Pharmaceutical Analytical Chemistry I - Lecture 6

Pharmaceutical Analytical Chemistry I - Lecture 6

• Cations of Group II: This group is classified as Copper-Arsenic or Hydrogen sulfide group.
• Group II (A): Contains Copper subgroup cations: Cu²⁺, Pb²⁺, Hg₂²⁺, Bi³⁺, Cd²⁺
• Group II (B): Contains Arsenic subgroup cations: As³⁺, Sb³⁺, Sn²⁺
• Group Reagent: H₂S gas in an acidic medium (0.2–0.3N HCl).
• Group II Cations as Sulphides:
  • CuS, HgS, and PbS: Black precipitate
  • Bi₂S₃: Brown precipitate
  • CdS: Canary yellow precipitate
  • As₂S₃: Yellow precipitate
  • SnS₂: Orange precipitate

Precipitation of Group II

• Adjust Acidity: Adjust the filtrate's acidity from Group I to 0.2–0.3 N HCl using crystal violet indicator until a bluish-green color appears.
• Warm and Pass H₂S: Warm the solution and pass H₂S gas until complete precipitation.
• Boil and Filter: Boil the mixture and filter the precipitate.
• Wash the Precipitate: Wash the precipitate with 5% NH₄NO₃ and pass H₂S.
• Result: Group II "A" and "B" cations precipitate as sulfides
• Filtrate: Contains elements of groups III and IV.

Notes on Adjusting HCl Acidity

• Too high HCl ( > 0.3N): Decreases the amount of sulfide ions, leading to incomplete precipitation of lead, cadmium, and tin as sulfides.
• Too low HCl ( < 0.2N): Increases the amount of sulfide ions and leads to precipitation of Group IV cations.

Additional Notes

• Heating After H₂S Addition: H₂S is slightly soluble in water; therefore, heating the solution increases the solubility of H₂S and its concentration.
• Washing Precipitate with NH₄NO₃: This step removes soluble cations from other groups (III–VI) from the solid precipitate.
• Why Not HNO₃ or H₂SO₄? HNO₃ oxidizes H₂S to free sulfur and dissolves the sulfide precipitate; H₂SO₄ precipitates Group V as sulfate along with Group II sulfides.

Separation of Group II A and B

• Procedure: Add 2 ml of 2M NaOH and a few drops of ammonium polysulfide to the ppt. Heat in a boiling water bath for 3 minutes while stirring, then filter.
• Result: The ppt will contain Group II A cations (Copper subgroup) while the filtrate contains Group II B cations (Arsenic subgroup).

Analysis of Group II A

• Procedure: Centrifuge the precipitate and treat it with dilute HNO₃ . Centrifuge again and treat the new solution with H₂SO₄. Centrifuge again.
• Result: The individual cations within group II A can now be identified

Identification and Confirmation of Bi³⁺

• Procedure: Wash the precipitate of Bi(OH)₃ with water, and dissolve it in the minimum volume of dilute HCl. Pour a small portion of this solution into a large volume of water.
• Result: White ppt of BiOCl will form.

Identification and Confirmation of Cu²⁺ and Cd²⁺

• Procedure: The filtrate containing both Cu²⁺ and Cd²⁺ cations in the form of soluble amine complexes will be tested.
• Result: Individual cations will be identified using specific tests for each.

Confirmation for Cu²⁺

• Procedure: Add acetic acid to decompose copper amine complex (blue color disappears). Add K₄[Fe(CN)₆] to precipitate a reddish brown Cu₂[Fe(CN)₆]. Or add solution of KI to produce a white precipitate of Cu₂I₂ in a brown solution.
• Result: Test confirms the presence of Copper.

Confirmation for Cd²⁺

• Procedure: Add KCN dropwise until the blue copper amine complex color disappears. Warm the solution and add H₂S to precipitate canary yellow CdS.
• Result: Test confirms the presence of Cadmium.

Cations of Group III

• Group Name: Iron group or ammonium hydroxide group
• Cations: Fe²⁺, Fe³⁺, Al³⁺, Cr³⁺
• Precipitation: Hydroxides are precipitated using concentrated NH₄OH with NH₄Cl.
• Appearance after precipitation: Reddish brown, white gelatinous, greenish

Analysis of Group III

• Precipitation: Centrifuge the solution obtained from Group II. Boiled to remove H₂S. Then add HNO₃ and NH₄Cl. Add NH₄OH.
• Separation: This step depends on the amphoteric nature of Al(OH)₃ and Cr(OH)₃, which are soluble in NaOH, and the insolubility of Fe(OH)₃ and MnO(OH)₂.
• Procedure: Centrifuge.
• Filtrate: Used for analysis of Groups IV and higher.
• Precipitate: Used for analysis of Group III cations.

Notes on Group III

• Removing H₂S: Remove H₂S to avoid Group IV sulfide precipitation.
• Adding NH₄Cl: This lowers NH₄OH ionization to prevent Zn²⁺, Co²⁺, Ni²⁺, and Mg²⁺ hydroxides from precipitating along with Group III.
• Oxidizing Fe²⁺ to Fe³⁺: Fe(OH)₃ precipitates better than Fe(OH)₂.

Additional Notes for Group III

• Boiling Excess NH₄OH: Chromium forms an ammine complex in cold solution that decomposes on boiling to reform Cr(OH)₃.

Analysis of Group III Cations

• Procedure: Treat the hydroxide precipitate with excess NaOH to separate the cations. The filtrate contains AlO₂⁻ and CrO₄²⁻, and the ppt are Fe(OH)₃ and MnO(OH)₂.
• Oxidizing Chromium: Oxidize chromium to chromate using H₂O₂.
• Result: The individual cations of group III are identified.

Identification and Confirmation of Fe³⁺

• Procedure: Add KCNS to the solution and divide it into two parts.
• Result: Blood red color for Fe³⁺.
• Alternative Test: Add K₄[Fe(CN)₆] solution to the other part to produce a Prussian blue precipitate.

Identification and Confirmation of Mn²⁺

• Procedure: Add 50% HNO₃ and one gram of red lead to the solution for 1-2 minutes.
• Result: Pink or purple permanganate color will appear.

Identification and Confirmation of Al³⁺

• Procedure: Add 1 gm of solid NH₄Cl to the solution, boil until only a faint odor of NH₃ remains.
• Result: White gelatinous Al(OH)₃ will precipitate.
• Additional Test: Aluminum can also be detected using Aluminon.

Identification and Confirmation of Cr³⁺

• Procedure: Perform Perchromic acid test by adding dilute H₂SO₄, ether, and 1 ml of H₂O₂. Perform the lead acetate test by adding acetic acid and lead acetate solution.
• Result: A deep blue color (ether layer) and a yellow precipitate (PbCrO₄) confirms the presence of chromium.

This quiz covers the cations of Group II in Pharmaceutical Analytical Chemistry, focusing on the precipitation methods and characteristics of various cations. Key subgroups include Copper and Arsenic, with detailed procedures for identifying precipitates formed in the presence of H₂S. Test your knowledge on the classification and behavior of these important cations.