Chapter 4-5 vr.2.0

• Qualitative analysis


  Method of analysis aimed to determine the types of ions present in a solution
• Electrolytes


  Substances composed of ions, or which produce ions in aqueous solution
• Methods employed in qualitative analysis


  • Flame test
  • Reagent-based
• Flame test


  • Heat gives energy to elements and ions, causing them to emit light at a characteristic color or emission spectrum

  
• Reagent-based


  • Utilizes chemical interaction with specific reagents / chemical solution
• Basis for classification of cations in qualitative analysis

  • Solubility of their chlorides, sulfides, and carbonates

  
• Group I cations


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

  Hydrochloric acid (HCl)
• Group I reactions


  • Form insoluble chlorides

  
• Solubility


  Maximum concentration to which a solution can be formed
• Lead (II)


  • Bluish-grey metal with high density
  • Forms white precipitate of lead chloride with hydrochloric acid
  • Forms yellow precipitate of lead chromate with potassium chromate
  • Forms yellow precipitate of lead iodide with potassium iodide
• Mercury (I)

  • Silver-white, liquid metal
  • Forms white precipitate of mercury(I) chloride with hydrochloric acid
  • Ammonia solution converts the precipitate into a mixture of mercury (II) amidochloride and mercury metal
  • Mercury vapour is extremely poisonous
• Silver (I)


  • White, malleable, and ductile metal
  • Forms white precipitate of silver chloride with hydrochloric acid
  • Dilute ammonia solution dissolves the silver chloride precipitate to form the diammine-argentate complex ion
• Group II cations

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

  Hydrogen sulfide (H2S) - in gas form or saturated aqueous solution
• Group II precipitates

  • Mercury (II) sulfide - black
  • Copper (II) sulfide - black
  • Cadmium sulfide - yellow
  • Bismuth (III) sulfide - brown
  • Arsenic (III) sulfide - yellow
  • Arsenic (V) sulfide - yellow
  • Antimony (III) sulfide - orange
  • Antimony (V) sulfide - orange
  • Tin (II) sulfide - brown
  • Tin (IV) sulfide - yellow
• Group II subgroups

  Copper subgroup - sulfide precipitates insoluble in ammonium polysulfide

  Arsenic subgroup - sulfide precipitates soluble in ammonium polysulfide, forming thiosalts
• Arsenic subgroup


  • Possess amphoteric character: oxides form salts with both acids and bases
• Mercury (II)


  • Initially forms white precipitate of mercury(II) chlorosulfide, which then reacts with more hydrogen sulfide to form black mercury(II) sulfide
• Bismuth (III)


  • Insoluble in hydrochloric acid, but dissolves in oxidizing acids

  Forms brown precipitate of bismuth sulfide with hydrogen sulfide

  Forms black precipitate of bismuth metal when reduced by sodium tetrahydroxostannate(II)

  Forms white precipitate of bismuth(III) hydroxide with sodium hydroxide
• Copper


  • Exists in two series of compounds: Copper(I) and Copper(II)

  Copper(I) compounds are colorless, Copper(II) compounds are colored
• Formation of black precipitate

  1. Sodium hydroxide reacts with bismuth (III) ions to form bismuth (III) hydroxide

  2. Bismuth (III) hydroxide is reduced by tetrahydroxostannate (II) ions to form bismuth metal and hexahydroxostannate (IV) ions
• Bismuth (III) (Bi3+)


  Group II cation
• The reagent must be freshly prepared, and the test must be carried out in cold solution
• Reaction of bismuth (III) ions with sodium hydroxide

  Bi3+ + 3 OH- ⇄ Bi(OH)3 ↓
• Copper (Cu)


  Group II cation
• Copper (Cu) is a light-red metal which is soft, malleable, and ductile. It melts at 1038 °C.
• Copper (I) compounds


  Derived from the red copper (I) oxide Cu2O and contain the copper (I) ion - Cu+. These compounds are colorless.
• Copper (II) compounds

  Derived from the black copper (II) oxide, CuO and contain the copper (II) ion - Cu2+. Copper (II) salts are generally blue both in solid, hydrated form and in dilute aqueous solution.
• In analytical practice, only the copper (II) ion is important
• Reaction of copper (II) ions with hydrogen sulfide


  Cu2+ + H2S ⇄ CuS ↓ + 2H+
• Reaction of copper (II) ions with sodium hydroxide

  Cu2+ + 2 OH- ⇄ Cu(OH)2 ↓
• Dehydration of copper (II) hydroxide


  Cu(OH)2 ↓ ⇄ CuO ↓ + H2O
• Cadmium (Cd)


  Group II cation, a silver-white, malleable and ductile metal that melts at 321ºC. All compounds of cadmium are toxic!
• Reaction of cadmium (II) ions with hydrogen sulfide

  Cd2+ + H2S ⇄ CdS ↓ + 2H+
• The cadmium sulfide precipitate is insoluble in potassium cyanide (POISON), which distinguishes cadmium ions from copper.
• Arsenic (As)


  Group II cation, a steel-grey, brittle solid with a metallic luster. All compounds of arsenic are toxic!
• Arsenic (III) compounds


  Derived from the amphoteric arsenic trioxide AS2O3, which yields salts both with strong acids and with strong bases.
• Arsenic (V) compounds


  Derived from arsenic pentoxide, As2O5 which is the anhydride of arsenic acid, H3AsO4.
• Reaction of arsenic (III) ions with hydrogen sulfide

  2 As3+ + 3 H2S ⇄ As2S3 ↓ + 6H+