Qualitative Analysis : Analysis of Zero & First Group

Analysis of Zero Group

Radical – NH4+

Gp. Reagent – NaOH solution

Principle: Ammonium salts are decomposed by alkali and NH3 is liberated can be identified by different methods.

 

Experiment Observation Inference
1. Mixture is heated with NaOH solution Smell of NH3 NH4+ possible
2. On bringing glass rod moistned conc. HCl on the mouth of test tube White fumes are formed NH4+  confirm
3. On bringing phenolphthale in paper on the mouth of test tube in exp 1  Paper turns pink NH4+  confirm
4. On (adding Nessler’s reagent in aqueous solution of mixture. Brown ppt. is obtained NH4+  confirm
5. On bringing filter paper, soated in turmeric solution on the mouth of test tube Paper turns brown NH4+  confirm
6. On bringing filter paper moistned with Hg2 (NO3)2 on the mouth of test tube in experiment 1. Paper turns black NH4+  confirm 

 


NOTE:

  • Nessler’s reagent is alkaline solution of K2HgI4. To prepare it KI solution is added in HgCl2 till red ppt. formed are not dissolved. Then solution is made alkaline by adding NaOH or KOH. The alkali present in the solution liberates NH3 from ammonium salt and K2HgI4 reacts with NH3.
  • Some ammonium salts (like-ferrous ammonium sulphate) liberate NH3 only on strong heating with NaOH.

 

Reactions and Explanations:

1. NH4Cl + NaOH \underrightarrow { \quad \Delta \quad } NaCl + H2O + H2O + NH3­ ↑

Pungent smell is of ammonia liberated.

2. NH3 + HCl → NH4Cl

                Ammonium chloride

                   (white fumes)

3. NH3 turns phenolphthalein paper pink as its aqueous solution is alkaline.

4. NH4Cl + 4KOH + 2K2HgI4 → NH2 (Hg)O(Hg)I +7KI+ KCl + 3H2O

                                                          Iodide of Millon’s base

                                                          (Brown ppt.)

5. 2NH3 + Hg2 (NO3)2 → \underbrace { Hg\downarrow +Hg(N{ H }_{ 2 })NO_{ 3 } }_{ Black } +N{ H }_{ 4 }NO_{ 3 }

 

Analysis of Group – I

                   Radicals – Pb2+, Ag+, Hg22+

                   Group reagent – dil. HCl

Principle: (i) AgCl, PbCl2 and Hg2Cl2 are insoluble in cold water while other chlorides are soluble

(ii) Among these chlorides, only PbCl2 is soluble in hot water and can be separated by filtering hot solution.

(iii) In AgCl and Hg2Cl2, only AgCl forms soluble complex [Ag(NH3)2Cl] excess of NH4OH.

(iv) Hg2Cl2 gives black ppt. of aminochloride with NH4OH.

Procedure: In cold original solution, dil. HCl is added drop wise. Formation of white ppt. indicates the presence of gp. I These precipitates are filtered and residue obtained is used to analyse gp. I radicals while filtrate is used for analysis of gp. II.

 

Residue: White ppt. may be PbCl2, AgCl or Hg2Cl2. After washing ppt. are boiled in a beaker with water and filtered white hot.
Filtrate: (Pb2+ soluble in hot water). It may contain Pb2+. Divide the filtrate in four parts:

(i) Cool under tap water, formation of white ppt. of PbCl2 again indicates Pb2+

(ii) Yellow ppt. (PbCrO4) are obtained when K2CrO4 is added in hot solution. This yellow ppt. is soluble in NaOH but insoluble in CH3COOH

(iii) Yellow ppt. (PbI2) are obtained when KI is added in filtrate. Yellow ppt. are soluble in hot water.

(iv) White ppt. (PbSO4) are obtained if dil. H2SO4 is added in hot filtrate and mixture is cooled. White ppt. is soluble in CH3­COONH4 solution. Pb2+ confirm.

 Residue: (Ag2+, Hg22+ insoluble in hot water). Residue is treated with warm NH4OH is filtered.
Residue: Black residue may contain Hg2+ and ppt. may be of [Hg + Hg(NH2)Cl]. This ppt, is dissolved in aqua-regia and is boiled till it is semi-dry. Boiling is done to evaporate most of the acids. Water is added and it is divided into three parts.

Part – 1: Red ppt. (HgI2) is obtained, on adding KI in hot solution, ppt. is soluble in excess KI.

