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Questions and Answers
What is the primary reason for adjusting the acidity of HCl to 0.2 – 0.3 N?
What is the primary reason for adjusting the acidity of HCl to 0.2 – 0.3 N?
What happens if the acidity of HCl is less than 0.2 N during the precipitation process?
What happens if the acidity of HCl is less than 0.2 N during the precipitation process?
Why is it necessary to heat the solution after adding H2S?
Why is it necessary to heat the solution after adding H2S?
What color precipitate is produced by the reaction of sulphides with cadmium?
What color precipitate is produced by the reaction of sulphides with cadmium?
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Why is 5% NH4NO3 used to wash the precipitated sulphides?
Why is 5% NH4NO3 used to wash the precipitated sulphides?
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What is the consequence of using HNO3 instead of HCl in this context?
What is the consequence of using HNO3 instead of HCl in this context?
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How is group II 'A' separated from group II 'B' after precipitation?
How is group II 'A' separated from group II 'B' after precipitation?
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What precipitate color is associated with the reaction of As2S3?
What precipitate color is associated with the reaction of As2S3?
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Why should H2SO4 not be used instead of HCl for precipitation?
Why should H2SO4 not be used instead of HCl for precipitation?
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What color does the amine complex of Cu2+, [Cu(NH3)4]2+, display?
What color does the amine complex of Cu2+, [Cu(NH3)4]2+, display?
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What forms when acetic acid is added to the copper amine complex?
What forms when acetic acid is added to the copper amine complex?
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Why is KCN added dropwise when confirming Cd2+ ions?
Why is KCN added dropwise when confirming Cd2+ ions?
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When confirming Cu2+ with KI, what is the observed precipitate formed?
When confirming Cu2+ with KI, what is the observed precipitate formed?
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What is the role of NH4Cl before the precipitation of group III cations?
What is the role of NH4Cl before the precipitation of group III cations?
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What type of precipitants are formed for the cations in group III?
What type of precipitants are formed for the cations in group III?
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Which precipitant is formed from the reaction of Cd2+ with H2S?
Which precipitant is formed from the reaction of Cd2+ with H2S?
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What happens to H2S if it is not removed before adding NH4OH to group III?
What happens to H2S if it is not removed before adding NH4OH to group III?
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What form do Cu2+ and Cd2+ take in the filtrate before testing?
What form do Cu2+ and Cd2+ take in the filtrate before testing?
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What is the color of the precipitate formed from Fe3+ in group III?
What is the color of the precipitate formed from Fe3+ in group III?
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What is the primary reason for oxidizing Fe2+ into Fe3+ before the precipitation of group III hydroxides?
What is the primary reason for oxidizing Fe2+ into Fe3+ before the precipitation of group III hydroxides?
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What happens to Cr(OH)3 upon boiling after adding excess NH4OH?
What happens to Cr(OH)3 upon boiling after adding excess NH4OH?
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What is indicated by the formation of a blood red color when Fe3+ reacts with KCNS?
What is indicated by the formation of a blood red color when Fe3+ reacts with KCNS?
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What color is produced when Mn2+ reacts with red lead in the presence of 50% HNO3?
What color is produced when Mn2+ reacts with red lead in the presence of 50% HNO3?
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What occurs when Al(OH)3 is dissolved in HCl along with sodium acetate and aluminon?
What occurs when Al(OH)3 is dissolved in HCl along with sodium acetate and aluminon?
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What is produced when performing the perchromic acid test with Cr3+?
What is produced when performing the perchromic acid test with Cr3+?
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What compound forms when CrO42- reacts with Pb2+ in the presence of acetic acid?
What compound forms when CrO42- reacts with Pb2+ in the presence of acetic acid?
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What is the purpose of boiling the solution after adding excess NH4OH?
What is the purpose of boiling the solution after adding excess NH4OH?
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Which ion is indicated by the presence of a yellow precipitate upon addition of Pb2+ and chrome products?
Which ion is indicated by the presence of a yellow precipitate upon addition of Pb2+ and chrome products?
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What is the resulting change of color indicating manganese when Mn2+ is treated under acidic conditions with red lead?
What is the resulting change of color indicating manganese when Mn2+ is treated under acidic conditions with red lead?
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Study Notes
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.
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Description
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.
