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Tests for Anions

Tests for Anions. Need to know test for Chloride Sulfate/sulfite carbonate/hydrogen carbonate nitrate phosphate (NB know confirmatory test too!). Chloride. Add AgNO 3 Get white ppt Confirm = ppt dissolves in dilute ammonia Equation needed Ag + + Cl -  AgCl (  ).

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Tests for Anions

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  1. Tests for Anions Need to know test for • Chloride • Sulfate/sulfite • carbonate/hydrogen carbonate • nitrate • phosphate • (NB know confirmatory test too!)

  2. Chloride • Add AgNO3 • Get white ppt • Confirm = ppt dissolves in dilute ammonia • Equation needed • Ag+ + Cl - AgCl ()

  3. Sulfate/sulfite • Add BaCl2 • Get white ppt with both sulfate and with sulfite • To Distinguish you… • add dil HCl to white ppt • ppt remains = sulfate • ppt dissolves = sulfite (sulfite doesn’t stay white) Equation needed !!

  4. Sulfate/sulfite Equations • With barium chloride • Ba2+ + SO42- BaSO4() • Ba2+ + SO32- BaSO3() • Distinguishing test • BaSO4 + HCl  no reaction • BaSO3 SO32- + 2H+  SO2 + H20 (ppt dissolves)

  5. CO32- /HCO3- • Add dil. HCl (or any acid) • Get bubbles of CO2 (limewater milky) with each • To Distinguish you…. • add MgSO4 to fresh solution • Get white ppt. immediately = carbonate white ppt on heating = hydrogen carbonate Equation needed !!

  6. CO32- /HCO3- EQUATIONS • With dilute HCl • CO32- + 2H +  CO2() + H2O • HCO3- + H +  CO2() + H2O • Limewater • Ca(OH)2 + CO2  CaCO3 () + H2O Milky white

  7. CO32- /HCO3- • Distinguishing test • Mg 2++ CO32-  MgCO3 () white • Mg 2+ + HCO3- Mg(HCO3- )2 soluble • Mg(HCO3- )  MgCO3 () + CO2() + H2O (when heated) white

  8. Nitrate • Brown Ring Test • Add fresh FeSO4 At slant add conc. H2SO4 drop wise • Get brown ring at junction of 2 layers No equation needed

  9. Phosphate • Add ammonium molybdate • Add 5 drops of conc. nitric acid (warm the solution in water bath 40oC) • Get yellow ppt No equation needed Confirm: Goes colourless when add dilute NH3

  10. Why do we need to know? • Nitrates and phosphates are present in fertilisers and detergents • Excess nitrates & phosphates runoff into rivers and lakes • This fertilises the algae – causing algal bloom – lake goes green • Algae die • Dead algae are decomposed by aerobic bacteria • This uses up oxygen • Fish and other animals die • Eutrophication = excess nutrients in water

  11. Questions • You add ammonium molybdate when testing for ? • Phosphates • You add silver nitrate when testing for ? • Chlorides • You use Magnesium sulfate for? • Distinguishing between carbonates and hydrogencarbonates • (white ppt formed immediately means carbonate)

  12. Questions • BaCl2 is used to test for which two anions? • Sulfates and sulfites • You add conc.nitric acid when testing for ? • phosphates • You add dilute HCl in which 4 anion tests • Gives CO2 with both carbonates and hydrogencarbonates • As distinguishing test for sulfates and sulfites – both gave white ppt with BaCl2 • (sulfate – ppt unchanged)

  13. Questions • AgNO3 is used to test for ? • Chloride • You add conclusion can you draw if a white ppt forms when BaCl2 is added to a solution ? • Solution is either a sulfate or sulfite • What is the confirmatory test for phosphates? • The yellow ppt(from addition of ammonium molybdate, conc HNO3 and gentle heating ) dissolves in dilute ammonia solution (NH4OH)

  14. The End!

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