Salt Analysis — Anions Why anion analysis matters Anion (negative ion) identification is a core lab skill and a NEET-favorite. In a typical unknown salt, you first screen for the anion because many cation tests are chosen or masked depending on the anion present. The classic scheme groups anions by their behavior with acids and with specific reagents so that you can move quickly from a first observation (like a gas or a precipitate) to a firm confirmation. A tidy bench helps you spot quick observations—bubbles, color changes, precipitates—without confusion. Label acids (dil. H2SO4, conc. H2SO4), AgNO3, BaCl2, FeSO4, ammonium molybdate, and ammonia. The three groups of anions (the workflow map) Think of the scheme as three doors: - Door 1 (dilute acid): Add dilute HCl/H2SO4. If a gas appears, identify it by smell and simple papers (lime water, lead acetate, starch-iodide) and then confirm. - Door 2 (concentrated H2SO4): If no gas with dilute acid, try conc. H2SO4 in the cold carefully—certain anions give characteristic fumes (halides, nitrate). - Door 3 (special reagents): If no gas with acids, use specific reagents (Ba2+ for sulfate, ammonium molybdate for phosphate, ethanol + conc. H2SO4 for borate, Ca2+ for fluoride). Group Classification of anions by first test Group Add this Typical anions What you first see Dilute acid group Dilute H2SO4 or HCl CO3 2− , SO3 2− , S 2- , NO2 − , CH3COO − Gas with distinct smell/behavior (bubbles) Concentrated H2SO4 group Conc. H2SO4 (cold, carefully) Cl - , Br - , I - , NO3 − Fumes: HCl (white fumes with NH3), Br2 (reddish-brown), I2 (violet), brown ring (NO3− confirmatory with FeSO4) Special-reagent group Specific reagents SO4 2− , PO4 3− , BO3 3− , F - No gas; characteristic precipitate or flame/odor test Three-group flow: Dilute acid → gases (CO2/SO2/H2S/NO2, ester odor). Concentrated H2SO4 → halide/nitrate fumes. Special reagents → SO4/PO4/BO3/F− confirmations. tip General safety: Work in a fume hood for conc. H2SO4 and NOx/H2S. Smell by wafting only. Always acidify before adding BaCl2 or AgNO3 to avoid false positives from CO3 2− /OH−. Dilute acid group — Anions that give gases Carbonate ( CO3 2− ) Test: Add dilute H2SO4 or HCl to the solid or solution. Effervescence of carbon dioxide (CO2; SMILES: O=C=O) appears. Pass the gas into lime water (Ca(OH)2; SMILES: O[Ca]O): it turns milky due to CaCO3, then clears in excess CO2 due to soluble Ca(HCO3)2. Evolution of CO2 from carbonate with acid (must-see effervescence). Lime water turns milky due to CaCO3. Milkiness dissolves in excess CO2 forming soluble calcium hydrogencarbonate. Sulfide ( S 2- ) Test: On adding dilute HCl, rotten-egg smelling hydrogen sulfide (H2S; SMILES: S) is evolved. Confirm with moist lead acetate paper (turns black due to PbS) and with AgNO3 (black Ag2S). Liberation of H2S from sulfide with acid. Blackening of lead acetate paper by H2S. Black silver sulfide with AgNO3 confirms S 2- . Sulfite ( SO3 2− ) Test: With dilute H2SO4, suffocating sulfur dioxide (SO2; SMILES: O=S=O) is evolved. Observations: decolorizes acidified KMnO4 (purple to colorless), reduces acidified K2Cr2O7 (orange to green), and turns lime water milky (CaSO3) which then dissolves in excess due to formation of bisulfite. Evolution of SO2 from sulfite with acid. Lime water turns milky due to calcium sulfite. Milkiness dissolves in excess SO2 (bisulfite formation). Nitrite ( NO2 − ) Test: With dilute mineral acid, brown fumes of nitrogen dioxide (NO2) are observed (NO formed first gets oxidized by air to NO2). Starch-iodide paper turns blue because liberated iodine (from nitrous acid oxidation of I−) forms the starch–iodine blue complex. Acetate (CH3COO−; from sodium acetate CH3COONa, SMILES: CC(=O)[O-].