Chapter 25 Chemical analysis by TEACHING CARE Online coaching and tuition classes

Chapter 25 Chemical analysis by TEACHING CARE Online coaching and tuition classes

File name : Chapter-25-Chemical-analysis.pdf

 

 

Analytical chemistry deals with qualitative and quantitative analysis of substances.

Qualitative analysis : In qualitative inorganic analysis, the given compound is analysed for the basic and acid

 

radicals (i.e., the cations and the anions), that it contains. For example zinc blend is analysed for the ions that it contains.

Zn2+

and

S2-

 

 

(1)  Colourless gases

  • Tests for CO2 : It is colourless and odourless It gives white ppt. with lime water which dissolves on

 

passing excess of CO2 . Ca(OH)2 + CO2  ® CaCO3  ¯ +H 2 O ;     CaCO3 + CO2 + H 2O ® Ca(HCO3 )2

 

Limewater

White ppt.

White ppt.

Excess

So lub le

 

  • Test for CO : It is colourless and odourless It burns with a blue flame.

Note : ® CO is highly poisonous gas.

2CO + O2 ® 2CO2

 

  • Test for O2 : It is colourless and odourless It rekindles a glowing splinter.
  • Tests for H2 S : It is a colourless gas with a smell of rotten It turns moist lead acetate paper black.

(CH3 COO)2 Pb + H 2 S ® 2CH3 COOH + PbS

Black

  • Tests for SO2 : It is a colourless gas with a suffocating odour of burning It turns acidified

 

K 2 Cr2 O7 solution green.

3SO2  + K2Cr2O7  + HSO4  ® K2 SO4  + Cr2 (SO4 )3 + H2O

Green

 

  • Tests for NH3 : It is a colourless gas with a characteristic ammonical It gives white fumes of

 

NH4 Cl

with HCl ,

NH3 + HCl ®

NH4Cl . With Nessler’s reagents, it gives brown ppt.

White fumes

 

2K2 [HgI 4 ] + NH 3 + KOH ® NH 2 HgOHgI + 7KI + 2H 2 O

 

Nessler s reagent

Iodine of Millons base

( Brown ppt )

 

It gives deep blue colour with CuSO4

solution, CuSO4  + 4 NH3  ® [Cu(NH3 )4 ]SO4  .

Deep blue

NH 3

dissolves in water

 

to give

NH 4OH, which being basic, turns red litmus blue,

NH3  + H2O ® NH4 OH NH + + OH .

 

4
  • Tests for HCl gas : It is colourless gas with a pungent irritating It turns moist blue litmus paper

 

red i.e., it is acidic in nature. It gives white ppt. with

AgNO3

solution. This white ppt. is soluble in

NH 4 OH.

 

HCl + AgNO3 ®

AgCl + HNO3 ;

White ppt.

AgCl + 2NH 4 OH ® [Ag(NH 3 )2 ] + 2H 2 O .

Soluble

 

  • Test for

CH3 COOH

vapours : These vapours are colourless with a vinegar like smell.

 

(2)  Coloured gases

  • Tests for Cl2 : It is a greenish yellow gas with a pungent In small quantity it appears almost

colourless. It bleaches a moist litmus paper, Cl2 + H 2O ® 2HCl + [O]; Colour + [O] ® Colourless. Blue litmus

paper first turns red and then becomes colourless.

  • Tests for Br2 : Brown vapours with a pungent It turns moist starch paper yellow.
  • Tests for I2 : Violet vapours with a pungent sm1 It turns moist starch paper blue.

 

 

  • Tests for NO2 : Brown coloured pungent smelling It turns moist starch KI paper blue

2KI + 2NO2 ® 2KNO2 + I 2 ; I 2 + Starch ® Blue colour.

It turns ferrous sulphate solution black, 3FeSO4 + NO2 + HSO4 ® Fe2 (SO4 )3 + FeSO4 . NO+ H2O

Black brown

It involves the following steps : (1) Preliminary tests (2) Wet tests for acid radicals and (3) Wet tests for basic radicals.

