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QUALITATIVE ANALYSIS

QUALITATIVE ANALYSIS. Solubility & Colour Tests for Cations Tests for Anions Tests for Gases Tests for Oxidising / Reducing Agents. What is QA ?. The process of determining the identity of an unknown substance, based on a series of chemical tests.

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QUALITATIVE ANALYSIS

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  1. QUALITATIVE ANALYSIS Solubility & Colour Tests for Cations Tests for Anions Tests for Gases Tests for Oxidising / Reducing Agents

  2. What is QA ? • The process of determining the identity of an unknown substance, based on a series of chemical tests. • Salts consist of cations (+ ions) and anions (- ions).

  3. Solubility & Colour of salts • The solubility of a salt can help us narrow down the possible identity of the salt. • Soluble : All nitrates, hydrogen carbonates, & sodium, potassium & ammonium salts. Most sulphates, chlorides, bromides & iodides. A few carbonates & hydroxides.

  4. Insoluble : Most carbonates & hydroxides (except Na+, K+ & NH4+ ); PbSO4 , BaSO4 , CaSO4 , AgCl , PbCl2 , AgBr, PbBr2, AgI, PbI2.

  5. The colour of a salt also can help narrow down the possible identity of the salt. • E.g. : • Blue : Cu 2 + / Co2+ salts • Green : Cu2+ / Ni2+ salts • Pale Green : Fe2+ salts • Reddish brown : Fe3+ salts / Cu metal • Grey : Metals • Black : Metal oxides / sulphides i.e. CuO , CuS

  6. Action of Heat • On heating a solid, things to observe are : • Change in colour of solid • Change in state • Effervescence ( Test for gas evolved ) • The colour changes involved & the gases liberated will give us a guide to the identity of the cations / anions present.

  7. Change in colour : • White solid Yellow White • Zinc ions likely to be present. • White solid Orange Yellow • Lead (II) ions likely to be present.

  8. Gas liberated : • Carbon dioxide Carbonate / Combustion of fuel or carbon • Hydrogen Metal - Acid Reaction • Oxygen Nitrates / Oxides of less reactive metals / Hydrogen peroxide

  9. Ammonia Ammonium salts • Chlorine Chlorides / Chlorates • Sulphur dioxide Sulphites / Sulphates

  10. Testing for Cations • Add sodium hydroxide solution until excess OR • Add aqueous ammonia until excess • Observe for the presence of a precipitate & note its colour; then the solubility of the ppt in excess alkali.

  11. Testing for Cations

  12. Testing for Cations

  13. Testing for Anions • Carbonates : • All carbonates react with dilute acids (HCl & H2SO4) to form salt, water & CO2 gas. • Add dilute acid to a portion of the unknown solid & look for effervescence. Bubble the gas formed into limewater. If the gas is CO2, a white ppt is formed with limewater (aq calcium hydroxide).

  14. For Chlorides : • Chloride ions react with aqueous silver nitrate to form a white ppt which is insoluble in dilute nitric acid, but soluble in aqueous ammonia. • To a portion of the solution of the unknown, add dilute nitric acid followed by aqueous silver nitrate. Formation of a white ppt indicates the presence of chloride ions. Ag+(aq) + Cl-(aq) AgCl(s)

  15. For Iodides : • Iodide ions react with aqueous silver nitrate / aqueous lead (II) nitrate to form a yellow ppt of silver iodide or lead (II) iodide respectively. The ppt is insoluble in both dilute nitric acid & aqueous ammonia.

  16. To a portion of the solution of the unknown, add dilute nitric acid followed by aqueous silver nitrate / aqueous lead (II) nitrate. Formation of a yellow ppt indicates the presence of iodide ions. Ag+(aq) + I- (aq) AgI(s) Pb2+(aq) + 2I-(aq) PbI2(s)

  17. For Sulphates: - To a portion of the solution of the unknown, add dilute nitric acid followed by aqueous barium nitrate OR dilute hydrochloric acid followed by aqueous barium chloride. • Formation of a white ppt indicates the presence of sulphate ions. Ba 2+(aq) + SO42-(aq) BaSO4(s)

  18. For Nitrates : • To a portion of the solution of the unknown, add aqueous sodium hydroxide, followed by aluminium foil / Devarda’s alloy. Warm the mixture gently and test for evolution of ammonia gas. • Evolution of a colourless pungent gas that turns moist red litmus blue indicates the presence of nitrate ions.

  19. Test for Reducing Agents • Using Acidified Potassium Manganate (VII) solution • To a sample of the solution of the unknown substance, add an equal portion of aqueous acidified potassium manganate (VII). • If the acidified potassium manganate (VII) solution is decolourised (No purple colouration remaining), the unknown solution contains a reducing agent.

  20. Test for Reducing Agents • Using Acidified Potassium Manganate (VII) solution • Note the colour of the resulting solution. • i.e. Solution X decolourises aq. acidified potassium manganate (VII). Solution X turns from pale green to yellow. (Resulting solution is yellow)

  21. Test for Reducing Agents • Using Acidified Potassium Dichromate (VI) solution • To a sample of the solution of the unknown substance, add an equal portion of aqueous acidified potassium dichromate (VI). • If the acidified potassium dichromate (VI) solution turns from orange to green, the unknown solution contains a reducing agent.

  22. Test for Oxidising Agents • Using Acidified Potassium Iodide solution • To a sample of the solution of the unknown substance, add an equal portion of aqueous acidified potassium iodide. • If the solution turns brown (and formation of a black ppt on standing), the unknown solution contains an oxidising agent.

  23. Test for Oxidising Agents • Using Iron (II) Sulphate solution • To a sample of the solution of the unknown substance, add an equal portion of aqueous iron (II) sulphate. • If the pale green solution turns yellow (oxidation of iron (II) ions to iron (III) ions), the unknown solution contains an oxidising agent.

  24. Tests for Gases • Hydrogen: - Insert a lighted splint into a tube of the gas. If the gas extinguishes the lighted splint with a ‘pop’ sound, the gas is hydrogen gas.

  25. Tests for Gases • Oxygen: - Insert a glowing splint into a tube of the gas. If the gas relights a glowing splint, the gas is oxygen gas.

  26. Tests for Gases • Carbon dioxide: - Bubble the gas into a tube of aqueous calcium hydroxide (limewater). If the gas forms a white precipitate with limewater, the gas is carbon dioxide gas.

  27. Tests for Gases • Chlorine: - Place a piece of moist blue litmus paper at the mouth of a tube of the gas. If the gas turns moist blue litmus red and bleaches it, the gas is chlorine gas.

  28. Tests for Gases • Ammonia: - Place a piece of moist red litmus paper at the mouth of a tube of the gas. If the gas turns moist red litmus blue, the gas is ammonia gas.

  29. Tests for Gases • Sulfur dioxide: - Dip a piece of filter paper into aqueous acidified potassium manganate(VII) and insert the paper into a tube of the gas. If the gas turns purple acidified potassium manganate (VII) colourless (if the gas decolourises purple acidified potassium manganate(VII)), the gas is sulfur dioxide gas.

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