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Introduction to Analytical Chemistry

Ch. 2Std 11

Easy Overview

You're a chemical detective handed a mysterious white powder. Analytical chemistry teaches you to identify what's present (qualitative) and how much (quantitative). This chapter covers systematic qualitative inorganic analysis — identifying cations and anions in a sample through a sequence of group reagents and confirmatory tests. You start with preliminary examination: color (blue=Cu²⁺, green=Ni²⁺/Fe²⁺), solubility, flame test (Na=yellow, K=lilac, Ca=brick red), and dry heating. Then systematic cation analysis: Group 1 (HCl → Ag⁺, Pb²⁺, Hg₂²⁺), Group 2 (H₂S in acid → Cu²⁺, Cd²⁺, etc.), Group 3 ((NH₄)₂S → Co²⁺, Ni²⁺, Fe³⁺, Al³⁺, etc.), Group 4 ((NH₄)₂CO₃ → Ba²⁺, Sr²⁺, Ca²⁺), Group 5 (self → Na⁺, K⁺, Mg²⁺, NH₄⁺). Anion analysis starts with H₂SO₄ tests (dilute and concentrated) followed by individual confirmatory tests. The entire process relies on solubility products and the common ion effect for selective precipitation.

Preliminary Examination of the Salt

Color: white/colorless (Na⁺, K⁺, Ca²⁺, Ba²⁺, Mg²⁺, Al³⁺, Zn²⁺, NH₄⁺), blue (Cu²⁺), green (Ni²⁺, FeSO₄, CuCl₂), pink/red (Co²⁺, Mn²⁺), yellow (Fe³⁺, chromates). Test solubility in water and dil. HCl. Smell: ammonium salts smell of NH₃ when moistened. Dry heating: decrepitation (NaCl, KCl), sublimation (NH₄Cl, I₂), color change (CuSO₄·5H₂O blue→white), gas evolution (CO₂, O₂, NO₂).

Flame Test — Electronic Fingerprints

Clean Pt/Ni wire with conc. HCl. Dip in sample, hold in non-luminous flame. Na⁺ = golden yellow (persistent), K⁺ = lilac (cobalt glass to filter Na), Ca²⁺ = brick red, Sr²⁺ = crimson, Ba²⁺ = apple green, Cu²⁺ = emerald green. Positive = strong evidence; negative doesn't rule out.

Systematic Cation Analysis — Group 1 (HCl Group)

Ag⁺, Pb²⁺, Hg₂²⁺ precipitate as white chlorides with dil. HCl. Hot water: PbCl₂ dissolves; AgCl and Hg₂Cl₂ remain. Hot water + K₂CrO₄ → yellow PbCrO₄ (Pb²⁺ confirmed). Residue + NH₃: AgCl dissolves as [Ag(NH₃)₂]⁺; Hg₂Cl₂ turns black (Hg + HgNH₂Cl). Acidify with HNO₃ → white AgCl (Ag⁺ confirmed).

Systematic Cation Analysis — Group 2 (H₂S Group)

Precipitated as sulfides by H₂S in acidic medium. 2A (Cu²⁺, Cd²⁺, Hg²⁺, Bi³⁺, Pb²⁺) and 2B (As³⁺, Sb³⁺, Sn²⁺, Sn⁴⁺). Yellow ammonium sulfide dissolves 2B as thio-salts. Dissolve 2A in dil. HNO₃: Cu²⁺ + excess NH₃ → deep blue; Cd²⁺ + H₂S (NH₃) → yellow CdS; Bi³⁺ + sodium stannite → white→black.

Systematic Cation Analysis — Group 3 ((NH₄)₂S Group)

Make alkaline with NH₃, add (NH₄)₂S. Precipitates: Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ (sulfides); Fe²⁺/Fe³⁺, Al³⁺, Cr³⁺ (hydroxides). Dissolve in dil. HCl. Ni²⁺ + DMG → rose-red. Co²⁺ + SCN⁻ → blue complex (amyl alcohol). Fe³⁺ + KSCN → blood-red; + K₄[Fe(CN)₆] → Prussian blue. Al³⁺ + NH₃ → gelatinous white, soluble in excess NaOH. Cr³⁺ + NaOH + Br₂ → yellow CrO₄²⁻; + Pb²⁺ → yellow PbCrO₄.