(1)  Preliminary tests

  • Physical examination : It involves the study of colour, smell, density
  • Dry heating : Substance is heated in a dry test

Observation                                                      Result

(a)        A gas or vapour is evolved.                                        Compounds with water of crystallisation Vapour, evolved, test with litmus paper.                                     Ammonium salts, acid salts, and hydroxides.

(usually accompanied by change of colour)

The vapour is alkaline.                                                          Ammonium salts.

The vapour is acidic.                                                              Readily decomposable salts of strong acids.

Oxygen is evolved                                                                     Nitrates,chlorates and certain oxides.

Dinitrogen oxide                                                                       Ammonium nitrate or nitrate mixed with an ammonium salt.

 

Dark-brown or reddish fumes (oxides of nitrogen), acidic in reaction.

Nitrates and nitrites of heavy metals.

 

CO2 is evolved, lime water becomes turbid.           Carbonates or hydrogen carbonates.

NH 3 is evolved which turns red litmus blue.      Ammonium salts.

 

SO2 is  evolved,  which  turns     acidified

K2Cr2O7 green, decolourises fuschin colour.

H2S is evolved, turns lead acetate paper black, or cadmium acetate yellow.

Cl2 is evolved, yellowish green gas, bleaches litmus paper, turns KI – starch blue, poisonous.

Br2 is evolved (reddish brown, turns fluorescent paper red).

Sulphates and thiosulphates.

 

 

Hydrates,      sulphides      or     sulphides      in      the prescnce of water.

Unstable chlorides e.g., copper chlorides in the presence of oxidising agents.

 

Bromides in the presence of oxidising agents.

 

I 2 is evolved, violet vapours condensing to  Free iodine and certain iodides

2

 

 

 

Action of heat on different compounds : Many inorganic salts decomposes on heating, liberating characteristic gases. A few such reactions are as follows,

 

2HgO ¾¾D®

(Re d )

2Hg

( Silvery deposit )

  • O2

2Pb3 O4

¾¾D ® 6PbO+ O2

(Yellow)

2PbO2

¾¾D ® 2PbO+ O2

 

 

CuCO

¾¾D ® CuO + CO

(Re d)

 

ZnO

¾¾D ®

ZnO

  • CO

(Brown)

 

CuSO4 .5H2O ¾¾D ® CuSO4 + 5H2O

 

3

(Green)

2                                3

(White)

Yellow(hot)            2

White (cold )

(Blue)

(White)

 

CuSO4 ¾¾D ® CuO + SO3

2Zn(NO3 )2  ¾¾D ® 2ZnO + 4 NO2 + O2

2FeSO4  ¾¾D ® FeO3  + SO2  + SO3

2AgNO3  ¾¾450¾°¾C ® 2Ag + 2NO2  + O2

2Ag 2O ® 4 Ag + O2

2Cu(NO3 )2 ® 2CuO + 4 NO2  + O2

 

(White)                                    (Brown)

 

2Ag 2 CO3  ® 4 Ag + 2CO2  + O2

2Pb(NO3 )2 ® 2PbO + 4 NO2 + O2

(NH 4 )2 Cr2 O7  ® N 2  + Cr2 O3 + 4 H 2 O

 

(Orange)                          (Green)

 

2NaHCO3  ® Na2CO3 + CO2 + H2O

2NaNO3 ® 2NaNO2 + O2

2Mg(NO3 )2 ® 2MgO + 4 NO2 + O2

2CaSO4 .2H2O  ®  2CaSO4 .H2O+ 2H2O

(Plaster of Paris)

NH4 HCO3  ® NH3 + CO2 + H2O

MgCO3 ® MgO + CO2

2Ca(NO3 )2  ® 2CaO + 4 NO2  + O2

2AlCl3 .6H2O ® Al2O3  + 6HCl + 9H2O

CaCO3 ® CaO + CO2

2NH3  ¾¾Red¾h¾ot ® N2 + 3H2

Al 2 (SO4 )3  ¾¾Red¾h¾ot ® Al 2 O3  + 3SO3

2BeSO4  ¾¾D ® 2BeO + 2SO2  + O2

 

3                                       2         2

2AgNO   ¾¾350o¾C ® 2AgNO   + O

2MgSO4

¾¾D ® 2MgO + 2SO2

  • O2

2ZnSO4

¾¾D ® 2ZnO + 2SO2

  • O2

 

