QC Lesson 4

• Precipitimetry


  Precipitation Titrimetric Analysis - Based on the formation of relatively insoluble substances (precipitates) to cause the reaction to go to completion quantitatively
• Silver Nitrate


  • Most widely used precipitating agent/ standard solution
  • Used for determination of halides
  • ARGENTOMETRY if the titrant used is AgNO3 standard solution
• Precipitimetry


  1. Endpoint maybe determined by: Cessation of precipitation or appearance of turbidity

  2. Use of internal indicators

  3. Instrumental methods (potentiometry/amperometry)
• Indicators Used

  • Ferric Ammonium Sulfate (ferric alum)
  • Potassium Chromate
• Adsorption Indicators


  • Examples: Dichlorofluorescein (DCF), Eosin Y, Tetrabromophenolphthalein ethyl ester (TEE)
  • Used in analysis of halides by direct titration w/ AgNO3 as standard solution
  • Are weak acids
  • Endpoint best seen in diffused-light condition
• Fajans Method


  • Direct titration using AgNO3 as titrant
  • Uses an adsorption indicator, an organic compound that adsorbs onto or desorbs from the surface of the solid
  • Rapid, accurate, and reliable
  • Standardization of Silver nitrate is an example of Fajans titration
• Volhard Method


  • A form of residual or back-titration
  • Complete precipitation of insoluble silver salts with the addition of excess silver nitrate to precipitate
  • Followed by titration of the unreacted silver nitrate with ammonium thiocyanate
  • Ferric alum as indicator (reddish-brown endpoint)
• Mohr Method


  • Potassium chromate serves as the indicator for the argentometric titration of neutral halides
  • 2Ag+ + CrO4-2 → Ag2CrO4 (brick red)
  • Rarely used because Cr(VI) is a carcinogen
• Methods

  • Fajans (Direct, Adsorption Indicators, AgNO3 as titrant, Colored ppt)
  • Volhard (Residual, Ferric Ammonium Sulfate (ferric alum), NH4SCN, Reddish brown complex due to FeSCN2+)
  • Mohr (Direct, Potassium Chromate, Seldom used-carcinogenic, Red ppt)
• Assays


  • Assay of Phenylmercuric Nitrate for Mercury Content (Fajans, C5H5Hg, 0.1N NH4SCN, Ferric Alum TS)
  • Assay of Iopanoic Acid Tablets (Fajans, C11H12I3NO2, 0.05N AgNO3, TEE)
  • Assay of Benzyltrimethyammonium Chloride (Fajans, C6H5CH2N(CH3)3Cl, 0.1 N AgNO3, DFC)
  • Assay of Sodium Chloride (Volhard, NaCl, 0.1 N AgNO3, 0.1N NH4SCN, Ferric Alum TS)
  • Assay of Iodide Content in Povidone Iodine (Volhard, Iodide, 0.1 N AgNO3, 0.1N NH4SCN, Ferric Alum TS)
  • Assay of Sodium Lauryl Sulfate for Sodium Chloride Content (Mohr, NaCl, 0.1 N AgNO3, K2CrO4 TS)
• Sodium Tetraphenylboron Titrations


  • Uses Na(C6H5)4B to precipitate organic nitrogen compounds (alkaloids, amines and quaternary salts), ammonium, potassium, and silver ions
  • Uses chloroform as extraction indicator, Endpoint: Colorless
• Complexometry


  Metal ion reacts with a suitable ligand to form a complex, and the end point is determined by an indicator or an appropriate instrumental method
• EDTA


  • Most widely used titrant
  • Available as dihydrate of sodium salt
  • A hexadentate ligand
  • Commonly used in the concentration of 0.05 M
• EDTA Titrations

  • Forms 1:1 ratio with metals, regardless of charge
  • Applied to the determination of virtually every metal cation except alkali metals
• MASKING


  • Control over interferences can be done by MASKING
  • pH regulation
  • Addition of masking agent (an auxiliary ligand that preferentially forms highly stable complexes with the potential interfering ion)
• Auxiliary Complexing agents (Masking Agents)

  • Triethanolamine - suppress Al EDTA complex in the presence of a Mg EDTA complex
  • Thioglycols - inactivate metals such as Hg & Cu
  • Ammonium Fluoride - mask Ca, Mg and Al to allow titration of Zn
  • Others: Ascorbic acid, citrates and tartrates
• EDTA Titrations - Indicators

  • HNB, eriochrome black T, azo dyes, phthaleins
  • Qualities of good quality indicators: sharpness of color change at the end point, stability of the indicator for the metal ion, Stability constant smaller than that of the metal – EDTA complex
• EDTA Titrations - Types

  • Direct (Ca+2, Mg+2, Zn+2)
  • Residual (For Aluminum and Bismuth compounds, Back-titrant: Zinc sulfate, Indicator: Dithiozone, Solvent: Organic solvent, alcohol)
• FERRIC CHLORIDE TITRATION

  • For sodium fluoride determination
  • Titrant: Ferric chloride TS
  • Indicator: Potassium thiocyanate
  • End point: red color
• Water Hardness


  • The capacity of cations in water to form sparingly soluble products that cause "scum" in the sink or bathtub
  • Expressed in terms of CaCO3 or Ca content
  • Types: Temporary (due to presence of bicarbonates), Permanent (due to presence of chlorides and sulfates)
• Atoms and Ions
• The 3 subatomic particles are proton, neutron, and electron
• The charge of an electron is negative
• A positively charged ion is called a cation
• Cations are formed due to loss of electrons, becoming positively charged
• The net charge of an atom is generally neutral/zero
• Anion


  Negative charged ion
• Cation


  Positively charged ion
• Gegenion


  A counterion
• Zwitterion


  A bipolar ion
• Cations are formed


  Loss of electrons, becoming positively charged
• Anions are formed


  Gain of electrons, becoming negatively charged
• Atom


  • Composed of proton (+), neutron, electron (-)
• Oxidation-Reduction (Redox) Reactions


  Electrons are transferred from one reactant to another
• Valence Increase


  Loss of Electrons
• Valence Decrease


  Gain of Electrons
• Reduction


  Valence decrease, gain of electrons, oxidizing agent
• Oxidation


  Valence increase, loss of electrons, reducing agent
• Oxidation-Reduction (Redox) Reactions involve a change in valence of the reactants
• Atoms in elemental form have zero valence