[Na+]) Two useful confirms: - Neutral FeCl3 gives a blood-red complex [Fe(CH3COO)3] (discharged on adding mineral acid). FeCl3: SMILES: O=Fe(=O)=O with ligands; commonly used as aqueous solution. - Ester test: Warm the salt with ethanol (SMILES: CCO) + conc. H2SO4 + a pinch of oxalic acid; fruity smell of ethyl acetate (SMILES: CCOC(=O)C) confirms acetate. CO3 2− CO2 Colorless, odorless Turns lime water milky, clears in excess SO3 2− SO2 Colorless, choking Decolorizes acidified KMnO4; K2Cr2O7 orange → green S 2- H2S Rotten-egg Blackens lead acetate paper; AgNO3 → black Ag2S NO2 − NO/NO2 Brown fumes (NO2) Blue on starch-iodide paper CH3COO − No characteristic gas Fruity odor on esterification Ethyl acetate smell; FeCl3 blood-red complex Gases on adding dilute acid — quick recognition Anion Gas evolved Color/smell Quick test Anion All acidic gases look alike; you can’t really tell CO2, SO2, and H2S apart. Each has a signature: CO2 is odorless and turns lime water milky then clears; SO2 is choking and decolorizes acidified KMnO4/K2Cr2O7 and also turns lime water milky then clears; H2S smells like rotten eggs and blackens lead acetate paper. Concentrated H2SO4 group — Halides and nitrate When no gas with dilute acid, carefully add conc. H2SO4 (cold). Observe fumes, then perform specific confirmations, especially with silver nitrate and ammonia. Chloride (Cl−) Conc. H2SO4 liberates pungent HCl gas (SMILES: Cl), which gives dense white fumes with ammonia (NH3; SMILES: N). With AgNO3 in presence of dilute HNO3, a curdy white AgCl precipitate forms; it dissolves in aqueous NH3 (forms [Ag(NH3)2]+). Chromyl chloride test is a hallmark confirmation for Cl−. Conc. H2SO4 liberates HCl gas from chlorides. White AgCl with acidified AgNO3. AgCl dissolves in NH3 due to complex formation. Chromyl chloride test: red-brown vapors of CrO2Cl2 form only with chloride. Chromyl chloride test: red-brown CrO2Cl2 fumes evolved from a hot test tube containing K2Cr2O7 + solid chloride + conc. H2SO4; fumes led into NaOH. Later, Pb(CH3COO)2 gives yellow PbCrO4. 2026-05-26T17:05:54.445Z Chromyl chloride test apparatus: test tube with solid NaCl + K2Cr2O7 + conc. H2SO4 emitting red-brown CrO2Cl2 fumes; delivery tube into NaOH beaker turning yellow (chromate). Labels: reagents, fumes, NaOH receiver, later PbCrO4 step. Clean 2D vector, arrows in red, white background. gpt-image-2 Bromide (Br−) and Iodide (I−) With conc. H2SO4, HBr and HI are first formed; they get oxidized to halogens. Observations: - Br−: Reddish-brown Br2 fumes (SMILES: BrBr). AgNO3 gives pale yellow AgBr, sparingly soluble in NH3. - I−: Violet I2 vapors (SMILES: I[I]). AgNO3 gives yellow AgI, insoluble in NH3. Starch with I2 gives deep blue color. Color comparison of silver halides: AgCl (white, dissolves in NH3), AgBr (pale yellow, partly soluble in NH3), AgI (yellow, insoluble in NH3). 2026-05-26T17:05:54.643Z Three side-by-side labeled test tubes with precipitates: AgCl (milky white, dissolves in NH3), AgBr (pale yellow, partial dissolution), AgI (canary yellow, no dissolution). Include small NH3 addition icons. Clean vector style, neutral palette. gpt-image-2 Nitrate (NO3−): the brown ring test Procedure: To the nitrate solution, add freshly prepared FeSO4 (aq). Carefully pour conc. H2SO4 down the side so it forms a lower acid layer. A brown ring appears at the junction due to the nitrosyl complex [Fe(H2O)5(NO)] 2+ . This distinguishes nitrate from nitrite (which gives brown NO2 fumes directly with dilute acid). Key step in brown