Systematic Cation Analysis — Group 4 ((NH₄)₂CO₃ Group)

Ba²⁺, Sr²⁺, Ca²⁺. Add NH₄Cl + NH₃ + (NH₄)₂CO₃. NH₄Cl prevents Mg(OH)₂ ppt. Dissolve white carbonates in dil. acetic acid. Ba²⁺ + K₂CrO₄ → yellow BaCrO₄ (even in acetic acid). Sr²⁺ → crimson flame, no ppt with K₂CrO₄ in acetic acid. Ca²⁺ + ammonium oxalate → white CaC₂O₄; brick-red flame.

Systematic Cation Analysis — Group 5 (Self Group)

Na⁺, K⁺, Mg²⁺, NH₄⁺ — not precipitated by any group reagent. Test NH₄⁺ from original: Nessler's reagent → brown color; or NaOH heat → NH₃ (smells, blue litmus). Na⁺ → golden yellow flame. K⁺ → lilac flame (cobalt glass). Mg²⁺ + NaOH → white Mg(OH)₂, soluble in NH₄Cl.

Analysis of Anions — Preliminary Tests

Dil. H₂SO₄: CO₃²⁻ → CO₂ (limewater milky); SO₃²⁻ → SO₂ (K₂Cr₂O₇ green); S²⁻ → H₂S (lead acetate black); NO₂⁻ → brown NO₂. Conc. H₂SO₄: Cl⁻ → HCl (NH₃ white fumes); Br⁻ → brown Br₂; I⁻ → violet I₂; NO₃⁻ → brown NO₂.

Confirmatory Tests for Anions — CO₃²⁻, HCO₃⁻, SO₃²⁻, S²⁻

CO₃²⁻: dil. HCl → CO₂, milky limewater. HCO₃⁻: also CO₂ but MgSO₄ test — CO₃²⁻ gives immediate MgCO₃, HCO₃⁻ needs heating. SO₃²⁻: dil. HCl → SO₂ turns K₂Cr₂O₇ green, KMnO₄ colorless. S²⁻: dil. HCl → H₂S turns lead acetate black; sodium nitroprusside → purple.

Confirmatory Tests for Anions — Cl⁻, Br⁻, I⁻, NO₃⁻

Cl⁻: AgNO₃ → white AgCl, soluble in NH₃, insoluble in HNO₃. Br⁻: AgNO₃ → pale yellow AgBr, partially NH₃-soluble; Cl₂ + CCl₄ → red-brown Br₂ layer. I⁻: AgNO₃ → yellow AgI, NH₃-insoluble; Cl₂ + CCl₄ → violet I₂ layer. NO₃⁻: brown ring test — FeSO₄ + conc. H₂SO₄ layered → brown [Fe(H₂O)₅(NO)]²⁺.

Confirmatory Tests for Anions — SO₄²⁻, PO₄³⁻, CH₃COO⁻, C₂O₄²⁻

SO₄²⁻: BaCl₂ → white BaSO₄, insol. in dil. HCl/HNO₃. PO₄³⁻: ammonium molybdate + conc. HNO₃ + warm → canary yellow. CH₃COO⁻: ethanol + conc. H₂SO₄ + warm → fruity ethyl acetate. C₂O₄²⁻: CaCl₂ → white CaC₂O₄, insol. in acetic acid, sol. in mineral acids.

Dry Heating Tests

Heat dry salt. Decrepitation: NaCl, KCl. Sublimation: NH₄Cl (white), I₂ (violet), As₂O₃ (white). Color changes: CuSO₄·5H₂O (blue→white), FeSO₄·7H₂O (green→brown). Gas: CO₂ (carbonates), O₂/NO₂ (nitrates), H₂O (hydrated salts — condenses on cool tube).