Part – II: White ppt. obtained on addition of SnCl2 which turns grey in excess of SnCl2­.

part – III: Add Cu-turning and deposition of grey deposits on surface of Cu confirms Hg22+. Hg2+ confirm

 Filtrate: It may contains silver amino chloride [Ag(NH3)2]Cl. Divide it into three parts:

Part – I: on adding excess of dil. HNO3 a white ppt. is obtained.

Part – II: Yellow ppt. (AgI) is obtained on adding KI-solution and ppt. is insoluble in excess of NH4OH.

Part – III: Black ppt. is obtained on passing H2S and ppt. is insoluble in NH4­OH white soluble in dil. HNO3 Ag+ confirm. 

 

NOTE:

(i)     Pb2+ is not completely precipitated in gp-I because PbCl2 is partially soluble in water. It is completely precipitated in gp-II.

(ii)    HCl should not be added in excess otherwise PbCl2 may form soluble complex with hot HCl.

(iii)   When solution  is prepared in conc. HCl, then on dilution, white ppt, may be formed even in the absence of gp-I. They may be oxychlorides of Sb, Bi or Sn.

 

Reactions and Explanations:

1. On adding dil HCl, insoluble chlorides are precipitated.

[Pb(NO3)2 + Hg2(NO3)2 + 2AgNO3] + 2HCl → (PbCl2↓ + Hg2Cl2↓ + AgCl↓) + 2HNO3

                   Soluble Pb2+. Hg2+, Ag+ ions                            White ppt.

2. In hot water only PbCl2 is soluble.

3. Test for Pb2+:

(i) White ppt. of PbCl2 dissolves in hot water and reappears on cooling.

          (ii) PbCl2 + K2CrO4 → PbCrO4↓ + 2KCl

          (Hot & soluble)  Lead chromate    (yellow)

          PbCrO4 + 4NaOH → Na2PbO2 + Na2CrO4 + 2H2O

          Yellow ppt.         Sodium plumbite        (soluble)

          (iii) PbCl2 + 2KI → PbI2 + 2KCl

          Hot & soluble      Lead iodide

                   (yellow)soluble in hot water

          (iv) PbCl2 + H2SO4 → PbSO4↓ + 2HCl

          Hot & soluble    Lead sulphate

                                      (white ppt.)

          PbSO4 + 2CH3COONH4 → (CH3COO)2Pb + (NH4)2SO4

                   Lead acetate                   (water soluble)

 

4. Test for Hg22+:

(i) On adding NH4OH on residue (Hg2Cl2 + AgCl), only AgCl dissolves and tests are perfomed from solid residue Hg2Cl2 gives black ppt. with NH4OH.

          Hg2Cl2 + 2NH4OH → [Hg(NH2)Cl + Hg] + NH4Cl + 2H2O

Black ppt.                   Mercury amino chloride     

(ii) Black ppt, is dissolves in aqua-regia.

          \underbrace { H{ NO }_{ 3 }+3HCl }_{ Aqua-regia } → NOCl + 2Cl + 2H2O

          2Hg(NH2)Cl + 6Cl → 2HgCl2 + 4HCl + N2

                   Hg  + 2Cl → HgCl2

(iii) Reaction with KI yields red ppt.

                   HgCl2 + 2KI → HgI2↓   +    2KCl

                   Mercury iodide            (Red ppt.)

                   HgI2 + 2KI → K2[HgI4]

                   Red ppt.

(iv) Reaction with SnCl2:

          2HgCl2 + SnCl2 → Hg2Cl2 + SnCl4

                                White ppt.

          Hg2Cl2 + SnCl2 → 2Hg + SnCl4

                               Grey

(v) Reaction with Cu:

          HgCl2 + Cu → Hg + CuCl2

                             Grey

 

 5. Tests for Ag+:

          (i) AgCl dissolves in NH4OH.

AgCl + 2NH4OH → [Ag(NH3)2]Cl  + 2H2O

          White ppt.          Diammine silver (I) chloride

                                                (soluble) 

          (ii) Reaction with dil. HNO3:

          [Ag(NH3)2].Cl + 2NHO3 → AgCl↓ + 2NH4NO3

                                      White ppt.

          (iii) Ag(NH3)2Cl + KI → AgI↓ + KCl + 2NH3

                                     Silver iodide

                                      (yellow ppt.)

          (iv) 2Ag(NH3)2Cl + H2S → Ag2S↓ + 2HCl + 4NH3

                                        Black ppt.

          3Ag2S + 8HNO3 → 6AgNO3 + 2NO + 4H2O + 3S

          Black ppt.              Soluble