Qualitative Analysis : Acid radicals-Analysis of Category-3

 

Analysis of Category – 3: Acid Radicals

This group includes the acid radicals which are not decomposed either by dilute or conc. acids.

 

Experiment/Observation Interference
1. Sulphate (SO42–) : water extract is acidified with dil. HCl and BaCl2 is added in it. A white ppt. which is insoluble in conc. HCl or conc. HNO3 is obtained. White ppt. is of BaSO4 and salt contains (SO42–: (sulphate)
Reactions:
 Na2SO4 + BaCl2 → BaSO↓ + 2NaCl White ppt. of BaSO4 is obtained
2. Borate (BO33–) : Smell amount of salt is heated with ethyl alcohol and conc. H2SO4 and the fumes formed burns with green edged flame. Vapour or ethyl borate burns with green edged flame and borate (BO33–) is confirmed.
Reactions:
2Na3BO3 + 3H2SO4 → 3Na2SO4 + 2H3BO3

H3BO3 + 3C2H5OH → (C2H5)3BO3 + 3H2O

Vapour of ethyl borate burns with green edged flame
3. Phosphate (PO43–) Substance/mixture is dissolved in small amount of conc. HNO3 and ammonium molybadate is added in it. A canary yellow ppt. is obtained Yellow ppt. is of ammonium phosphomolybdate and (PO43–) is confirmed.
Reactions:
Ca3(PO4)2 + 6HNO3 → 3Ca(NO3)2 + 2H3PO4

H3PO4 + 12(NH4)2MoO421HNO

(NH4)3.PO4.12MoO↓+ 21NH4NO3 + 12H2O

Yellow ppt. is of ammonium phosphomolybdate. 
4. Fluoride (F)  
(i) Pungent smelling gas is evolved. Sides of test tube becomes greasy and oily droplets appear in solution. Gas may be HF and F (fluoride) may be present.
(ii) In a dry test tube salt/mixture and sand are taken in ratio 1 : 2 conc. H2SO4 is added. On heating white fumes are a waxy substance deposits on tube. F is confirmed
Reactions:
(i) NaF + H2SO4 → NaHSO4 + HF ↑­ HF is colourless and pungent smelling substance.
(ii) SiO2 + 4HF → SiF4 + 2H2O SiF4 is waxy substance.

 

NOTE:

(i)     In the testing of sulphate, if Ag and Pb cations are present then on adding BaCl2, chlorides of Ag or Pb get precipitated. To avoid it Ba (NO3)2 must be used in place of BaCl2.

(ii)    In the test of (SO42–), acidification is necessary to decompose carbonates, if present and dil. HCl is used for the purpose, conc. HCl or HNO3 cannot be sued here, otherwise BaCl2 or Ba (NO3)2 may precipitate. If carbonates are not decomposed then Ba (CO3)2 may be precipitated.   

(iii)   In the test of borate, methyl alcohol (CH3OH) can also be sued in place of ethyl alcohol. The test must be performed in test tube and not china dish, because if test is being performed in china dish and Cu2+ or Ba2+ salts are present, they may come in contact of flame and they also give green flame.

(iv)   The test prescribed for phosphate is also given by arsenate (AsO43–) but only on boiling the content with ammonium molybdate. So the test of phosphate must also done with filtrate of second group.    

Na­2HAsO4 + 12(NH4)2MoO4 + 23HNO3 → (NH4)3 [AsO4(Mo12O36)] + 2NaNO3 + 21NH4NO3 + 12H2O

(v)    The ppt. of ammonium phosphomolybdate dissolves in excess of phosphate so ammonium molybdate solution must be used in excess.

(vi)   In the test of phosphate, thick precipitates are obtained, if solution is heated with NH4NO3 solution at 400C.

(vii)  In the test of phosphate, HCl interferes, so it must be boiled off.

(viii) Reducing radicals like SO32–, S2– etc. interface in test of phosphate and converts MO6+ to molybdenum blue (MO3O8.xH2O). To avoid it, solution must be boiled with conc. HNO3 for sometime.

 

Qualitative Analysis : Soda Extracts Preparation