(COO)2 Sn ¾¾D ® SnO + CO2  + CO

NH 4 NO3 ® N 2 O + 2H 2 O Li2 CO3 ® Li2 O + CO2

CaC2O4 ® CaCO3 + CO 2KClO3  ® 2KCl  + 3O2 (COO)2 Fe ® FeO + CO + CO2

NH 4 NO2  ® N 2 + 2H 2 O

2FeCl 3 ® 2FeCl 2 + Cl2

2KMnO4 ® K2 MnO4  + MnO2 + O2

 

MgCl 2 . 6H 2O ® HgCl 2 + Hg

NH 4 Cl ® NH 3 + HCl

2LiNO3

® Li2

O + 2NO2

+ 1 O

2

2

 

Hg(NO3 )2  ® Hg + 2NO2  + O2

2CuCl2  ¾¾D ® Cu2 Cl2  + Cl 2

2Co(NO3 )2  ¾¾D ® 2CoO + 4 NO2  + O2

 

4 K 2 Cr2 O7 ® 4 K 2 CrO4 + 2Cr2 O3 + 3O2

2Zn(NH 4 )PO4  ¾¾D ® Zn2 P2 O7  + H 2 O + 2NH 3

ZnCl 2 . 2H 2 O ¾¾D ® Zn(OH)Cl + HCl + H 2 O

2Mg(NH 4 )PO4  ¾¾D ® Mg 2 P2 O7  + H 2 O + 2NH 3

K 4 Fe(CN)6  ¾¾D ® 4 KCN + Fe + 2C + N 2

2(ZnCl 2 .H 2 O) ¾¾D ® Zn2 OCl 2  + 2HCl + H 2 O

 

 

 

  • Flame test

Characteristic flame colour : Certain metals and their salts impart specific colours to Bunsen burner flame.

  • Pb imparts pale greenish colour to the
  • Cu and Cu salts impart blue or green colour to the
  • Borates also impart green colour to the flame.
  • Ba and its salts impart apple green colour to the
  • Sr imparts crimsen red colour to the
  • Ca imparts brick red colour to the
  • Na imparts yellow colour to the
  • K imparts pink-violet (Lilac) colour to the
  • Li imparts crimsen-red, Rb imparts violet and Cs imparts violet colours to the
  • Livid- blue flame is given by As, Sb and

 

  • Borax bead test : The transparent glassy bead the colour produced gives some idea of cation present in

(NaBO2 + B2 O3 ) when heated with inorganic salt and

 

Colour of bead in oxidising flameColour of bead in reducing flameBasic radical present
Greenish when hot, blue in cold.Red and opaqueCu
Dark green in hot and coldSameCr
Deep – blueDeep blueCo
Yellow when hotGreenFe
Violet in hot and coldColourlessMn
Brown in coldGrey or black or opaqueNi

 

Microcosmic salt bead test :   Microcosmic salt,

Na(NH 4 )HPO4 .4 H 2O

is also used to identify certain

 

cations just like borax. When microcosmic salt is heated in a loop of platinum wire, a colourless transparent bead of sodium metaphosphate is formed.

Na (NH 4 )HPO4 .4 H 2 O ® Na(NH 4 )HPO4 + 4 H 2 O ;  Na(NH 4 )HPO4  ® NaPO3 + NH 3 + H 2 O

 

 

 

NaPO3 + CuO ® NaCuPO4 (Blue);  NaPO3 + CoO ® NaCoPO4 (Blue);  NaPO3  + Cr2 O3  ® NaPO3 .Cr2 O3  (Green)

  • Charcoal cavity test

 

(a) Compound fused in cavity directly
Nature and colour of beadCation
Yellow, brittle beadBi 3+
Yellow, soft bead which marks on paperPb 2+
White, brittleSb3+
White yellow when hotZnO
White garlic odourAs2O3
BrownCdO
Grey metallic particles attracted by magnetFe, Ni, CO
Maleable beadsAg and Sn (White),Cu (Red flakes)

 

 

(b)  Compound mixed with

Na 2 CO3

Crystalline

 

Sustance Decrepitates

Salts, NaCl, KCl ;

Substance deflagrates

Oxidising agents like ; Substance infusible, perform test (a)

NO , NO chlorates

 

3          2

 

  • Cobalt Nitrate test

 

ColourCompositionResult
Blue residue Green residue Pink dirty residue

Blue residue

CoO. Al2 O3 CoO. ZnO CoO. MgO

NaCoPO4

Al ZnO MgO

PO 3 in absence of Al.

