, the equilibrium is bestrepresented by the cumulative formation reaction shown here.2+2+Zn ( aq) + 3NH ( aq) Zn(NH ) ( aq) β = 7.2×103 3 33To see how we incorporate this cumulative formation constant into a ladderdiagram, we begin with the reaction’s equilibrium constant expression.6Because K 3 is greater than K 2 , which isgreater than K 1 , the formation of themetal-ligand complex Zn(NH 3 ) 32+ ismore favorable than the formation of theother metal ligand complexes. For thisreason, at lower values of pNH 3 the concentrationof Zn(NH 3 ) 32+ is larger thanthat for Zn(NH 3 ) 22+ and Zn(NH 3 ) 2+ .The value of b 3 isb 3 = K 1 × K 2 × K 3