ANACHEM LEC: FINALS

• GROUP 4 cations contains : Barium, Strontium, Calcium
• it is also called the Alkaline Earth

  and Ammonium carbonate
• group reagent is NH4Cl + NH4OH + NH42CO3

  or namely

  Ammonium chloride , Ammonium Hydroxide, Ammonium carbonate
• they precipitated as CARBONATE in alkaline medium
• the three cations belong to the same group in PERIODIC TABLE. therefore have similar CHARACTERS and their SEPARATION is somewhat difficult
• difficult of separation because of the: COLOR of PRECIPITATE
• WHat is the procedure for separation?

  Centrifugate from group 3B

  Evaporated nearly to DRYNESS

  add NH4CL

  add NH4OH

  HEAT

  add NH4CO3

  HEAT to 60 degrees Celsius

  Centrifuge

  Residue : Group 4 cations : White ppt

  Centrifugate : group 5
• Why do we evaporate the centrifugate from group 3 B?

  to expel H2s to prevent oxidation of S(sulfur) to SO4^2 (sulfate) and prevent precipitation of Barium and Strontium as SO4^2 (sulfate). CaSO4 (Calcium Sulfate) has higher Ksp so it needs MORE SO4 (sulfate) ion concentration to precipitate
• Why do we add a moderate amount of NH4+ ?

  because excess ammonium (NH4+) will decrease carbonate ion concentration.

  = NH4+ + CO3^2 -> NH3 + HCO3
• Why do we add a moderate amount of NH4+ ?

  solution must be a little basic. if acidic, the carbonate will change to HCO3. which are SOLUBLE= CO3 + H+ -> HCO3

  HCO3 is Bicarbonate which is soluble
• Why do we use NH4Cl?

  By common ion effect , it decreases hydroxide(OH) ion concentration and carbonate ion(CO3) concentration to prevent precipitation of Magnesium (Mg) as Magnesium hydroxide (Mg(OH)2) or Magnesium carbonate (MgCO3)

  to coagulate the precipitate
• Why we use NH4OH?

  it converts Ammonium bicarbonate (NH4CO3) to Ammonium carbonate (NH4)2CO3

  
  

  NH4HCO3 + NH3 -> (NH4)2CO3
• Why we use NH4OH?

  converts ammonium carbamate into carbonate

  NH4CO2NH2 + H2O -> (NH4)2CO3
• WHy we use NH4OH?

  TO make the medium faintly alkaline to prevent Bicarbonate (HCO3) formation as all metal bicarbonates are soluble
• Why we use NH4OH?

  ammonium carbonate is prepared from EQUIMOLAR concentration of ammonium bicarbonate NH4(HCO3)2 and ammonium carbamate NH4CO2NH2
• Why we heat to 60 degrees celsius and not boil?

  to convert HCO3 to CO3

  M(HCO3)2 -> MCO3 + CO2 + H2O
• Why we heat to 60 degrees celsius and not boil?

  Warming aids in the formation of more crystalline precipitate

  Warming aids in the conversion of carbamate to carbonate

  EXCESS HEAT cause loss of Ammonia : Carbonate will convert to Bicarbonate by pushing the reaction FORWARD
• Separation of GROUP 4 :

  Precipitate + acetic acid = precipitate will dissolve as acetate

  BaCO3 + 2 CH3COOH = Ba(CH3COO)2 + CO2 +H2O

  SrCO3 + 2 CH3COOH = Sr(CH3COO)2 + CO2 +H2O

  CaCO3 + 2 CH3COOH =Ca(CH3COO)2 + CO2 +H2O

  acetates are SOLUBLE
• TEST for Barium:

  1 to 2 drops of the solution

  add Potassium Chromate (K2CrO4) , if yellow ppt, Ba is present

  if Barium is present, add potassium chromate till solution acquires orange tint

  
  

  :

  Centrifuge -> residue: BaCrO4 (yellow ppt), centrifugate: Sr, Ca (red ppt) + add NH4OH + NH4Cl + NH42CO3 + HEAT 60 celsius -> centrifuge -> residue: SrCO3, CaCO3 (white ppt) , centrifugate : reject
• Test for barium:

  FLAME test

  ppt of BaCrO4

  add HCl

  = Soluble BaCl2 which is volatile in flame

  gives apple green color

  if barium is ABSENT, reject the portion used for test and the remainder of the sol. is used for strontium and calcium
• Test for barium:

  if we add excess K2CrO4, Strontium chromate and Calcium chromate will also precipitate, as the chromate of this group are INSOLUBLE. but Ksp of BaCrO4 < SrCro4 < CaCrO4 . so we add sufficient amount of K2Cro4 to precipitate barium but not strontium and calcium

  
  

  we also use acetic acid to convert part of potassium chromate to potassium dichromate to decrease chromate ion concentration
• Why do we not use HCl or HNO3 , instead of acetic acid?