4

 

 

  • Wet tests for acid radicals : Salt or mixture is treated with

HSO4

and also with conc.

H 2 SO4

 

separately and by observing the types of gases evolved. Confirmatory tests of anions are performed.

 

Observations with Dilute

H 2 SO4

 

 

 

 

 

 

 

 

 

Brown fumes

CaCO3  + H 2 O + CO2  ¾¾® Ca(HCO3 )2

soluble

 

2

NO – (Nitrite)                         Add                  KI                   and                  starch                 solution                 blue                  colour

 

 

 

 

 

 

 

 

 

 

 

Smell      of     rotten  eggs

2NaNO3  + H 2 SO4  ¾¾® Na 2 SO4  + 2HNO2 ;

HNO2 ¾¾® NO (colourless);

2NO + O2 (air) ¾¾® 2NO2 (brown);

2KI + H 2 SO4 + 2NHO2 ¾¾® K 2 SO4  + 2H 2 O + 2NO + I 2 ;

I 2 + starch ¾¾® blue colour

S 2- (sulphide)                      Gas turn lead acetate paper black

 

(H 2 S smell) on heating

Sodium  carbonate  extract   (SE)* +  sodium  nitroprusside  –  purple  colour,

 

Na 2 S + H 2 SO4  ¾¾® H 2 S + Na 2 SO4 ;

H 2 S + (CH3 COO)2 Pb ¾¾® PbS+ 2CH3 COOH ;

(black)

 

 

Na2S + Na2[Fe(CN)5 NO] ¾¾® Na4 [Fe(CN)5

NOS]

 

sodium nitroprusside      (purple)

 

 

Colourless           gas         with pungent smell of burning

SO 2-  (sulphite)                   Gas turns acidified    K 2 Cr2 O7   solution green [different from    CO2- ] since

 

3
3

sulphur

gas                           also                           turns                          lime                           water                         milky

Na2 SO3  + H 2 SO4  ¾¾D ® Na2 SO4  + H 2 O + SO2 ;

 

7

Cr2 O2 + 3SO2

+ 2H + ¾¾® 2Cr 3+ + 3SO2 + H

4

(green)

2O ;

 

 

 

 

 

 

Solution gives smell of vinegar

 

CH 3 COO

Ca(OH)2 + SO2 ¾¾® CaSO3

(milky)

 

Aq.             Solution           +              neutral

 

FeCl 3 ® blood              red             colour

 

(acetate)

3CH3 COONa + FeCl3  ¾¾® Fe(CH3 COO)3 + 3NaCl

 

 

White or yellowish white turbidity on warming

3

S2 O 2-

neutral                            (red)

 

Aq. Solution + AgNO3 ® white ppt. changing to black (viii) on warming ,

 

(thiosulphate)

Na2 S2 O3  + 2AgNO3  ¾¾® Ag 2 S2 O3 + 2NaNO3 ;

white ppt.

 

Ag 2 S2 O3  + H 2 O ¾¾® Ag 2 S + H 2 SO4

black ppt.

 

 

Observation with concentrated H 2 SO4

 

 

 

 

 

 

 

ObservationAcid RadicalConfiramatory Test
Colourless           pungentCl (chloride)Add MnO2 in the same test tube and heat–pale green Cl 2
gas giving white fumes gas (i)
with aq. NH 4 OH S.E.+ HNO3 + AgNO3 solution –white ppt. soluble in aq.
  NH 3 (ii)

 

6

 

 

 

 

 

Reactions Chloride : (i)

KCl + conc.H 2 SO4  ¾¾® KHSO4  + HCl  ;

HCl + NH 3 ¾¾®

NH 4 Cl

(white fumes)

 

4 HCl + MnO2  ¾¾D ® MnCl2  + Cl2  + 2H 2 O

 

 

(ii)

KCl + AgNO3  ¾¾® AgCl ¯ + KNO3  ;

white ppt.

