Salt Analysis Notes for NEET – Complete Chapter

Introduction

What is Salt Analysis?

Acid + Base → Salt + Water
e.g. HCl + NaOH → NaCl + H₂O

Salt dissociates into Cation (Basic radical — from base) and Anion (Acidic radical — from acid)
Salt Analysis = Detection of anions & cations from a given mixture
Also called Qualitative Analysis
Anion analysis is performed first, followed by cations — because interfering anions can interfere with cation tests

Interfering Anions (Removed First)

\(BO_3^{3-}\)
Borate

\(PO_4^{3-}\)
Phosphate

\(AsO_4^{3-}\)
Arsenate

\(C_2O_4^{2-}\)
Oxalate

\(F^-\)
Fluoride

\(SiO_4^{4-}\)
Silicate

Classification of Anions

Two Classes of Anions

Class A — Form volatile product with acids | Group Reagent: dil. HCl / dil. H₂SO₄ (Subgroup 1) or Conc. H₂SO₄ (Subgroup 2)
Class B — Don't form volatile product with acids | Identified by ppt reactions (Subgroup 1) or Redox changes (Subgroup 2)

Anions of Class A — Subgroup 1 (G.R. = dil. HCl / dil. H₂SO₄)

Anion	Formula	Gas Evolved	Observation
Carbonate	\(CO_3^{2-}\)	\(CO_2\uparrow\)	Colourless, odourless, brisk effervescence
Bicarbonate	\(HCO_3^-\)	\(CO_2\uparrow\)	Same as carbonate
Sulphite	\(SO_3^{2-}\)	\(SO_2\uparrow\)	Burning 'S' like gas, choking smell
Bisulphite	\(HSO_3^-\)	\(SO_2\uparrow\)	Same as sulphite
Sulphide	\(S^{2-}\)	\(H_2S\uparrow\)	Rotten egg-like smell
Nitrite	\(NO_2^-\)	\(NO_2\uparrow\)	Brown fumes (gas)
Acetate	\(CH_3COO^-\)	\(CH_3COOH\uparrow\)	Vinegar-like smell
Thiosulphate	\(S_2O_3^{2-}\)	\(S\downarrow + SO_2\uparrow\)	Sulphur ppt + choking gas

Anions of Class A — Subgroup 2 (G.R. = Conc. H₂SO₄)

Anion	Formula	Product	Observation
Fluoride	\(F^-\)	HF (clear soln)	HF + SiO₂ → Etching of glass
Chloride	\(Cl^-\)	HCl↑ (white fumes)	NH₄OH → Dense white fumes
Bromide	\(Br^-\)	\(Br_2\uparrow\) (brown vapour)	H₂O → Brown solution
Iodide	\(I^-\)	\(I_2\uparrow\) (violet fumes)	Violet fumes
Oxalate	\(C_2O_4^{2-}\)	CO + CO₂↑	Both gases evolved
Nitrate	\(NO_3^-\)	\(NO_2\uparrow\) (brown)	H₂O → NO + HNO₃ (colourless)
Borate	\(BO_3^{3-}\)	\(H_3BO_3\uparrow\)	Boric acid vapours

Anions of Class B

Subgroup 1 — By ppt Reactions

\(SO_4^{2-}\) (Sulphate), \(PO_4^{3-}\) (Phosphate), \(AsO_3^{3-}\) (Arsenite), \(As_2O_4^{3-}\) (Arsenate), \(CrO_4^{2-}\) (Chromate)

Subgroup 2 — By Redox Reactions

\(MnO_4^-\), \(Cr_2O_7^{2-}\), \(CrO_4^{2-}\)

Q: Which salt remains unaffected by Conc. H₂SO₄?

Ans: (d) None of these — NaCO₃, NaCl, NaBr all react with Conc. H₂SO₄

⚠️ Important Note

Subgroup-1 Anions can also react with Conc. H₂SO₄, but we use dilute as reagent for Subgroup-1.