ring formation: nitrosyl complex of Fe(II). Brown ring at the acid–aqueous interface confirms nitrate. Pour conc. H2SO4 carefully along the test tube wall. Fe2+ in strong acid reduces nitrate to NO; NO then binds to Fe2+ aquo complex to form the brown nitrosyl complex at the interface. In strongly acidic medium, Fe2+ reduces nitrate to nitric oxide (NO) at the cold interface. NO coordinates to [Fe(H2O)6]2+ replacing one H2O to give [Fe(H2O)5(NO)]2+, seen as the brown ring. Layering is critical: NO is generated where conc. H2SO4 meets FeSO4 solution, so the ring forms exactly at that boundary. Fresh FeSO4, cold, add H2SO4 along test tube wall. Brown ring test — what actually happens complex precursor Ferrous ion aquo complex hexaaquairon(II) nitric oxide ligand formed in situ Nitric oxide brown-ring species Nitrosyl complex pentaaquanitrosyliron(II) neet-alert Brown ring pitfalls: Use freshly prepared FeSO4; do not shake; pour conc. H2SO4 down the wall to create layers; perform cold. Brown gas with dilute acid indicates nitrite, not nitrate. The classic brown ring forms for nitrate due to [Fe(H2O)5(NO)] 2+ . Nitrite gives brown NO2 fumes with dilute acid instead of the ring. Any nitrogen-containing anion gives the brown ring. Special-reagent group — No gas with acids Sulfate ( SO4 2− ) Test: To the solution acidified with dilute HCl, add BaCl2. A dense white precipitate of BaSO4 forms, insoluble in conc. HCl/HNO3. Acidification prevents interference from CO3 2− / SO3 2− . White BaSO4 confirms sulfate; the precipitate is insoluble in strong acids. Phosphate ( PO4 3− ) Test: Warm the solution with ammonium molybdate ((NH4)2MoO4) in nitric acid medium. A canary-yellow crystalline precipitate of ammonium phosphomolybdate forms. Formation of yellow ammonium phosphomolybdate (schematic stoichiometry). Close-up of a test tube showing bright yellow crystalline precipitate forming upon heating a colorless solution; labels: sample + HNO3 + (NH4)2MoO4; temperature 60 C ; white background, vector style. gpt-image-2 Yellow crystalline ammonium phosphomolybdate forming on gentle warming with ammonium molybdate in nitric acid medium. 2026-05-26T17:05:55.305Z Borate ( BO3 3− ) Test: Mix the solid/solution with ethanol (SMILES: CCO) and conc. H2SO4, warm gently to form triethyl borate (B(OEt)3; SMILES: B(OCC)(OCC)OCC). On ignition, it burns with a green-edged flame. 2026-05-26T17:05:55.206Z Green-edged flame of ethyl borate confirming borate anion. gpt-image-2 Small spirit lamp/flame test loop with a test tube emitting a green-edged flame labeled 'ethyl borate'. Include reagents: ethanol + conc. H2SO4 + borate salt. Clean vector, dark flame background, labels outside. Fluoride (F−) Simple check: Add CaCl2 to give white CaF2, insoluble in acetic acid (distinguishes from CaCO3/CaSO3). Do not generate HF with conc. H2SO4 in the open—hazardous and etches glass. Anion High-yield confirmatory tests to remember Anion Confirmatory test Observation Cl− Chromyl chloride (K2Cr2O7 + conc. H2SO4 + dry chloride) Red-brown CrO2Cl2 fumes; yellow PbCrO4 after absorption in NaOH and adding Pb2+ Cl−, Br−, I− AgNO3 in presence of dil. HNO3; then add NH3 AgCl white (soluble in NH3); AgBr pale yellow (sparingly soluble); AgI yellow (insoluble) NO3− Brown ring (FeSO4 + layered conc. H2SO4) Brown ring at interface ([Fe(H2O)5NO] 2+ ) SO4 2− BaCl2 after acidifying White BaSO4 insoluble in conc. HCl/HNO3 PO4 3− Ammonium molybdate in HNO3, warm Yellow crystalline ammonium phosphomolybdate BO3 3− Ethanol + conc. H2SO4 (ethyl borate flame) Green-edged flame They differ in color and solubility in ammonia: AgCl is white and