AgCl + aq. 2NH 3  ¾¾®[Ag(NH 3 )2 ]Cl

soluble

 

 

 

(iii) Chromyl  chloride  test  :  Chloride + K 2 Cr2 O7 (solid) + conc. H 2 SO4  ¾¾he¾a¾t ®

reddish brown vapours of

 

chromyl-chloride (CrO2Cl2 ). Pass these vapours into

NaOH,

when yellow

Na2 CrO4

solution is formed. On adding

 

CH 3 COOH and (CH 3 COO)2 Pb, yellow ppt. of lead chromate (PbCrO4 ) is formed.

 

 

KCl + conc.H 2 SO4  ¾¾D ® KHSO4  + HCl ;

K2 Cr2 O7  + 2H 2 SO4  ¾¾D ® 2KHSO4  + 2CrO3  + H 2 O

conc.

 

 

CrO3 + 2HCl ¾¾® CrO2 Cl2  + 2H 2 O ;  CrO2 Cl2  + 4 NaOH ¾¾® Na2 CrO4  + 2NaCl + 2H 2 O

 

Na2 CrO4  + (CH3 COO)2 Pb ¾¾® PbCrO4  ¯+ 2CH3 COONa

yellow ppt.

 

 

Bromide : (iv)

KBr + conc. H 2 SO4  ¾¾D ® KHSO4  + H ;

4 HBr + MnO2  ¾¾D ® Br2  + 2H 2 O + MnBr2

 

(v)

NaBr + AgNO3  ¾¾® AgBr ¯ + NaNO3 ;

pale yellow ppt.

AgBr + aq. 2NH 3  ¾¾®[Ag(NH 3 )2 ]Br

partially soluble

 

 

 

 

(vi) Layer Test :

S.E. + Cl2

water  + CHCl3 ¾¾sh¾ak¾e ®

yellowish orange colour in CHCl 3

layer

(CS2 or

 

CCl 4

can be taken instead of CHCl3 );

2NaBr + Cl2 ¾¾® 2NaCl +

Br2

orange yellow (soluble in CHCl3 )

 

 

 

In case of

I , violet colour of I 2

in CHCl3 layer, 2NaI + Cl2  ¾¾® 2NaCl + I2

(violet)

 

Iodide : (vii)

KI + conc.H 2 SO4  ¾¾D ® KHSO4  + HI ;

2HI  +  H 2SO4 ¾¾®

I 2

(violet)

+ 2H 2 O + SO2

 

 

Nitrate : NaNO3 + H 2 SO4  ¾¾® NaHSO4  + HNO3

 

4 HNO3  ¾¾® 4 NO2  + O2 2H 2 O  ;

brown fumes

Cu + 4 HNO3  ¾¾® Cu(NO3 )2  + 2NO2  + 2H 2 O

 

 

 

(viii) Ring test : To water extract (all

NO

are water soluble) add freshly prepared

FeSO4 solution and then

 

3

conc.

HSO4

carefully by the side of the test- tube. A dark brown ring of [Fe (H 2 O)5 NO]2+ SO2

at the interface

 

4

between the two liquids is formed.

2NaNO3 + H 2 SO4  ¾¾® 2NaHSO4  + 2HNO3 ;

2HNO3 + 6Fe SO4  + 3H 2 SO4  ¾¾® 3Fe 2 (SO4 )3 + 2NO + 4 H 2 O

4

[Fe (H 2 O)6 ] SO4  + NO ¾¾®[Fe(H 2 O)5 NO]2+ SO2  + H 2 O

 

Oxalate :

Na2 C2 O4  + H 2 SO4  ¾¾® Na2 SO4  + H 2 O + CO + CO2

 

CO burns with blue flame and CO2 turns lime water milky.

 

 

(ix) 5C2 O2 + 2MnO + 16H + ¾¾®10 CO

+ 2Mn2+ + 8H  O

 

4                     4

(violet)

2                               2

colourless

 

 

 

(x) CaCl2  + NaC2 O4  ¾¾® CaC2 O4  ¯+ 2NaCl

white ppt.