General Rules of Solubility

Ksp Concept

Ksp↑ → Solubility increases
Ksp↓ → Solubility decreases → ppt formation tendency increases
w/I = Water Insoluble | w/s = Water Soluble | s.s = Sparingly Soluble (partial)

Anion	Solubility Rule	Important Exceptions
\(CO_3^{2-}\)	All w/I except Low φ & NH₄⁺ (Na⁺, K⁺, Rb⁺, Cs⁺)	Li₂CO₃ → s.s
\(SO_3^{2-}\)	Same as \(CO_3^{2-}\)	—
\(NO_3^-\)	All w/s (due to N-Bonding)	NaNO₃ (s.s)
\(HSO_3^-\)	Same as \(HCO_3^-\)	—
\(S^{2-}\)	All w/I except IA & IIA & NH₄⁺	Most insoluble (very low Ksp): Hg²⁺, Pb²⁺, Cu²⁺, Cd²⁺, Bi³⁺
\(OH^-\)	All w/I except Low φ, Ba, Sr, Ca, NH₄⁺	Al³⁺, Fe²⁺, Co²⁺ → most insoluble
\(NO_3^-\) & \(NO_2^-\)	All w/s	AgNO₂ → s.s
\(CH_3COO^-\)	All w/s	Except Ag⁺, Hg₂²⁺, Cu₂²⁺
\(SO_4^{2-}\)	Most w/s	Except Ba, Sr, Ca, Ag, Hg, Pb
\(Cl^-/Br^-/I^-\)	All w/s	Except Ag⁺, Hg₂²⁺, Pb²⁺
\(NH_4^{4+}\)	All salts w/s	HgI₂ → w/I (Red ppt), Hg₂Cl₂ → w/I, HgCl₂ → w/s

Important Points — ppt Dissolution

\(CO_3^{2-}\downarrow\) (w/I) + excess CO₂ → \(HCO_3^-\) (Bicarbonate) [w/s]
\(SO_3^{2-}\downarrow\) (w/I) + excess SO₂ → \(HSO_3^-\) (Bisulphite) [w/s]
Generally all ppts are soluble in strong acids like HCl & HNO₃, except BaSO₄ & HgS
Acid-Base reactions dissolve many ppts
Complex formation also dissolves ppts

ppt Dissolution — Acid-Base Reactions

Common Hydroxide ppt Dissolution

Al(OH)₃↓ (white gelatinous) → HCl → Soluble

Al(OH)₃↓ → NaOH (excess) → Na[Al(OH)₄] Soluble

Cr(OH)₃↓ (green) → HCl → Soluble

Cr(OH)₃↓ → NaOH → Soluble

Fe(OH)₃↓ (red brown) → HCl → Soluble

Fe(OH)₃↓ → NaOH → Soluble then Insoluble

Mg(OH)₂↓ → HCl → Soluble

Mg(OH)₂↓ → NaOH → Insoluble

ppt Dissolution — Complex Formation

ppt	Reagent	Complex Formed
CuCN↓ (white)	Excess CN⁻	\([Cu(CN)_4]^{3-}\) (Soluble)
Ni(CN)₂↓ (green)	Excess CN⁻	\([Ni(CN)_4]^{2-}\) (Soluble)
Co(CN)₂↓ (brown)	Excess CN⁻	\([Co(CN)_6]^{4-}\) (Soluble)
AgCl↓ (white)	Excess NH₃/CN⁻	\([Ag(CN)_2]^-\) & \([Ag(NH_3)_2]^+\) (Soluble)
Ni(OH)₂↓ (green)	Excess NH₃	\([Ni(NH_2)_6]^{2+}\) (Soluble)

MDJ — Coloured ppt Rule

If no d-block cation in salt → ppt is generally white
But S²⁻ of As, Sn, Sb are coloured | PbS → Black
If d-block cation has d¹⁰ configuration → generally white ppt | But AgBr, AgI, CdS, HgS have colour

Confirmatory Tests for Anions

Preparation of Soda Extract

For water-insoluble salts (contains Na⁺, Low φ salts, water-soluble)
Soda extract = Sodium carbonate extract: 1 Part Salt + 3 Parts Na₂CO₃ + H₂O → heat → Filtrate [NaX]
Na₂CO₃ ka Na⁺ salt ke cation ko replace karta hai, anion charge nahi hoga
MX + Na₂CO₃ → NaX (w/s) + MCO₃ (w/I)
Note: For CO₃²⁻/HCO₃⁻ salts, soda extract is NOT prepared

① CO₃²⁻ (Carbonate)

🧪 (A) Acid Test

\(CO_3^{2-}\) + dil. H₂SO₄ → \(CO_2\uparrow\)

\(CO_2 + Ca(OH)_2/Ba(OH)_2\) → Milkiness (CaCO₃/BaCO₃↓)

+ Excess CO₂ → Milkiness disappears (Ca(HCO₃)₂/Ba(HCO₃)₂)

🧪 (B) ppt Test

\(Na_2CO_3 + BaCl_2/CaCl_2\) → BaCO₃↓ (white) / CaCO₃↓ (white)