dissolves; AgBr is pale yellow and partly dissolves; AgI is yellow and does not dissolve. All silver halide precipitates look the same. Environmental link: Elevated NO3−/NO2− and PO4 3− / SO4 2− in water can signal contamination and eutrophication. High nitrate in infant drinking water risks methemoglobinemia ("blue baby" disorder). remember Brown ring = nitrate; brown gas with dilute acid = nitrite. Chromyl chloride works only for chloride (Br−/I− get oxidized to Br2/I2; no CrO2Cl2). Always acidify before BaCl2 (to avoid false BaCO3/BaSO3). For halides, learn both color and NH3 solubility order: AgCl > AgBr >> AgI. NEET traps and quick fixes Use this three-group map as your mental checklist before touching any reagent. 2026-05-26T17:05:54.320Z Silver halide comparison under identical lighting to train your eye for NEET: AgCl (white), AgBr (pale yellow), AgI (yellow), with NH3 solubility cues. gpt-image-2 Four-panel mini reference: three labeled precipitate vials and a panel showing 'NH3 added' with dissolution extent bars. Clean textbook vector style. Memorize the look: a sharp brown band at the acid–aqueous boundary is the signature of nitrate. Systematic identification of negative ions in a salt using group reagents and confirmatory tests. Anion analysis Anions that liberate characteristic gases with dilute HCl/H2SO4 ( CO3 2− , SO3 2− , S 2- , NO2−, CH3COO−). Dilute acid group Anions that respond to conc. H2SO4 with distinctive fumes (Cl−, Br−, I−, NO3−). Concentrated acid group Nitrate identification using FeSO4 and layered conc. H2SO4 to form [Fe(H2O)5(NO)] 2+ at the interface. Brown ring test Specific test for chloride forming CrO2Cl2 red-brown fumes; not given by Br−/I−. Chromyl chloride test Yellow crystalline precipitate formed from phosphate with ammonium molybdate in nitric acid. Ammonium phosphomolybdate Green-edged flame due to burning of triethyl borate formed from borate + ethanol + conc. H2SO4. Ethyl borate flame test AgCl dissolves in NH3, AgBr partly, AgI not—crucial to distinguish halides. Silver-halide solubility CO2/SO2 passed through Ca(OH)2 turns it milky; excess gas clears the milkiness via soluble bicarbonate/bisulfite. Lime-water test Detects oxidizers that liberate iodine, forming a blue complex with starch (used with nitrite tests). Starch-iodide paper Detects H2S by formation of black PbS. Lead-acetate paper Nutrient enrichment ( PO4 3− , NO3−) of water bodies causing algal blooms and oxygen depletion. Eutrophication Key terms, quickly Start with a tiny sample: add dilute H2SO4. If gas evolves, trap/waft and test (lime water, lead acetate, starch-iodide). If no gas: add conc. H2SO4 carefully in the cold; observe characteristic fumes. Halides: perform AgNO3 test (acidified), then NH3 solubility check; for Cl−, do chromyl chloride to confirm. For nitrate: do the brown ring test with fresh FeSO4 and layering. If still unknown: move to special reagents—BaCl2 (acidified) for sulfate, ammonium molybdate (HNO3) for phosphate, ethanol + conc. H2SO4 for borate, CaCl2 for fluoride. Cross-check with one more independent confirmatory test if available. Note interferences (carbonate/sulfite) and repeat with acidification where required. Fast workflow to identify the anion Mohr method (titrimetric) also confirms chloride using K2CrO4 indicator—kept for volumetric analysis (separate module), but do know the name for theory-based questions. tip Once you see a group test result, you can skip the confirmatory test. Preliminary observations can overlap (e.g., lime-water milkiness from both CO2 and SO2). Always run a specific confirmatory test to avoid misidentification.