1. Chapter 17: Complexation Reactions & Titrations
CHE 321: Quantitative Chemical Analysis
Dr. Jerome Williams, Ph.D.
Saint Leo University

2. Overview Complexation Reactions Ligands Complexation Equilibria
Classification
Chelates & Macrocycles
Complexation Equilibria
Complexometric Titrations
EDTA

3. Complexation Reactions
Complexation reactions are widely used in analytical chemistry
Applications include
Spectrophotometric determinations
Gravimetric
Precipitation titrations

4. Ligands
Ligand - ion or molecule that forms a covalent bond with a cation or neutral metal atom by donating a pair of electrons which are then shared by the two.
Ligands function in the same fashion as a Lewis base (electron pair donor).
Coordination Number – number of covalent bonds a cation forms with electron donors.

5. Ligands: Classification
Unidentate one donor group (electron pair)
Example: Ammonia complexation with Copper (II)
Bidentate two donor groups (electron pair)
Example: Glycine with Copper (II)
Tridentate three donor groups (electron pair)
Tetradentate four donor groups (electron pair)
Example: EDTA
Higher order ligands possible (penta-, hexa-, etc.)

6. p401

7. Ligands: Chelates & Macrocycles
Chelate is produced when metal ion coordinates with two or more donor groups of a single ligand to form a five or six membered heterocyclic ring.
Macrocycles are a type of complex formed between metal ions and cyclic organic compounds (e.g., crown ethers).

8. p401

9. Complexation Equilibria
Consider reaction sequence Cu (II) and NH3
Cu2+ + NH3 → Cu(NH3) 2+
Cu NH3 → Cu(NH3)2 2+
Cu NH3 → Cu(NH3)3 2+
Cu NH3 → Cu(NH3)4 2+
Complexations occur stepwise; equilibrium constants are termed formation constants.

10. Figure 17-1 p406

11. Table 17-1 p407

12. Table 17-2 p408

13. Figure 17-2 p409

14. Figure 17-3 p410

15. Figure 17-4 p411

16. Table 17-3 p414

17. EDTA EDTA Ethylenediaminetetraacetic acid
Most widely used complexometric titrant
Hexadentate ligand; free acid (H4Y) available primary standard
Disodium salt (Na2H2Y) most often used in experimental work. Behaves like an amino acid.
Reagent combines with metals in 1:1 ratio.
Often found in food preservatives & biological samples

18. p414

19. Figure 17F-1 p416

20. Figure 17-1a p416

21. Figure 17-1b p416

22. Figure 17-1c p416

23. Figure 17-1d p416

24. Figure 17-1e p416