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Questions and Answers
Which reagent is used to precipitate cations in acid medium?
Which reagent is used to precipitate cations in acid medium?
- Hydrochloric acid (correct)
- Sodium hydroxide
- Ammonium chloride
- Hydrogen sulphide (correct)
What is the formula for barium chromate?
What is the formula for barium chromate?
- Ba(CrO4)2
- BaCrO4 (correct)
- Ba3(CrO4)2
- BaSO4
Iron can be oxidized from Fe2+ to Fe3+ using concentrated HNO3.
Iron can be oxidized from Fe2+ to Fe3+ using concentrated HNO3.
True (A)
How is iron weighed after drying?
How is iron weighed after drying?
Al3+ can be precipitated directly without any pH control.
Al3+ can be precipitated directly without any pH control.
What agents can be used for determining Al3+?
What agents can be used for determining Al3+?
Iron must be present in the form of ______ before determination.
Iron must be present in the form of ______ before determination.
What occurs when Al(OH)3 is treated with excess NaOH?
What occurs when Al(OH)3 is treated with excess NaOH?
Study Notes
Cation Groups
- Cations are classified into six groups based on their reactivity with specific reagents.
- Each group reagent separates cations from a solution for further analysis.
- Group 1: Hydrochloric acid (HCl)
- Group 2: Hydrogen sulphide (H2S) in an acidic medium
- Group 3: Ammonium chloride (NH4Cl) and Ammonium hydroxide
- Group 4: Hydrogen sulphide (H2S) in a basic medium
- Group 5: NH4Cl + NH4OH + Ammonium carbonate (NH4)2CO3
- Group 6: Has no specific reagent
Separating Barium, Strontium, and Calcium
- Barium is first precipitated as chromate.
- Strontium can be separated from Calcium by treating their acetate salts with excess concentrated ammonium sulfate ((NH4)2SO4) in a faintly ammoniacal solution.
- Calcium forms the soluble (NH4)2[Ca(SO4)2]
- Alternatively, a saturated solution of calcium sulfate (CaSO4) can be used.
- Strontium sulfate (SrSO4) precipitates as a white solid.
Applications
Determination of Cations
Determination of Iron as Fe2O3
- Iron must be present as Fe3+.
- If Fe2+ is present, it must be oxidized to Fe3+ by boiling with concentrated HNO3.
- The Fe3+ solution is precipitated as hydrous oxide using a slight excess of NH4OH.
- The precipitate is then washed with NH4NO3, dried, ignited, and weighed as Fe2O3.
Notes on Iron Determination
- Avoid washing Fe2O3.xH2O with NH4Cl, as it converts some Fe2O3 to FeCl3, which is lost during drying.
- Iron can be precipitated using organic precipitants like cupferron, ignited, and weighed as Fe2O3.
- It can also be precipitated by 8-hydroxyquinoline (oxine) as oxinate.
- Organic hydroxy compounds (citrate or tartarate) must be absent, as they form soluble complexes with iron.
Determination of Aluminum as Al2O3
- Aluminum ion (Al3+) is precipitated as Al(OH)3 using ammonia in the presence of NH4Cl or a methyl red (M.R.) indicator..
- The precipitated Al(OH)3 is washed with either NH4NO3 or NH4Cl, filtered, dried, ignited and weighed as Al2O3.
Notes on Aluminum Determination
- NH4Cl or M.R. is used to control the pH of the medium and prevent the dissolution of amphoteric Al(OH)3 in excess NH4OH.
- Al3+ can be precipitated as aluminum oxinate at pH 4-10 using 8-hydroxyquinoline.
Determination of a Mixture of Fe3+ and Al3+
- Treat the mixture with excess NaOH. Non-amphoteric Fe(OH)3 precipitates, while the amphoteric Al(OH)3 dissolves as AlO2-.
- The Fe(OH)3 residue is determined as described in the "Determination of Iron" section.
- Al3+ is determined in the combined filtrate and washing after acidification, where Al3+ is produced and determined as described in the "Determination of Al3+" section.
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Description
Explore the classification of cations into six distinct groups based on their reactivity with specific reagents. This quiz delves into the techniques for separating Barium, Strontium, and Calcium, along with their applications in cation determination.
