Qualitative Analysis : ANALYSIS OF BASIC RADICALS Introduction

 

ANALYSIS OF BASIC RADICALS

Analysis of basic radicals: Analysis of basic radicals involves following steps:

(i)      Preparation of aqueous solution or original solution.

(ii)     Separation of basic radicals in different groups.

(iii)    Analysis of the ppt. obtained for each group.

(iv)    When substance is changed into original solution, its constituents are ionized and each basic radicals a specific route of tests is used.

(A)    Preparation of Original Solution

Transparent, aqueous solution of salt is known as original solution and it is prepared by dissolving slat/mixture in suitable solvent. To prepare original solution, following solvents are used strictly in the order given below:   

(i)      Cold water, (ii) Hot water, (iii) Dilute HCl, (iv) conc. HCl

To prepare original solution small quantity of substance is shaken with cold, distilled water. It substance dissolves, whole substance is dissolved in water to prepare original solution. If substance remains insoluble then, mixture is heated and solubility is tested. 

If substance is insoluble in hot water also then a few drops of conc. HCl are added.

If substance remains insoluble in dil. HCl also, then we try to dissolve in dil. HNO3.

If substance does not dissolve in the solvents used in above steps, then it is dissolved in conc. HCl. If it remains insoluble even in conc. HCl, then a paste of substance and conc. HCl is made in a porcelein dish and this mixture is heated till it becomes semi-solid. Again conc. HCl is added to make paste and is heated again to get a semi-solid. The procedure is repeated twice or thrice and semi-solid thus obtained is dissolved in distil water. The solution is filtered and filtrate is as original solution. If some insoluble part is obtained that is neglected and is not used. The filtrate obtained as original solution is necessarily diluted with water.

 

NOTE:

(i)     Original solution may be coloured but always transparent.

(ii)    Conc. HNO3, conc. H2SO4 or aqua-regia can’t be used to prepare original solution because.

  • If O.S. is prepared in HNO3 then in the analysis of gp. II, H2S passed will get oxidized into colloidal S.

H2S + 2HNO3 → 2NO2 + 2H2O + S

  • If O.S. is prepared in H2SO4, then Ba2+, Pb2+, Sr2+ or Ca2+ will get precipitated as insoluble Sulphates.
  • If O.S. is prepared in aqua-regia, then some sulphides of gp- II will not precipitate in gp II as they are soluble in it.

(iii)   If O.S. is prepared in conc. HCl, then its dilution is necessary before use. If not diluted then Ba2+ may precipitate as BaCl2, Gp- II sulphides are not precipitated completely precipitated as PbCl2 is soluble in conc. HCl.

(iv)   O.S. must be prepared in distilled water because ordinary water may contain some cations/anions.

(v)    If observed carefully, one may get some ideas regarding cations that may be present in the mixture.

(a)     If O.S. is coloured then transition metal ions (like-blue Cu2+(aq), green Fe2+ (aq), or Cr2+, or Cr2+(aq). brown Fe2+(aq) etc.) will be present. If O.S. is colourless than these ions will be absent.

(b)     If O.S. turns pink from blue, when water is added, then Co2+ is present.

(c)      If O.S. is prepared in dil. HCl, then gp. I (Pb2+, Ag+ and Hg2+) will be absent.

(d)     If white ppt, is obtained when O.S. is prepared in dil. HCl then Pb2+ may be present there. If this white ppt. is soluble in hot water and insoluble in cold water then it is PbCl2.

(e)     If white ppt. is obtained on dilution of O.S. prepared in conc. HCl then:

(a1) substance may contain Pb2+, because PbCl2 is soluble in conc. HCl and insoluble in dil HCl.

(a2) substance may contain Bi3+, Sb3+ or Sn2+. Because their chlorides give white insoluble oxy-chlorides, confirmation tests can be made as follows – 

 

Test for Pb2+:

(i)      White ppt. disappears on heating and reappears on cooling

(ii)     Yellow ppt. (PbI2) is obtained when KI is added in hot solution

 

Test for Bi3+:

Bi3+ will be confirmly present if yellow ppt. is obtained on adding conc. HNO3 followed by thiourea solution, in the white ppt. obtained after dilution.

 

Test for Sb3+:

The white ppt. obtained is dissolved in conc. HCl. Solution is diluted and H2S is passed in the solution. Orange ppt. confirms Sb3+.

 

(B)   Division of Basic Radicals into Different Groups and Their Separation

The original solution prepared for analysis of basic radicals is treated with specific reagents (known as group reagent) to separate the cations. On the basis of salt precipitated, all the salts have been classified into zero to VI gp. Cations of each group can be precipitated by specific group reagent.

 

Groups or Basic Radicals

S.No. Group Basic radical Group reagent Precipitated compound Colour of ppt.
1. Zero group NH4+ Dil. NaOH NH3 gas __
2. I group  

 

 

(i) Pb2+

(ii) Ag+

(iii) Hg22+

Dil. HCl Chlorides of respective cation

(i) PbCl2

(ii) AgCl

(iii) Hg2Cl2

 

 

White

White

White

3. II group (A) Cu-sub group

(i) Hg2+

(ii) Pb2+

(iii) Bi3+

(iv) Cu2+

(v) Cd2+

(B) As-sub-group

(i) As3+

(ii) Sb3+

(iii) Sn2+

(iv) Sn4+

H2S in presence of dil. HCl

 

 

 

 

H2S in presence of dil HCl

 

 

 

(i) HgS

(ii) PbS

(iii) Bi2S3

(iv) CuS

(v) CdS

 

 

(i) As2S3

(ii) Sb2S3

(iii) SnS

(iv) SnS2

 

 

Black

Black

Yellow

Black

Yellow

 

 

Yellow

Orange

Brown

Yellow

4. III group (i) AI3+

(ii) Fe3+

(iii) Cr3+

NH4OH in presence of

NH4Cl

(i) AI(OH)3

(ii) Fe(OH)3

(iii) Cr(OH)3

White

Red

Brown

Green

5. IV group (i) Zn2+

(ii) Mn2+

(iii) Ni2+

(iv) Co2+

H2S in presence of NH4­OH (i) ZnS

(ii) MnS

(iii) NiS

(iv) CoS

White

Buff

Black

Black

6. V group (i) Ca2+

(ii) Sr2+

(iii) Ba2+

(NH4)2CO3 in presence of NH4OH (i) CaCO3

(ii) SrCO3

(iii) BaCO3

White

White

White

7. VI group Mg2+ Na2HPO4 Mg(NH4)PO4 White

 

Qualitative Analysis : Analysis of Zero & First Group