Chapter 17: Complexation Reactions & Titrations

CHE 321: Quantitative Chemical AnalysisDr. Jerome Williams, Ph.D.

Saint Leo University

Overview

• Complexation Reactions• Ligands– Classification– Chelates & Macrocycles

• Complexation Equilibria– Complexometric Titrations– EDTA

Complexation Reactions

• Complexation reactions are widely used in analytical chemistry

• Applications include– Spectrophotometric determinations– Gravimetric– Precipitation titrations

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.

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.)

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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).

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Complexation Equilibria

• Consider reaction sequence Cu (II) and NH3

• Cu2+ + NH3 → Cu(NH3) 2+

• Cu2+ + 2 NH3 → Cu(NH3)2 2+

• Cu2+ + 3 NH3 → Cu(NH3)3 2+

• Cu2+ + 4 NH3 → Cu(NH3)4 2+

• Complexations occur stepwise; equilibrium constants are termed formation constants.

Figure 17-1 p406

Table 17-1 p407

Table 17-2 p408

Figure 17-2 p409

Figure 17-3 p410

Figure 17-4 p411

Table 17-3 p414

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

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Figure 17F-1 p416

Figure 17-1a p416

Figure 17-1b p416

Figure 17-1c p416

Figure 17-1d p416

Figure 17-1e p416