+ HCl/HNO₃ or excess CO₂ → Soluble

🧪 (C) Test by AgNO₃

\(CO_3^{2-} + AgNO_3\) → Ag₂CO₃↓ (pale yellow)

+ dil. HNO₃ → Soluble (AgNO₃) | + HCl → White ppt (AgCl↓) | + Excess NH₃ → Soluble \([Ag(NH_3)_2]^+\)

🧪 (D) Test by HgCl₂

\(CO_3^{2-} + HgCl_2\) → Red-Brown ppt (3HgO·HgCO₂↓)

② S²⁻ (Sulphide)

🧪 (A) Acid Test

\(S^{2-}\) + dil. HCl/dil. H₂SO₄ → \(H_2S\uparrow\) (rotten egg-like smell)

Filter Paper Test: Pb(CH₃COO)₂ → PbS↓ (Black) | Cd(CH₃COO)₂ → CdS↓ (Yellow)

🧪 (B) ppt Test

AgNO₃ → Ag₂S↓ Black

Pb(NO₃)₂ → PbS↓ Black

Cd(CH₃COO)₂ → CdS↓ Yellow

Hg(NO₃)₂ → HgS↓ Black — Insoluble in HCl & HNO₃

🧪 (C) Redox Test (S²⁻/H₂S is Reducing Agent)

IO₃⁻ → I₂ Violet

Cr₂O₇²⁻ (orange) → Cr³⁺ Green

MnO₄⁻/H⁺ (purple) → Mn²⁺ Colourless

Fe³⁺ (yellow) → Fe²⁺ Green

Cl₂ water (yellow-green) → Cl⁻ Colourless

Br₂ water (brown) → Br⁻ Colourless

🧪 (D) Special Test — SNP Test

SNP = Sodium Nitroprusside = Na₂[Fe(CN)₅NO]

CNP (red-brown) + S²⁻ → STNP Na₂[Fe(CN)₅NOS] Violet

T stands for Thio

③ NO₂⁻ (Nitrite)

Key Fact

All NO₂⁻ are w/s except AgNO₂. NO₂⁻ can work as both Reducing Agent (R.A.) and Oxidising Agent (O.A.)

🧪 (A) Acid Test

\(NO_2^- + dil. H_2SO_4\) → NO₂↑ Brown vapours

🧪 (B) ppt Test

\(NO_2^- + AgNO_3\) → AgNO₂↓ (white ppt)

🧪 (C) Redox Test

K₂Cr₂O₇/H⁺ (orange) → Cr³⁺ Green

KMnO₄/H⁺ (purple) → Mn²⁺ Colourless

Br₂ (brown) → Br⁻ Colourless

🧪 (D) Special Test — Brown Ring Test

NO₂⁻(aq) + dil. HCl/dil. H₂SO₄/CH₃COOH + FeSO₄(aq) → Brown ring [Fe(H₂O)₅NO]SO₄

Reactions: NO₂⁻ + H⁺ → HNO₂ → HNO₃ + NO | NO + FeSO₄(freshly prepared) → Brown ring complex

Test for NO₃⁻ (Nitrate) — Subgroup 2

Group Reagent: Conc. H₂SO₄

All Nitrates are w/s — do not give ppt test

🧪 (A) Acid Test

\(NO_3^- + Conc. H_2SO_4\) → NO₂↑ Brown or Red-Brown

Note: If Cu turnings added → where NO₂ evolving, intensity of NO₂ increases

🧪 (C) Special Test — Brown Ring Test

\(NO_3^- + H^+\) (from Conc. H₂SO₄) → HNO₃

\(HNO_3 + FeSO_4\) → Fe₂(SO₄)₃ + NO

\(FeSO_4(aq) + NO\) → \([Fe(H_2O)_5NO]SO_4\) Brown Ring Complex

Interfering Anions for Brown Ring Complex

NO₂⁻, ClO₃⁻, CrO₄²⁻, Br⁻, I⁻ — removal required before Brown Ring Test

NO₂⁻ removal by Sulphamic acid: NO₂⁻ + H₂N-SO₂OH → N₂↑ + SO₄²⁻ + H₂O

NO₃⁻ + Sulphamic acid → Unaffected

Q: Brown Ring test of NO₂⁻ is possible in presence of NO₃⁻?

Ans: True ✓

Q: Brown Ring test of NO₃⁻ is possible in presence of NO₂⁻?