  because they are strong acids which convert chromate ion to dichromate ions and NO PRECIPITATE is formed.
• Identification of Strontium and Calcium:

  Residue dissolve in acetic acid

  add Ammonium sulfate

  Heat

  Centrifuge -> Residue: Strontium ; Centrifugate: Calcium

  Strontium acetate + ammonium sulfate -> Strontium Sulfate (SrSO4) (white ppt) + 2 CH3COONH4
• Identification of Strontium and Calcium:

  the test depends on that Calcium can form DOUBLE SALT with ammonium sulfate which is SOLUBLE

  Calcium + ammonium sulfate = Ammonium Calcium Sulfate [ (NH4)2Ca(SO4)2 ]

  if we used dilute H2SO4 instead of Ammonium sulfate, Both Strontium and Calcium will precipitate as Sulfate
• Test for calcium:

  solution

  add ammonium hydroxide (NH4OH)

  add ammonium oxalate [ (NH4)2C2O4 ]

  * C2O4(NH4)2 + Calcium -> C2O4Ca (white ppt) + 2 NH4Cl
• Test for calcium:

  solution

  add ammonium chloride (NH4Cl)

  add potassium ferrocyanide

  result in pale yellow precipitate

  *Calcium + NH4 + K4[Fe(CN)6] -> Triple Ferrocyanide [ CaNH4K[Fe(CN)6]
• Test for calcium and strontium:

  Flame test

  Calcium oxalate (CaC2O4) + HCl = CaCl2 , volatile in flame with BRICK RED color
• Test for calcium and strontium:

  Flame test

  Strontium Sulfate (SrSO4) is heated in the REDUCING ZONE of the flame = SrS + HCl = SrCl2 , volatile in flame with CRIMSON RED color
• Group 5 consists of : Magnesium, Sodium, Potassium, Ammonium
• Group 5 is called Alkali metal group

  and Soluble group because it has no group reagents

  • all Sodium, potassium ,ammonium salts are soluble
• Removal of traces of alkaline earth group:

  Centrifugate of group 4

  add ammonium oxalate to precipitate any CALCIUM

  add ammonium sulfate to precipitate any BARIUM or STRONTIUM

  Heat

  centrifuge -> Residue: Reject ; Centrifugate: Group 5
• Removal of traces of alkaline earth group:

  divide the centrifugate into two unequal portions

  The smaller portion : Test for Magnesium

  > solution + conc. HCl + Na2HPO4 ( Disodium phosphate)

  > Render alkaline excess by NH4OH = White Gel ppt

  * Mg + NH4 + PO4^3 -> MgNH4PO4 (white ppt)

  * Mg + OH excess NH4OH -> Mg(OH)2 (white ppt)
• Removal of traces of alkaline earth group:

  divide the centrifugate into two unequal portions

  The larger portion : Test for Sodium and Potassium

  > Before we test for Potassium, we must remove Ammonium

  because ammonium gives all POSITIVE tests for Potassium
• Removal of traces of alkaline earth group:

  Larger portion:

  Removal of Ammonium:

  Larger portion

  Heat

  add conc. HNO3

  Heat, evaporate to dryness

  Residue is dissolved in WATER and divided to 2 portions this treatment also remove Magnesium as Magnesium Oxide

  NH4 + HNO3 -> NH4NO3

  NH3NO3 --heat--> N2O + NH3
• Removal of traces of alkaline earth group:

  TEST for sodium :

  By potassium dihydrogen antimonate or potassium pyroantimonate

  > sol. + KH2SbO4 / KH2SbO7 = White ppt

  > KH2SbO4 + Na = NaH2SbO4 + K

  > K2H2Sb2O7 + 2 Na = Na2H2Sb2O7 + 2 K
• Removal of traces of alkaline earth group:

  TEST for sodium :

  By magnesium uranyl acetate = Yellow ppt

  > Na + HMg(UO2)3 (CH3COO)9 = NaMg(UO2)3(CH3COO)9 + H

  > NH4 + HNO3 -> NH4NO3
• Removal of traces of alkaline earth group:

  TEST for sodium :

  flame test

  > Golden Yellow color
• Removal of traces of alkaline earth group:

  TEST for Potassium

  By Sodium Cobaltic Nitrite = Yellow ppt

  > Na3[Co(NO2)6] + 3 K --> K3[Co(NO2)6] + 3 Na
• Removal of traces of alkaline earth group:

  TEST for Potassium:

  Flame test = Violet color in flame
• Removal of traces of alkaline earth group:

  TEST for Ammonium :

  must be tested on the original solid because we add NH4CL and NH4OH during the course of analysis

  > SOLID + NaOH -> NH3 Odor

  > NH4 + NaOH -> NH3 + H20