complexometric titration

• Complexometric titration

  Endpoint is the formation of a complex or a chelate
• Complexometric Titration

  Aka: complexometry, chelation ion formation, chelometry, and complex formation
• Complexometric titration


  1. Metal ion combines with a molecule that can donate an electron

  2. Results in the formation of a complex or a chelate
• Ligands


  Electron-pair donors that form coordinated covalent bonds with metal ions
• what are the types/classification of ligands

  • Monodentate
  • Multidentate
• tetradentate / hexadentate ligand

  • More satisfactory as titrants
  • Tend to form 1 to 1 complexes
• Complex


  Product containing 1 electron donating group
• Chelate


  Product containing 2 or more electron connecting groups
• EDTA


  Aka: ethylene diaminetetraacetate/ acetic acid
• Uses of EDTA


  • Antidote for lead poisoning and other heavy metal poisoning
  • Antidote for digoxin toxicity
  • Chelating agent in dentistry
  • Chelate therapy for administration of essential nutrients
• EDTA


  • Reacts with metal ions to form water soluble, stable complexes or chelate compounds
  • Reaction is rapid and quantitative with polyvalent ions
  • Monovalent metal ions yield relatively weak or unstable complexes
  • Ratio of 1 EDTA compound to 1 metal ion
  • Reactivity factor is always 1
• EDTA


  Hexaprotic acid and hexadentate molecule
• EDTA forms strong 1:1 complexes with metal ions
• Metal ion indicators


  • Compounds that change color when bound to a metal ion
  • Must bind metal ions less strongly than EDTA
• Factors influencing EDTA reactions

  • Activity of the metal ion
  • pH of the titration
  • Presence of interfering ions
  • Organic solvents
  • Neutral salts
• Disodium EDTA


  • Preferred over EDTA acids as standard solution
  • More water-soluble, non-hygroscopic and very stable
  • Chemical formula: C10H16N2O8, Molecular weight: 292.24g/mol
• Disodium EDTA


  • Glass-distilled is preferred (metal-free)
  • Best stored in polyethylene or glass containers previously prepared by boiling in alkaline 2% EDTA solution

  
• Indicators


  Organic compounds that form colored complex ions with metals in high dilution
• Qualities of indicators


  • Sharpness of color change at endpoint
  • Specificity for metal ions
  • Stability (indicator stability constant should be smaller than metal-EDTA complex)
• Common metal ion indicators

  • Eriochrome Black-T
  • Calmagite
  • Murexide
• Masking


  • Determination of a metal in the presence of another metal
  • Accomplished by adjusting the pH or using a masking agent
• Demasking


  The process of releasing a masked metal ion
• Auxiliary complexing agent


  Ligand that binds to a metal at high pH to prevent precipitation but is weaker than EDTA
• Methods of masking


  • pH changes
  • Masking agents
  • Other reagents
• Other reagents used for masking

  • Ascorbic acid
  • Citrates
  • Tartrates
• EDTA titration techniques


  • Direct titration
  • Back titration
  • Displacement titration
• Volumetric solutions, primary standards, direct assays, and residual titrations

  • Volumetric solution: EDTA, Disodium EDTA
  • Primary standard: CaCO3
  • Direct assays: Alumina, Magnesia sulphate, Calcium cyanamide, Magnesium Sulfate, Zinc Oxide
  • Residual titrations: Aluminum, AlOH, Glycobiarsol or bismuth content
• Indicators used


  • Hydroxyl-napthal blue
  • EBT
  • Dithiazone
• what are the types of ligands?

  unidentate, bidentate, tridentate, hexa
• what are the types of ligands?

  unidentate, bidentate, tridentate, hexadentate, chelating ligand