Solubility Product and Precipitation

Ksp = [A]ˣ[B]ʸ. Precipitate when IP > Ksp. Group 2: acidic medium low [S²⁻] → only very low Ksp sulfides ppt (CuS=6×10⁻³⁷). Group 3: higher pH → higher [S²⁻] → ZnS (2×10⁻²⁵) can ppt. Calculate IP and compare to Ksp.

Common Ion Effect in Qualitative Analysis

Group 2: H⁺ from HCl suppresses H₂S → low [S²⁻] → selective precipitation. Group 4: NH₄⁺ from NH₄Cl suppresses OH⁻ → prevents Mg(OH)₂ ppt. Without common ion effect, groups would not separate cleanly.

Analysis of Insoluble Salts

Salts insoluble even in dil. HCl (BaSO₄, SrSO₄, PbSO₄, CaF₂, AgX): fuse with Na₂CO₃. BaSO₄ + Na₂CO₃ (fusion) → BaCO₃ + Na₂SO₄. Dissolve BaCO₃ in dil. HCl. Test anion from filtrate. Alternatively: PbSO₄ in CH₃COONH₄; AgCl in NH₃.

Interference and Elimination of Interfering Ions

Fe³⁺ and Cu²⁺ interfere with NO₃⁻ brown ring test (give color). Remove Cu²⁺ by H₂S. PO₄³⁻ interferes with Groups 3-4 by forming phosphates. Add excess FeCl₃ — FePO₄ precipitates, carrying down other cations. CrO₄²⁻: yellow color — reduce to Cr³⁺, test separately.

Key Points

  • •Qualitative analysis identifies constituents; quantitative measures amounts
  • •Preliminary: color, solubility, flame test, dry heating
  • •Flame test: Na⁺ (yellow), K⁺ (lilac), Ca²⁺ (brick red), Sr²⁺ (crimson), Ba²⁺ (apple green), Cu²⁺ (green)
  • •Group 1 (HCl): Ag⁺, Pb²⁺, Hg₂²⁺ as chlorides; PbClâ‚‚ soluble in hot water
  • •Group 2 (Hâ‚‚S in acid): 2A and 2B separated by yellow ammonium sulfide
  • •Group 3 ((NHâ‚„)â‚‚S in NH₃): Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺, Fe₂⁺/Fe³⁺, Al³⁺, Cr³⁺
  • •Group 4 ((NHâ‚„)â‚‚CO₃): Ba²⁺, Sr²⁺, Ca²⁺ (Mg²⁺ stays soluble with NHâ‚„Cl)
  • •Group 5 (self): Na⁺, K⁺, Mg²⁺, NH₄⁺ — test NH₄⁺ from original
  • •Ni²⁺: rose-red with DMG. Fe³⁺: blood-red with KSCN
  • •Brown ring test: FeSOâ‚„ + conc. Hâ‚‚SOâ‚„ → brown [Fe(Hâ‚‚O)â‚…(NO)]²⁺
  • •Cl⁻ (white AgCl/NH₃-soluble) vs Br⁻ (pale yellow/partial) vs I⁻ (yellow/insoluble)
  • •Precipitation when IP > Ksp; common ion effect enables selective separation
  • •Insoluble salts: fuse with Naâ‚‚CO₃ → soluble carbonates
  • •Interferences: Fe³⁺, Cu²⁺ (brown ring); PO₄³⁻ (Groups 3-4)

Practice Questions

  • A white salt gives brick-red flame. With dil. HCl, it produces gas that turns limewater milky. Identify and write reactions.
  • A salt gives: blue solution, green flame, deep blue with NH₃. Identify the cation.
  • How to systematically analyze a mixture of Pb²⁺, Cu²⁺, and Fe³⁺?
  • Distinguish: Naâ‚‚CO₃ vs Naâ‚‚SOâ‚„; NaCl vs NaBr; BaClâ‚‚ vs CaClâ‚‚
  • Explain common ion effect in Group 2 and Group 4 separation.
  • Why is NHâ‚„Cl added in Group 4? What happens if omitted?
  • Explain brown ring test. Which ions interfere and how removed?
  • A salt is insoluble in water and dil. HCl. Describe the procedure to identify it.