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Tests for Anions

Need to know test for

• Chloride

• Sulfate/sulfite

• carbonate/hydrogen carbonate

• nitrate

• phosphate

• (NB know confirmatory test too!)

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Chloride

• Add

AgNO3

• Getwhite ppt

• Confirm = ppt dissolves in dilute ammonia

• Equation needed• Ag+ + Cl - AgCl ()

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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 !!

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Sulfate/sulfite

Equations • With barium chloride

• Ba2+ + SO4 2- BaSO4 ()• Ba2+ + SO3 2- BaSO3 ()

• Distinguishing test • BaSO4 + HCl no reaction

• BaSO3

SO3 2- + 2H+ SO2 + H20 (ppt dissolves)

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CO32- /HCO3

-

• Adddil. HCl (or any acid)

• Get bubbles of CO2 (limewater milky) with each

• To Distinguish you….• add

MgSO4 to fresh solution• Get

white ppt. immediately = carbonatewhite ppt on heating = hydrogen carbonateEquation needed !!

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CO32- /HCO3

-

EQUATIONS• With dilute HCl

• CO32- + 2H + CO2() + H2O

• HCO3- + H + CO2() + H2O

• Limewater • Ca(OH)2 + CO2 CaCO3 () + H2O

Milky white

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• Distinguishing test

• Mg 2+ + CO3

2- MgCO3 () white

• Mg 2+ + HCO3

- Mg(HCO3- )2 soluble

• Mg(HCO3- ) MgCO3 () + CO2() + H2O (when heated)

white

CO32- /HCO3

-

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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

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Phosphate

• Addammonium 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

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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

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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)

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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)

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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)

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The End!