Week 4 and 5

• Qualitative analysis


  Primarily concerned with solutions of electrolytes
• Electrolytes


  Substances composed of ions or which produce ions in aqueous solution
• Theory of qualitative analysis


  Studies the behavior of ions in solution
• Flame test method

  Heating elements and ions to emit light at a characteristic color or emission spectrum
• Flame test method

  1. Heat gives energy to elements and ions

  2. Causing them to emit light at a characteristic color or emission spectrum
• Reagent-based method


  Utilizes chemical interaction with specific reagents or chemical solutions
• Flame test


  • Used to identify the presence of certain metal ions in a compound
• Flame test colors

  • Sodium - Yellow
  • Potassium - Lilac or light purple
  • Calcium - Orange-red
  • Copper - Blue or green
• Cation classification


  • Classified into five groups based on their behavior against some reagents
• Group reagents

  Hydrochloric acid, hydrogen sulfide, ammonium sulfide, and ammonium carbonate
• Cation classification basis


  Based on whether a cation reacts with these reagents by the formation of precipitates or not
• Basis for cation classification


  Differences of solubilities of their chlorides, sulfides, and carbonates
• Group I cations

  • Ag+, Hg2 2+, Pb2+
• Group I reagents

  Chloride ion (Cl-) and potassium chromate (K2CrO4)
• Group I reactions

  • White precipitate of silver chloride (AgCl)
  • Yellow precipitate of lead chromate (PbCrO4)
• Group I cations


  • Lead (II), Mercury (I), Silver (I)
• Group I reagent

  Hydrochloric Acid (HCl)
• Group I reactions

  • White precipitates of lead chloride {PbCl2}, mercury(I) chloride {Hg2Cl2}, and silver chloride {AgCl}
• Group I cations

  Form insoluble chlorides
• Lead chloride is slightly soluble in water, so it is never completely precipitated by adding hydrochloric acid
• Solubility


  The maximum concentration to which a solution can be formed
• Lead (Pb)


  • Bluish-grey color
  • High density
• Lead solutions

  Lead nitrate (0.25 M) or lead acetate (0.25 M)
• Lead (Pb) reaction with dilute hydrochloric acid

  1. White precipitate forms in cold solution

  2. Precipitate is soluble in hot water

  3. Precipitate separates again in long, needle-like crystals when cooling
• Lead (Pb) reaction with potassium chromate


  • Yellow precipitate of lead chromate (PbCrO4) forms in neutral, acetic acid, or ammonia solution
• Mercury (I) chloride reaction with aqua regia


  Forms undissociated but soluble mercury(II) chloride
• Mercury vapor is extremely poisonous
• Recommended amount of mercury vapor to be used in the test is not more than 0.1 gram
• Silver (Ag)


  • White, malleable, ductile, high density, melts at 960 ºC
• Silver(I) ions reaction with dilute hydrochloric acid or soluble chlorides


  Formation of white precipitate of silver chloride (AgCl)
• Silver chloride precipitate reaction with concentrated hydrochloric acid


  Precipitate can be dissolved, forming a dichloroargentate complex
• Silver(I) ion solution


  Silver nitrate (0.1 M)
• Silver chloride equilibrium with water


  Equilibrium shifts back to the left and the precipitate reappears when diluted
• Silver chloride precipitate reaction with dilute ammonia solution


  Forms diammine-argentate complex ion: Ag(NH3)2+
• Silver chloride equilibrium with dilute acids


  Neutralizes the excess ammonia, causing the precipitate to reappear
• Group II cations


  • Lead, mercury, bismuth, copper, cadmium, and tin
• Separation of Group II cations from Group I cations


  By adding dilute hydrochloric acid
• Confirmatory test for lead cation

  • Addition of potassium chromate, resulting in a yellow precipitate of lead chromate
• Group II cations

  • Mercury (II), Bismuth (III), Copper (II), Cadmium (II), Arsenic (III and V), Antimony (III and V), and Tin (II and IV)
• Group II reagent

  Hydrogen sulfide (H2S) in gas form or saturated aqueous solution