AG
Tests for Anions
Need to know test for
• Chloride
• Sulfate/sulfite
• carbonate/hydrogen carbonate
• nitrate
• phosphate
• (NB know confirmatory test too!)
AG
Chloride
• Add
AgNO3
• Getwhite ppt
• Confirm = ppt dissolves in dilute ammonia
• Equation needed• Ag+ + Cl - AgCl ()
AG
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 !!
AG
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)
AG
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 !!
AG
CO32- /HCO3
-
EQUATIONS• With dilute HCl
• CO32- + 2H + CO2() + H2O
• HCO3- + H + CO2() + H2O
• Limewater • Ca(OH)2 + CO2 CaCO3 () + H2O
Milky white
AG
• 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
-
AG
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
AG
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
AG
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
AG
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)
AG
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)
AG
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)
AG
The End!