Ans: False ✗

Tests for Cl⁻, Br⁻ & I⁻ (Halides)

🧪 (A) Acid Test

\(Cl^-\) + Conc. H₂SO₄ → HCl↑ (white fumes) + NH₄OH → Dense white fumes

\(Br^-\) + Conc. H₂SO₄ → Br₂↑ (brown vapour) + MnO₂ (O.A.) → Intensity increases

\(I^-\) + Conc. H₂SO₄ → I₂↑ (violet vapour) + MnO₂ (O.A.) → Intensity increases

🧪 (B) ppt Test with AgNO₃

\(Cl^- + AgNO_3\) → AgCl↓ White | CN⁻ → Soluble | S₂O₃²⁻ → Soluble | NH₃ → \([Ag(NH_3)_2]^+\) Soluble

\(Br^- + AgNO_3\) → AgBr↓ Pale Yellow | CN⁻ → Soluble | S₂O₃²⁻ → Soluble | NH₃ → Partially soluble

\(I^- + AgNO_3\) → AgI↓ Yellow | CN⁻ → Soluble | S₂O₃²⁻ → Soluble | NH₃/NH₄OH → Insoluble

AgF doesn't give ppt. AgCl, AgBr, AgI: covalent character increases → solubility in polar solvent like water & NH₃ decreases

🧪 (B) ppt Test with Pb(NO₃)₂

\(Cl^-\) → PbCl₂↓ (white) — Soluble in warm water

\(Br^-\) → PbBr₂↓ (white) — Soluble in hot water

\(I^-\) → PbI₂↓ (yellow) — Soluble in boiling water

Special Test for Cl⁻ — Chromyl Chloride Test

Given only by Ionic Chlorides (having ionic character)
CH₃Cl₂/CNCl₂/CCl₄ → NOT give this test (covalent)
AgCl/HgCl/SnCl₄ → NOT give this test (covalent character increase)
SnCl₂/NaCl/KCl/CaCl₂/BaCl₂ → Give this test ✓
\(Cl^-\) + Conc. H₂SO₄ + Solid K₂Cr₂O₇ → CrO₂Cl₂↑ (Red-Brown vapour) [Chromyl Chloride] + NaOH → Na₂CrO₄ (Yellow solution)

Special Test for Br⁻ & I⁻ — Layer Test

Based on oxidizing strength of halogens: F₂ > Cl₂ > Br₂ > I₂

Br⁻ + Cl₂ → Br₂ (Brown)
I⁻ + Cl₂ → I₂ (Violet)
NaBr + Cl₂ (dropwise) + CCl₄ → Two layers: NaCl (white) top + Br₂ + CCl₄/CS₂ (Brown) bottom
Avoid excess Cl₂ water — it further oxidizes I₂⁰ into IO₃⁻
If Br⁻ & I⁻ both present: Violet layer comes first, followed by red-brown layer
If red-brown layer comes first → I⁻ absent

SO₄²⁻ (Sulphate) — Class B, Subgroup 1

🧪 ppt Test

(i) \(SO_4^{2-} + BaCl_2\) → BaSO₄↓ (white) | dil. HCl → Insoluble | dil. HNO₃ → Insoluble | Aqua Regia → Soluble

(ii) \(SO_4^{2-} + Pb(NO_3)_2\) → PbSO₄↓ (white) | NaOH → Pb(OH)↓ (Insoluble) | Excess NaOH → [Pb(OH)₄]²⁻ (complex-soluble)

Aqua Regia = HCl:HNO₃ = 3:1

Test for Cations — Dry Tests

Types of Dry Tests

Flame Test
Borax Bead Test
Phosphate Bead Test
Charcoal Cavity Test
Cobalt Nitrate Cavity Test

Platinum wire is used to make platinum loop in Flame Test, Borax Bead Test and Phosphate Bead Test.

① Flame Test

Li⁺

Crimson Red

Na⁺

Golden Yellow

K⁺

Violet

Rb⁺

Red Violet

Cs⁺

Blue

Be²⁺

No flame (high IE)

Mg²⁺

No flame (high IE)

Ca²⁺

Brick Red [Green through blue glass]

Sr²⁺

Crimson Red [Purple through blue glass]

Ba²⁺

Apple Green [Bluish-green]

② Borax Bead Test — For Transition Metal Cations

Mechanism

Na₂B₄O₇·10H₂O → (Δ) → Na₂B₄O₇ → (High temp) → NaBO₂ (Soda meta borate) + B₂O₃ (Boric oxide)

