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Complexometry

• Complexometry is a volumetric method of analysis
• It is used to determine the concentration of metal ions in a solution.
• The method is based on the formation of a complex between a metal ion and a ligand
• Metal ions act as Lewis acids, and ligands act as Lewis bases
• The metal ion is known as the central metal atom (M).
• The anion or neutral molecule is known as the ligand (L).
• The formation of the complex involves a coordination bond.
• Examples include: [Ag(NH3)2]+, [Fe(SCN)6]3-

Types of Complexing agents (Ligands)

• Unidentate (Monodentate) Ligand:

  • Attaches to the metal at one site, forming one coordinate bond
  • Examples: H2O, NH3, CN-, Cl-, I, Br
  • These ligands can donate one unshared electron pair

• Bidentate Ligand:

  • Attaches to the metal at two sites
  • Examples: Ethylenediamine, forming a Copper amine complex

• Tridentate Ligands:

  • Attaches to the metal at three sites
  • Example: Diethylene triamine

• Tetradentate Ligands:

  • Attaches to the metal at four sites
  • Example: Triethylenetetramine

• Multidentate Ligands (Chelating Agents):

  • Contain two or more donor sites, forming a ring structure with the metal ion
  • Example: EDTA (Ethylenediaminetetraacetic acid).

EDTA (Ethylenediaminetetraacetic acid)

• EDTA forms very stable and soluble complexes with many metal ions (1:1 stoichiometry)
• The disodium salt of EDTA (Na2H2Y. 2H2O) is commonly available.
• Coprecipitation errors are absent because of the solubility of the metal complexes.
• The endpoint of the titration can be easily determined using suitable indicators.
• Selectivity is achieved by adjusting the pH at which the titration is performed.

pH and Complexometry

• Different metal ions form stable complexes at different pH values.
• This allows for separation of metals during analysis.
• Specific pH ranges are used for titrating different groups of metal ions
  • First group (Trivalent and tetravalent cations) titrated at pH 1-3
  • Second group (Divalent metals) titrated at pH 4-6
  • Third group (Alkaline earth metals) titrated at pH 10

Indicators

• Metal-indicator complexes should be less stable than metal-EDTA complexes
• The color change of the indicator signals the endpoint of the titration
• Examples of metal indicators include Eriochrome Black T (EBT) and Murexide.

Applications

• Hardness of water: EDTA is used to determine the concentration of Ca2+ and Mg2+ ions in water, which are responsible for water hardness. It can be used in a titration analysis.
• Back titration: A known excess of EDTA is added, the residual EDTA is titrated against a standard solution, allowing the determination of the concentration of the analyte.
• Replacement titration: A metal ion to be determined is treated with a Mg-EDTA complex, allowing for the determination of the metal concentration

Masking Agents

• Masking agents are added to prevent interference from other metal ions during EDTA titrations.
• Some important masking agents include cyanide (KCN), triethanolamine , and fluoride.
• They react with interfering ions to form a more stable complex, preventing them from reacting with EDTA. This isolates the targeted metal so results are accurate.