Qualitative Analysis : Analysis of Group V Radicals

 

Analysis of Group V Radicals

Radicals : Ba2+, Sr2+ & Ca2+

Group reagent   : (NH4)2CO3 in presence of NH4Cl/NH4OH

Precipitate         : Carbonates of Ba2+, Sr2+ and Ca2+

Principle   : Carbonates of Ba, Ca & Sr are insoluble in NH4OH and they are separated as follows:

(i)      BaCrO4 is soluble in acetic acid while SrCrO4 and CaCrO4 are soluble

(ii)     CaSO4 forms soluble complex with (NH4)2SO4 while SrSO4 is insoluble.

Procedure: Filtrate of gp-IV is boiled with CH3COOH and then NH4OH is added followed by addition of (NH4)2CO3. Formation of white ppt. indicates the presence of gp-IV precipitates are filtered and washed with hot water and then treated with acetic acid. The ppt. are separated by filtration.

 

Residue: Yellow ppt. may be of BaCrO4. it is dissolved in dil. HCl and is heated to almost dryness. In flame test green or yellow-green flame is obtained.

Ba2+ confirm

Filtrate: It may contain Sr2+ or Ca2+. IN it (NH4)2SO4 is added in excess and boiled. After sometime ppt. is filtered.
Residue: White ppt. may be of Sr2+ and in flame test, red coloured flame is obtained.

Sr2+ confirm

Filtrate: White ppt. is obtained on adding ammonium oxalate. If ppt. is dissolved in dil. H2SO4 and KMnO4 solution is added dropwise, its colour disappear. In flame test brick red colour is obtained.

Ca2+ confirm.

 

NOTE:

  • For removed of H2S, CH3COOH must not be added in excess because in its excess, (NH4)2S may be formed which may be oxidized to SO42+ and so BaSO4, SrSO4 and CaSO4 may bet precipitated.
  • Carbonates of gp-V cations must be decomposed by dil. CH3COOH and not be stronger acids like HCl or H2SO4 etc. when conc. dil of H+ is high BaCrO4 will not be precipitated completely.
  • The filtrate of gp-IV must be colourless. If black, it may contains NiS which may be completely precipitated by adding CH3COOH.
  • in fifth group. NH4OH/NH4Cl must be added before addition of ammonium carbonate.
  • (i)      IN absence of NH4Cl, magnesium is precipitated as basis magnesium carbonate in fifth group. [4MgCO3. Mg(OH)2.5H2O]. It can be explained on the basis of common ion effect.
  • (ii)     Due to impurity of ammonium bicarbonate in ammonium carbonate, soluble Sr2+ and Ca2+ bicarbonates are formed if NH4OH is absent. NH4OH converts these bicarbonates into insoluble carbonates.
  •                     NH4HCO+ NH4OH → (NH4)2CO3 + H2O
  • The test must be carried out in the order Ba2+ → Sr2+ → Ca2+. Because like SrSO4, BaSO4 is also insoluble in acetic acid and like Ca2+ -oxalate, Sr2+ and Ba2+ oxalates are also insoluble.
  • Precipitation must be done in hot solution because in hot solution bicarbonates are changed to insoluble carbonates. Temperature should not be too high otherwise carbonates of gp-V will dissolve in NH4Cl and (NH4)2CO3 is auto-decomposed.
  • In place of (NH4)2CO3, K2CO3 or Na2CO3 can’t be used.

 

Reaction and Explanations:

Carbonates are precipitated as follows:

MCl2 + (NH4)2CO3 → MCO3↓ 2NH4Cl (M = Ca, Ba or Sr)

These carbonates are soluble in acetic acid.

MCO3 + 2CH3COOH → (CH3COO)2 M + H2O + CO2     

(M = Ca, Ba or Sr)

 

(a)    Tests for Ba2+:

          (i) (CH3COO)2 Ba + K2CrO4 → BaCrO4  + 2CH3COOK

Barium chromate                        (yellow ppt)

          (ii) BaCl2 is volatile and provides apple green colour to the flame.

 

(b)    Tests for Sr2+:

          (CH3COO)2Sr + (NH4)2SO4 → SrSO4 + 2CH3COONH4

                   Strontium sulphate                    (white ppt.)

          SrCl2 is volatile and provides crimson red colour to flame.

 

(c)     Tests for Ca2+:

          (CH3COO)2Ca + (NH4)2C2O4 → CaC2O4 + 2CH3COONH4

                   Calcium oxalate                             (white ppt.)

 

Qualitative Analysis : Analysis of Group VI