→ (Δ) → Coloured meta borate [e.g. Co(BO₂)₂: Blue | Cu(BO₂)₂: Green/Blue]

d¹⁰ cations do NOT give this test — transparent glassy bead

Ion	Oxidizing Flame Cold	Oxidizing Flame Hot	Reducing Flame Cold	Reducing Flame Hot
Co²⁺	Blue	Blue	Blue	Blue
Cr³⁺	Green	Green	Green	Green
Cu²⁺	Blue	Green	Red	Colourless
Fe³⁺	Yellow	Yellowish Brown	Green	Green
Mn²⁺	Red-Brown	Violet	Grey	Colourless

③ Phosphate Bead Test

Mechanism

NaNH₄HPO₄·4H₂O (Microcosmic salt) → (Δ) → NaPO₃ + NH₃↑ + H₂O↑ (Sodium metaphosphate bead)

CuO + NaPO₃ → NaCuPO₃ [Blue] | Colors are same as in Borax Bead Test

④ Charcoal Cavity Test

Results

Salt + Na₂CO₃ → (Δ) → Coloured metal oxide

Pb²⁺ → Grey | Cd²⁺ → Brown | As²⁺ → White (garlic odour) | Zn²⁺ → White

⑤ Cobalt Nitrate Cavity Test

When to use

If white residue obtained in Charcoal Cavity Test → add 2-3 drops Co(NO₃)₂ salt and heat

ZnO·CoO → Green | Al₂O₃·CoO → Blue | MgO·CoO → Pink

Classification of Cations & Group Tests

Group	Cations	Reagent	ppt & Colour
I	Ag⁺, Hg₂²⁺, Pb²⁺	dil. HCl	AgCl, Hg₂Cl₂, PbCl₂ White
IIA	Hg²⁺, Pb²⁺, Cu²⁺, Bi³⁺, Cd²⁺	H₂S in presence of dil. HCl	PbS, CuS, Bi₂S₃ Black; CdS, As₂S₃, As₂S₅, SnS Yellow; SnS₂, Sb₂S₃ Orange; SnS → Brown
III	Al³⁺, Fe²⁺/³⁺, Cr³⁺	NH₄OH in presence of NH₄Cl	Al(OH)₃ → white gel; Fe(OH)₃ → red-brown; Cr(OH)₃ → green
IV	Ni²⁺, Co²⁺, Mn²⁺, Zn²⁺	H₂S in presence of NH₄OH & NH₄Cl	NiS, CoS → Black; ZnS → white; MnS → Buff/flesh
V	Ba²⁺, Sr²⁺, Ca²⁺	(NH₄)₂CO₃ in presence of NH₄OH & NH₄Cl	BaCO₃, SrCO₃, CaCO₃ (white ppt)
VI	Mg²⁺, Na⁺, K⁺	No common reagent	—
VII (0)	NH₄⁺	—	—

Important Group Tests

🔬 Test for Fe³⁺ (Group III)

Fe(OH)₃↓ (red-brown) + Conc. HCl → FeCl₃ solution

K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃ Prussian Blue ppt

Excess K₄[Fe(CN)₆] → K-Fe[Fe(CN)₆] Prussian Blue soluble colloid

SCN⁻ or KSCN → [Fe(SCN)]²⁺ Blood Red colour

🔬 Test for Ni²⁺ (Group IV)

NiS↓ (black) + Aqua regia → NiCl₂ (soluble) [Green] + NH₄OH + dmg → Ni(dmg)₂↓ Rosy Red / Cherry-red ppt

🔬 Test for Group V — Ba²⁺, Sr²⁺, Ca²⁺

Original solution + NH₄Cl + NH₄OH + (NH₄)₂CO₃ → white ppt (BaCO₃/SrCO₃/CaCO₃)

Preserve small amount of ppt for Flame Test: Ca²⁺ → Brick red | Ba²⁺ → Apple green | Sr²⁺ → Crimson red

Part (a): K₂CrO₄ (K-chromate) → Yellow ppt of BaCrO₄↓ (Ba²⁺ confirmed)

Part (b): (NH₄)₂SO₄ (Am. sulphate) → White ppt of SrSO₄↓ (Sr²⁺ confirmed)

Part (c): (NH₄)₂C₂O₄ (Am. oxalate) → White ppt of CaC₂O₄↓ (Ca²⁺ confirmed)

🔬 Test for NH₄⁺ (Group 0)

Always use Original solution (not filtrate, as NH₄⁺ ions will be added in mixture)

Original solution + KOH/NaOH → NH₃(g) + Nessler's reagent [K₂[HgI₄] + KOH] → Red-Brown ppt

Test for NH₄⁺: Original + KOH/NaOH → (Δ) → NH₃(g) → Glass rod dipped in HCl → NH₄Cl (white fumes) | Nessler's reagent → HgO·Hg(NH₂)I (Brown ppt)