UNIT II_ LESSON 3 COMPLEXOMETRIC

• Complexometric titration


  A process in which a metal ions combines with a molecule that can donate an electron and result to a formation of a COMPLEX or CHELATE
• Complexometric titration


  • Useful for the determination of a mixture of a different metal ions in a solution
  • Used in the assay of substance that contains metals
• Example of metals

  • Ca
  • Mg
  • Zn
  • Al
  • Bi
• EDTA


  Ethylenediamine tetraacetic acid (most common electron donor)
• Ligands


  Electron-pair donor and form coordinated covalent bond with Metal ion
• Types of ligands


  • Monodentate (capable of donating ONLY ONE electron)
  • Multidentate (capable of donating ONE OR MORE electrons)
• Multidentate ligands


  • For tetradentate or hexadentate ligands, they are more satisfactory as ligands because they're reactions with cations are more completely or faster reaction because they tend to form 1:1 complexes
• Types of ligands


  • Unidentate (ligands with only one donor atom)
  • Bidentate (ligands with two donor atoms)
  • Tridentate (ligands with three donor atoms per ligand)
  • Hexadentate (ligands which have six donor atoms per ligand)
• Chelating ligands


  Multidentate ligand simultaneously coordinates to a metal ion through more than one site
• Complex


  Product that contains one electron connecting group only (metal + monodentate ligand)
• Chelate


  Products that contains 2 or more electron connecting groups (metal + multidentate ligand)
• EDTA titrations


  A new volumetric procedure for metal determination that is employing metal-ion indicators in the same manner that the pH indicators are used in acid-base titration
• EDTA


  • Capable in reacting with metal ions to form : water-soluble stable complex or chelate compound
  • The reaction is rapid and quantitative with polyvalent ions (Al3+, Bi3+, Ca2+, Hg2+, Mg2+)
  • Relatively weak and unstable complex with monovalent metal ions
  • Ratio: 1:1 (1 compound of EDTA is = to 1 ion of metal hence it produces rapid reactions)
  • Reactivity factor: 1
• EDTA


  • An example of hexaprotic acid (6 protons that could be given up)
  • An hexadentate molecule — it is a multidentate ligand wherein it contains 4 oxygen and 2 nitrogen
• Uses of EDTA


  • Antidote for Lead Poisoning and other heavy metal poisoning
  • Antidote for Digoxin Toxicity
  • Dentistry - as chelating agent for smear layer removal
  • EDTA Chelate therapy - as an aid for the administration of essential nutrients
• Metal-Ion Indicators


  Compounds that change color when bound to a metal ion
• Factors influencing EDTA reactions

  • Activity of the metal ions
  • The pH at which the titration is run
  • The presence of the interfering ions such as cyanide, citrate, tartrate, fluoride, and other complex-forming agents
  • Organic Solvents
  • Neutral Salts
• Disodium EDTA


  Preferred over the 3 acids in preparing standard solution in EDTA because: more water-soluble, non hygroscopic and very stable
• Indicators


  Organic Compounds which form colored complex ions with the metal in high dilution
• Indicator qualities


  • Sharpness of the change in color at endpoint (easy to see or determine the color change)
  • Specificity of the indicator
  • Stability — should be smaller than the metal EDTA complex
• Common metal ion indicators

  • Eriochrome Black-T (EBT)
  • Calmagite
  • Murexide (ammonium purpurate)
• Masking


  Term used to indicate the determination of the metal in the presence of another metal
• Demasking


  The process in which the masked substance regains its ability to enter into a particular reaction

  
• Methods of masking


  • pH changes
  • Masking agents
  • Others (Ascorbic Acids, Citrates, Tartrates)

  
• Ferric Chloride Titration


  • To determine the sodium chloride content of ferruc chloride solution
  • For sodium fluoride determination
  • Indicator: Potassium Thiocyanate
  • Titrant: FeCl3 solution
• EDTA titration techniques


  • Direct titration
  • Back titration
  • Displacement titration
• Summary


  • Volumetric solution: EDTA, Disodium EDTA
  • Primary standard: CaCO3
  • Direct assays: Aluminum & Magnesia Sol'n, Calcium gluconate, Magnesium sulfate, Zinc Oxide
  • Residual assays: Alum, AlOH, Glycobiorsal (Bi content)
  • Indicators: Hydroxynapthol blue, EBT, Dithizone

  
• Biosci (lab) - Prelim