Pharmaceutical Analytical Chemistry I - Lecture 7

Pharmaceutical Analytical Chemistry I - Lecture 7

• Cations of Group IV:
  • Group name: Zinc group or ammonium sulfide group
  • Contains: Zinc (Zn²⁺), Manganese (Mn²⁺), Cobalt (Co²⁺), Nickel (Ni²⁺)
  • Precipitated as sulfides: ZnS, MnS, CoS, NiS
  • Precipitate colours: ZnS - white, MnS - buff, CoS - black, NiS - black
  • Group reagent: H₂S in the presence of NH₄OH and NH₄Cl

Why Alkaline Medium?

• Alkaline medium is crucial to shift the ionization of H₂S forward, producing enough sulfide ions to precipitate Group IV cations as sulfides. This is due to the higher Ksp (solubility product) of Group IV sulfides compared to Group II sulfides, requiring a higher sulfide ion concentration.

Precipitation

• Filter the solution from Group III.
• Add NH₄Cl and NH₄OH until the solution becomes alkaline.
• Boil the solution and add H₂S until a complete precipitate forms.
• Boil further until no more ammonia odour.
• Filter the solution.
• The precipitate will be Group IV sulfides.
• Wash with 1% NH₄Cl solution containing a little (NH₄)₂S and filter again.
• The filtrate contains the remaining groups.

Analysis of Group IV - Separation

• ZnS and MnS dissolve readily in cold dilute hydrochloric acid (HCl), while CoS and NiS do not.
• Zn²⁺ is amphoteric, while Mn²⁺ is not.
• To oxidise Mn(OH)₂ into the more insoluble MnO₂ using H₂O₂ or Br₂ water.
• H₂S must be removed before boiling with NaOH and H₂O to prevent the formation of colloidal sulfur and to prevent reprecipitation of ZnS (lowest Ksp).

Analysis of Group IV - Confirmation of Zinc

• The centrifugate (obtained after boiling with excess NaOH) contains soluble zincate.

• Acidify it with acetic acid, divide into two portions.

• Add acetic acid to decompose the zincate complex and liberate free Zn²⁺.

• Perform the H₂S test: Zn²⁺ + H₂S → ZnS↓ (white precipitate)

• Perform the potassium ferrocyanide test: 2Zn²⁺ + [Fe(CN)₆]⁴⁻ → Zn₂[Fe(CN)₆]↓ (sky blue precipitate)

Analysis of Group IV - Confirmation of Manganese (Mn²⁺)

• The white precipitate of Mn(OH)₂ obtained after boiling with NaOH and further oxidation with H₂O₂ is dissolved in concentrated HNO₃ and H₂O₂.
• Boil and cool.
• Add solid red lead(II) and stir. Let it settle.
• The purple colour observed indicates the presence of permanganate (MnO₄⁻). 2Mn²⁺ + 5PbO₂ + 24H⁺ → 2MnO⁴⁻ + 5Pb²⁺ + 12H₂O

Analysis of Group IV - Confirmation of Cobalt and Nickel

• Add NH₄Cl and NH₄OH to make the solution slightly alkali and add dimethylglyoxime.
• Ni²+ will form a red precipitate with dimethylglyoxime.
• This test is carried out in the presence of $Co^{2+}$.
• The precipitate of $Co$ and $Ni$ is boiled with Aqua Regia till it dissolves.
• Test each cation individually.
• Vogel's reaction: HCl soln. + NH₄CNS + ether or amyl alcohol. A blue colour in the ethereal layer or alcoholic layer indicates Co²⁺.

Group V

• Members: Alkaline earth group (Ba²⁺, Ca²⁺, Sr²⁺)
• Group reagent: Ammonium carbonate in the presence of ammonium hydroxide and ammonium chloride
• Precipitation: White carbonates are formed (BaCO₃; white ppt. , CaCO₃; white ppt., SrCO₃; white ppt.)

Analysis of Group V - Heating at 60°C

• Convert any soluble bicarbonates to insoluble carbonates.
• Promote precipitation at low ion concentrations.
• Produce more crystalline precipitates.

Analysis of Group V - Separation and Confirmation of Cations (Ba²⁺, Sr²⁺, Ca²⁺)

• Ba²⁺: BaCrO₄ is insoluble; treat with diluted acetic acid to create BaCrO₄ precipitate
• Sr²⁺: Separate by precipitating it with solution of (NH₄)₂S₄ in slightly ammoniacal solution.
• Ca²⁺: The soluble CaSO₄ can be dissolved with ammonium oxalate.

Group VI

• Members: Soluble group (Mg²⁺, Na⁺, K⁺, NH₄⁺)
• Group reagent: No common precipitating reagent. The cations remain soluble.

Analysis of Group VI - Confirmation of Mg²⁺

• Mg²⁺ is confirmed by obtaining a white crystalline magnesium ammonium phosphate precipitate.

Confirmation of Sodium (Na⁺)

• Potassium antimonate test: Na⁺ + KSb(OH)₆ → NaSb(OH)₆ (white crystalline precipitate)
• Flame test: Sodium gives a golden yellow flame.

Confirmation of Potassium (K⁺)

• Sodium cobaltinitrite test: 3K⁺ + Na₃[Co(NO₂)₆] → K₃[Co(NO₂)₆] + 3Na⁺ (yellow precipitate)
• Perchloric acid test: K⁺ + ClO₄⁻ → KClO₄ (white precipitate)
• Flame test: Potassium produces a pale violet (lilac) flame

Confirmation of Ammonium (NH₄⁺)

• Sodium hydroxide test: NH₄⁺ + NaOH → NH₃↑ + H₂O + Na⁺ (ammonia gas is evolved). Test ammonia gas properties.