UNIT 2G2 - QUALITATIVE ANION ANALYSIS_2024

This unit focuses on the systematic identification of anions in unknown samples using qualitative analysis techniques. Students will learn to apply various methods such as precipitation reactions, colorimetric tests, and chromatography to isolate and identify specific anions, including:

  • Chlorides (Cl⁻)

  • Nitrates (NO₃⁻)

  • Sulfates (SO₄²⁻)

  • Carbonates (CO₃²⁻)

Additionally, the unit will cover the importance of safety protocols and proper lab techniques to ensure accurate results.

Anion Analysis

  • Definition: Anion analysis refers to the process of determining the anions present in a specific material or sample. This is an essential step in qualitative chemistry to identify the components of the sample.

Reactions of the Anions as Qualitative Analysis

Acetate Ion (C2H3O2)

  • Dilute HCl: May liberate acetic acid (Hac).

  • Conc’d H2SO4: Forms volatile acetic acid with the characteristic smell of vinegar.

  • BaCl2 Solution: No Visible Change (NVC).

  • AgNO3 Solution: NVC.

  • C2H5OH with Conc’d H2SO4: Forms Ethyl Acetate (CH3COOC2H5), noted for its fruity ester scent.

    • Reaction: CH3COOH + C2H5OH + H2SO4 → CH3COOC2H5 + H2O

  • FeCl3: Produces a red-brown precipitate.

Arsenate Ion (AsO4)

  • Dilute HCl: No Visible Effect (NVE).

  • BaCl2 in NH4OH: Forms a white precipitate of Barium Arsenate (Ba3(AsO4)2), which dissolves in acids and NH4Cl.

  • AgNO3: NVC.

  • C2H5OH with Conc’d H2SO4: Produces a dark-red precipitate of Ag3AsO4 which is soluble in acids and NH4Cl.

  • Magnesia Mixture: Precipitates white crystals of MgNH4AsO4.

  • H2S in HCl: Forms yellow precipitates, primarily As2S3 and As2S5, which are soluble in alkaline solutions.

  • Ammonium Molybdate: Yields yellow ammonium arsenomolybdate from HNO3 solution.

  • FeCl3 in Ammonium Acetate: Gives yellowish-white Ferric Arsenate, depending on acidity.

Arsenite Ion (AsO3)

  • Dilute HCl: NVE.

  • BaCl2 in NH4OH: White precipitate of Barium Arsenite (Ba3(AsO3)2).

  • AgNO3: Yellow precipitate of Ag3AsO3 from neutral solution.

  • H2S: Precipitates yellow As2S3.

  • Ammonium Molybdate Solution: Needs heat with HNO3 to produce a precipitate similar to Arsenate.

Borate Ion (BO2)

  • Dilute HCl: No Visible Effect (NVE).

  • BaCl2 Solution: White precipitate Ba(BO2)2, soluble in acid.

  • AgNO3 Solution: Forms white precipitate AgBO2 in slightly acidic conditions.

  • Reaction with Ethanol: Produces a green flame due to Triethyl Boron.

  • Turmeric Test: Turns turmeric paper from yellow to red/orange.

Bromide Ion (Br2)

  • Dilute HCl: NVE.

  • Conc’d H2SO4: Produces reddish-brown vapor when warmed.

  • BaCl2 Solution: NVE.

  • AgNO3 Solution: Forms pale yellow precipitate of AgBr, which is insoluble in 0.25M NH4OH.

Carbonate Ion (CO3)

  • Dilute HCl: Liberates CO2 gas and forms CaCO3 with lime water.

  • BaCl2 Solution: Forms white precipitate of BaCO3.

  • AgNO3 Solution: Produces a white precipitate of Ag2CO3, which turns brown upon boiling.

Chlorate Ion (ClO3)

  • Dilute HCl: NVE.

  • Conc’d H2SO4: Causes small explosions with yellow ClO2 gas produced.

  • BaCl2 Solution: NVE.

  • AgNO3 Solution: NVE.

Chloride Ion (Cl2)

  • Dilute HCl: NVE.

  • Conc’d H2SO4: Liberates colorless HCl gas.

  • AgNO3 Solution: Forms white precipitate AgCl which dissolves in 0.25M NH4OH.

Chromate Ion (CrO4)

  • Dilute HCl: Changes from yellow to orange via reaction with H+.

  • BaCl2 Solution: Yellow precipitate of BaCrO4, which is soluble in acids.

  • AgNO3 Solution: Produces Scarlet Silver Chromate, soluble in NaOH.

Nitrate Ion (NO3-)

  • Dilute HCl: NVE.

  • Conc’d H2SO4: Yields colorless HNO3 and brown gas NO2 is produced.

  • Brown Ring Test: Formation of brown ring with FeSO4 indicates nitrate presence.

Nitrite Ion (NO2-)

  • Dilute HCl: Decomposes to form brown NO2 gas.

  • AgNO3 Solution: Produces white precipitate AgNO2 in concentrated solutions.

  • Oxidizing agents: Easily oxidized to nitrate (NO3).

Oxalate Ion (C2O4)

  • Dilute HCl: NVE.

  • Conc’d H2SO4: Decomposes into gases.

  • CaCl2 Solution: Yields a white precipitate of CaC2O4.

Phosphate Ion (PO4)

  • Dilute and Conc’d HCl: NVE.

  • AgNO3 solution: Forms yellow precipitate of Ag3PO4, soluble in dil. HNO3.

Sulfate Ion (SO4)

  • Dilute HCl: NVE.

  • BaCl2 Solution: Immediate white precipitate of BaSO4, insoluble in acids.

Sulfite Ion (SO3)

  • Dilute HCl: Produces sulfur dioxide gas.

  • AgNO3 solution: White precipitate of Ag2SO3.

Sufide Ion (S2)

  • Dilute HCl: Evolution of H2S occurs.

  • AgNO3 solution: Black precipitate Ag2S formed.

Thiocyanate Ion (SCN)

  • Dilute HCl: NVE.

  • FeCl3: Forms blood-red complex due to Fe(SCN)6 ion formation.

Thiosulfate Ion (S2O3)

  • Dilute HCl: Decomposes to gases.

  • AgNO3: Forms white precipitate of Ag2S2O3, which can turn brown or black.

  • May your learning continue!