Complexometry in Quantitative Analysis

Complexometry

• A type of quantitative analysis where a compound forms a slightly ionized complex with a metal ion (Mn+) and an anion or neutral molecule (L)
• The metal ion is the central metal atom, acting as a Lewis acid (electron acceptor), while the anion or neutral molecule is the ligand (L), acting as a Lewis base (electron donor)
• The bond between Mn+ and L is a coordinate bond or dative bond

Coordinate Bond

• Similar to a covalent bond, but with the electrons donated from one atom to another
• Represented by an arrow (→) from the donor to the acceptor

Coordination Number

• The number of coordinate bonds formed between the central metal and ligand ion
• Characteristics:
  • Always an even number (2, 4, 6, ...)
  • Usually double the charge of the metal ion (e.g., Ag+ forms 2 coordinate bonds, Ni2+, Cu2+ form 4 coordinate bonds)
  • Higher valence metal ions form more stable complexes

Ligands

• Unidentate Ligands: attached to the metal at one site (e.g., I-, CN-, F-, H2O, NH3)
• Polydentate Ligands:
  • Bidentate Ligands: attached to the metal at two sites (e.g., ethylene diamine)
  • Tridentate Ligands: attached to the metal at three sites (e.g., diethylene triamine)
  • Tetradentate Ligands: attached to the metal at four sites (e.g., triethylene tetramine)

Chelation

• A type of complexation where a ligand possesses two or more donor groups (polydentate)
• Forms a ring structure compound called a metal chelate
• Polydentate ligands are preferred as titrants due to their advantages:
  • Formation of more stable complexes due to multiple points of attachment
  • Complete reaction in one step, leading to a sharp end point

Factors Affecting Stability of the Complex

Metal Factors

• Type of metal: metals with incomplete subshells (e.g., Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+) are strongest in attracting ligands and forming stable complexes
• Ionic radius: smaller ionic radius increases the stability of the complex
• Electric charge: higher electric charge carried by the metal ion produces stable complexes (e.g., Fe3+ > Fe2+)

Ligand Factors

• Basic character (ability of electron donation) of the ligand increases the stability of the complex
• Ionic radius: larger ionic radius of the ligand increases the stability of the complex

EDTA (Ethylene Diamine Tetracetic Acid)

• An amino polycarboxylic acid compound used as a ligand, complexing agent, or titrant for many metal ions
• Hexadentate ligand containing 4 oxygen and 2 nitrogen donors
• Reacts with metal ions in a 1:1 ratio
• Disodium salt of EDTA is freely soluble in water
• Non-selective chelating agent
• Stability of metal-EDTA complex affected by pH:
  • Decreases in acidic medium
  • Increases in slightly alkaline solution
  • Metal precipitates as hydroxide in strong alkaline medium

Detection of End Point

• Methods:
  • Metal indicators (metallochromic indicators)
  • Acid-base indicators
  • Specific indicators
  • Turbidity indicators
  • Instrumental methods

Metal Indicators (metallochromic indicators)

• Organic dyes that form colored complexes with metal ions
• Characteristics:
  • Complexation reaction between metal ions and indicators is reversible
  • Color of free form differs from the color of the complexed form
  • Stability of metal-indicator complex is lower than metal-EDTA complex
• Examples:
  • Murexide
  • Eriochrome Black T (EBT)

Complexometry is a type of quantitative analysis where a metal ion reacts with an anion or neutral molecule to form a complex. Learn about the role of metal ions, Ligands, and Lewis acids and bases in this process.