Group 1 Cation Analysis

• Qualitative analysis


  The ions in a mixture are separated by selective precipitation
• Selective precipitation


  The addition of a carefully selected reagent to an aqueous mixture of ions, resulting in the precipitation of one or more of the ions, while leaving the rest in solution
• Cations


  Typically divided into Groups, where each group shares a common reagent that can be used for selective precipitation
• Group I cations


  Ag+, Pb2+, and Hg2+2 as they are the first group separated from the larger mixture
• Separation and Confirmation of Group I Cations

  1. Precipitation of Group I Cations

  2. Separation of Pb2+ from Hg2+2 and Ag+

  3. Separation and Identification of Hg2+2

  4. Identification of Ag+
• Group I cations form insoluble chlorides, their separation from all other ions may be accomplished by the addition of 6 M HCl(aq)
• Lead(II) chloride can be separated from the other two chlorides based on its increased solubility at higher temperatures
• Silver chloride will dissolve in 6 M NH3(aq) since it forms a soluble complex ion with ammonia
• Mercury(I) chloride reacts with ammonia yielding a gray solid which is a mixture of black Hg(l) and white HgNH2Cl(s)
• The presence of Ag+ can be confirmed by the appearance of a white precipitate upon adding 6 M HNO3(aq)
• Chemicals


  • 0.1 M AgNO3(aq)
  • 0.2 M Pb(NO3)2(aq)
  • 0.1 M Hg2(NO3)2(aq)
  • 6 M HCl(aq)
  • 6 M CH3COOH(aq)
  • 1 M K2CrO4(aq)
  • 6 M NH3(aq)
  • 6 M HNO3(aq)
• Equipment


  • small test tubes
  • glass stirring rod
  • small 10-mL graduated cylinder
  • 250-mL beaker
  • Stand and ring clamp
  • wire gauze
  • small watch glass
  • dropper pipets
  • blue litmus paper
  • wash bottle filled with deionized water
  • Alcohol lamp
  • centrifuge
• All the Group I cations and the chromate anion are hazardous to the environment, thus all waste solutions containing these ions must be disposed of in the hazardous-waste container in the fume hood
• Good organizational skills and techniques are essential to get correct results in this lab
• Cross-contamination is one of the most common causes for false observations leading to incorrect conclusions
• Analysis of Known Mixture of Group I Cations

  1. Precipitation of Group I Cations

  2. Separation of Pb2+ from Hg2+2 and Ag+

  3. Separation and Identification of Hg2+2

  4. Identification of Ag+
• The formation of a bright yellow precipitate upon adding 1 M K2CrO4 to the supernatant solution is confirmation of the presence of the Pb2+ ion
• The appearance of a gray solid is confirmation of the presence of Hg2+2
• Formation of a white precipitate of AgCl in the acidic solution is confirmation of the presence of Ag+