COMPLEXATION TITRATION

• Complexation titration


  A form of volumetric analysis wherein a sample in solution is made to react with a standard solution acting as a ligand until the endpoint is reached
• Complexometric titrations


  Another term for complexation titrations
• Ligand


  A species with at least one unshared pair of electrons available for bond formation (e.g. H2O, NH3, X2, and most metal ions in water)
• Analyte


  A compound containing a polyvalent cation
• Polyvalent cations


  • Form more stable products compared to monovalent cations (Na, K)
  • Better to target polyvalent cations (Ca, Mg, Bi, Al – cations with +2 or +3)
• Complexometric titrations


  Useful for various analyses: Gravimetry, Spectrophotometry, Fluorometry
• Coordination compounds


  The product formed when a cation bonds with an electron donor species based on its coordination number
• Complex


  Produced when a metal ion coordinates with one donor group of a single ligand
• Chelate


  Produced when a metal ion coordinates with two or more donor groups of a single ligand
• Coordination number


  The number of covalent bonds that a cation tends to form
• Visually, you cannot distinguish between complex and chelate
• Masking agents


  Temporarily binds to non-target metals so that the ligand (titrant) will only be able to interact with the target metal
• Control of pH


  Metals are sensitive to pH changes; at certain pH, the metal will not interact with the titrant or inactive
• Standard solutions


  Serve as a ligand that binds the metal cations that are determined in complexometric analysis
• Categorization of ligands


  • By Nature: inorganic (e.g. EDTA) or organic (e.g. 8-hydroxyquinoline)
  • By the number of Electron-pair Donors: Unidentate or Polydentate
• Examples of ligands


  • H2O, NH3, Cl-1, EDTA, NTA, EGTA
• Dentate


  The number of atoms with which a ligand binds to a metal ion
• Unidentate


  Ligands with one donor group (e.g. H2O, NH3)
• Nitrilotriacetic Acid (NTA)


  A tetradentate aminopolycarboxylic acid standard solution, the second most common complexing agent used for titrimetry
• Ethyleneglycol bis(B-aminoethylether)-N,N,N',N'-tetraacetic Acid (EGTA)


  An aminopolycarboxylic acid standard solution and an ether analog of EDTA, selectively titrates Ca in the presence of Mg ions
• Ethylenediamine Tetra Acetic Acid (EDTA)


  A hexadentate aminopolycarboxylic acid standard solution that is very stable in dilute solutions but sensitive to pH and selective with reactivity with metal ions
• ZnSO4


  A standard solution used for residual or indirect titration, specifically used together with dithizone (C13H12N4S) as the indicator
• Auxiliary complexing agents


  Ligands that bind metal ions effectively and prevent their precipitation, but can still be given up when EDTA is added, allowing multiple metals to be titrated in alkaline solution with EDTA
• Masking


  The process of determining the concentration of metals in the presence of another metal
• Different masking agents


  • KCN (Cyanide Ion)
  • Triethanolamine (TEA – C6H15NO3)
  • Thioglycols
  • Ammonium Fluoride (NH4F)
  • Ascorbic acid, Citrates, and Tartrates
• Complexation indicators


  Weak complexing agents that exhibit different colors in the presence of different metal ions
• Masking Agents


  Substances used to prevent interference from other ions during complexation titrations
• Different Masking Agents


  • KCN (Cyanide Ion)
  • Triethanolamine (TEA - C6H15NO3)
  • Thioglycols
  • Ammonium Fluoride (NH4F)
  • Ascorbic acid, Citrates, and Tartrates
• KCN (Cyanide Ion)


  Permits titration of Mg and Ca ions in the presence of Cd, Co, Cu, Ni, Zn, and Pd as these metals will form stable CN-1 complexes
• Triethanolamine (TEA - C6H15NO3)

  Provides sufficient alkalinity to prevent complexation of Al-EDTA in the presence of Mg-EDTA, allowing Mg determination
• Thioglycols


  Allow complexation of Zn at pH 6 by inactivating Hg and Cu which interact with sulfhydryl groups (-SH)
• Ammonium Fluoride (NH4F)


  Masks Ca, Mg, and Al to allow titration of Zn
• Ascorbic acid, Citrates, and Tartrates


  Can be used to mask Sb (antimony) in an analyte solution
• Complexation Indicators


  Weak complexing agents that exhibit different colors in their complexed and free form
• Different Complexation Indicators


  • Eriochrome Black T (EBT)
  • Calmagite
  • Arsenazo I
  • Xylenol Orange
• Eriochrome Black T (EBT)


  One of the oldest and most widely used indicators, exclusive at pH 7-11. EBT is wine red color (complexed) and turns blue color (free) at the endpoint
• Calmagite


  Structurally similar to EBT with better stability in aqueous solutions
• Arsenazo I


  An excellent indicator for EDTA titrations with the rare earth metals, can resist the presence of small amounts of Fe+3 during Ca and Mg titrations
• Xylenol Orange


  Commonly used in a tetrasodium salt form and can be used in acidic solutions. It will appear red (complexed) and turns yellow (free) at the endpoint
• Direct Titration


  1. The complexing agent reacts directly with the sample in which the metal analyte is present

  2. Mostly applied for the determination of Ca, Mg, and Zn