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.

This quiz covers the cations of Group IV in Pharmaceutical Analytical Chemistry, focusing on the zinc group and the necessary conditions for precipitation. It discusses the key reagents and procedural steps, along with the importance of an alkaline medium in the precipitation process. Test your knowledge of these essential concepts.