QC Lab

• Precipitimetry


  A class of reactions that require the formation of relatively insoluble substances or precipitates to cause the reaction to go to sufficient completion to be quantitative in nature
• Standard Solutions


  • 0.1 N Silver Nitrate (Argentimetric analysis)
  • 0.1 N Ammonium thiocyanate
• Indicators used in the official volumetric precipitation assays

  • Ferric ammonium sulfate TS
  • Potassium chromate TS
  • Dichlorofluorescein (DCF) TS
  • Eosin Y TS (Tetrabromofluorescein)
  • Tetrabromophenolpthalein ethyl ester (TEE) TS
• Ferric ammonium sulfate TS


  Used in both direct and residual titrations employing standard ammonium thiocyanate solution. The thiocyanate reacts with silver or mercuric ions present to form a white precipitate of silver or mercuric thiocyanate but as soon as all the silver or mercury has been precipitated, the thiocyanate ion reacts with ferric ammonium sulfate to form a red ferric thiocyanate. Red color = first appearance marks the end point of the reaction.
• Potassium chromate TS


  Forms a red precipitate of silver chromate against the white background of white silver chloride
• Adsorption indicators


  These indicators are used in the analysis of halides by direct titration with silver nitrate solution. They are weak organic acids that vary in strength so that some are more effective in acidic media than others. The end point of the titration is indicated when the color of the silver halide precipitate changes abruptly because of the adsorbed indicator anions.
• Dichlorofluorescein (DCF) TS


  Dissolve 100mg in 60ml ROH, add 2.5ml of 0.1 N NaOH, mix and dilute to make 100ml
• Eosin Y TS (Tetrabromofluorescein)


  Dissolve 50mg in 10ml water
• Tetrabromophenolpthalein (TEE) TS


  Dissolve 100mg in 90ml glacial acetic acid and add glacial acetic acid to make 100ml
• Methods employed in precipitation titrations for establishing the equivalence point

  • Formation of a non-turbid or clear supernatant solution
  • Formation of a colored complex ion
  • Formation of a colored secondary precipitate
  • Formation of a colored adsorption compound
• Types of precipitation titrations (based on the determination of end point)

  • FAJANS
  • VOLHARD
  • MOHR
  • LIEBIG
  • GAY-LUSAAC
• FAJANS


  Involves the change of color on the surface of the precipitate
• VOLHARD


  Involves the formation of an in soluble red colored complex
• MOHR


  Formation of a second precipitate which is a different color of the primary precipitate
• LIEBIG


  Formation of a blue colored complex
• GAY-LUSAAC


  Cessation of precipitation or turbidity
• Precipitimetry Assays

  • Assay of Phenylmercuric nitrate for mercury content
  • Assay of Sodium Lauryl Sulfate for Sodium Chloride Content
  • Assay of Iopanoic Acid Tablets
  • Assay of Benxyltrimethyl ammonium chloride
• Precipitimetry Assays (Volhard)


  • Assay of sodium chloride
  • Assay for Iodide content in Providone Iodine
• End Point


  Cessation of precipitation and Faint pink colored solution is obtained
• Each mL of 0.1 N silver nitrate is equivalent to 58.44 mg of NaCl
• Compleximetry


  Quantitative analysis of inorganic pharmaceutical products containing polyvalent metal ions using metal-ion indicators with the introduction of disodium ethylenediaminetetraacetate (EDTA)
• Good EDTA titration indicator

  • Sharpness of color change in the end point
  • Specificity of the indicator for the metal ion
  • Stability constant smaller than that of the metal-EDTA complex
• Masking


  • Indicates the determination of a metal in the presence of another metal
  • It is capable of entering complexation reaction
  • It can be accomplished by adjusting the pH of the titration medium or using auxiliary complexing agents (masking agents)
• EDTA Titration Indicators


  • Azo dyes
  • Phthaleins
  • Triphenylmethane dyes
  • Eriochrome black T
  • Hydroxynaphthol blue
  • Xylenol orange
  • Dithizone
• Volumetric Solutions

  • EDTA
  • Mercuric nitrate
  • Mercuric acetate
  • Zinc sulfate
• Types of Titration

  • Direct (Ca+2, Mg+2, Zn+2, CaCO3, ZnO)
  • Residual (Al+3, Bi+3)
• Masking Agent


  Elements Masked
• EDTA


  • A hexadentate ligand with six points at which it can attach to the central atom
  • Each of the acetic acid groups can give up a proton, yielding an anion with 4 negative charges
  • Each of the two nitrogens has a pair of non-bonding electrons