CaC2 O2 decolourises acidified

KMnO4 .

 

Specific test in solution :

  • Sulphate : E. add dil. (to decompose

 

3

CO 2 until reaction ceases). Add

BaCl 2 solution. White ppt.

 

insoluble in conc.

HNO3 ,

BaCl 2  + NaSO4  ¾¾® BaSO4  ¯+ 2NaCl

white ppt.

 

  • Borate : lgnite the mixture containing borate, splinter –green edged flame of ethyl borate.

H 2SO4 . And ethanol in a china-dish with a burning

 

2Na3 BO3  + 3H 2 SO4  ® 2H 3 BO3  + 3Na 2 SO4 ;

(conc.)

H3 BO3  + 3C2 H5 OH ¾¾D ®

ethanol

(C2 H5 O)3 B

burns with green flame (volatile)

  • 3H 2 O

 

 

In presence of Cu2+ , perform this test in a test tube since Cu 2+ salts are not volatile.

  • S.E. + HNO3 + ammonium molybdate Heat, yellow crystalline ppt. confirms

 

Na3 PO4  + 12(NH4 )2 MoO4  + 24 HNO3  ¾¾D ® (NH4 )3 PO4 . 12MoO3  + 21NH4 NH3  + NaNO3  + 12H2O

yellow ppt.

 

 

 

Arsenic also gives this test. Hence presence of phosphate should also be checked after group II.

  • Fluoride : Sand +salt (F ) +conc. H 2SO4 ; heat and bring a water wetted rod in contact with vapours

at the mouth of the test tube. A white deposit on the rod shows the presence to F

NaF  H 2 SO4 ¾¾D ® NaHSO4 + HF SiO2 + 4 HF ¾¾D ® SiF4 + 2H 2O ; 3SiF4 4 HO ¾¾® 2HSiF6  + HSiO4

white

  • Wet tests for basic radicals : Analysis of Basic Radicals
GroupGroup reagentBasic radicalComposition and colour of the precipitate
IDilute HCl

 

 

 

H 2 S      in presence of dilute HCl

 

 

 

 

 

 

 

NH 4 OH in presence of

NH 4 Cl

 

 

H 2S in presence of NH 4 OH

 

 

 

(NH 4 )2 CO3 in

presence of NH 4 OH

NaHPO4

NaOH

Ag +AgCl : white ü Chloride

PbCl 2 : white ï insouble

ý

HgCl 2 : white ï cold dilute HCl

þ

 

HgS : black     ü

PbS : black      ï

ï

Bi 2 S3 : black  ï

CuS : black     ï Sulphides

ï

CdS : yellow   ï insoluble in

ý

As 2 S3 : yellow ï dilute HCl

ï

Sb 2 S3 : orangeï SnS : brown     ï

ï

SnS 2  : yellow  ïþ

Fe(OH)3 :       ü

reddish brown ï Hydroxides are insoluble in

ï

Cr(OH) : greený NH OH

3                ï       4

Al(OH3  : white ïþ

ZnS :            ü

greenish white ï

ï Sulphides are insoluble in

MnS : buff       ý NH  OH

CoS : black    ï       4

ï

NiS : black      ïþ

BaCO3 : whiteü

SrCO3 : white ïCarbonates are insoluble

ý

CaCO3 : whiteï

þ

Mg (NH 4 )PO4 :   White

Ammonia gas is evolved

 Pb 2+
 Hg 2+
IIHg 2+
 Pb 2+
 Bi 3+
 Cu 2+
 Cd 2+
 As 3+
 Sb 3+
 Sn2+
IIIFe 3+
 Cr 3+
 Al 3+
 

IV

 

Zn 2+

 Mn2+
 Co 2+
 Ni 2+
V 

Ba 2+

 Sr 2+
 Ca 2+
VIMg 2+
VIINH +

4

Chemical reactions involved in the tests of basic radicals

Group I : When dil. HCl is added to original solution, insoluble chlorides of lead, silver mercurous mercury are precipitated.

 

 

 

Pb(NH 3 )2 + 2HCl ¾¾® PbCl 2 + 2HNO3 ; AgNO3  + HCl ¾¾® AgCl + HNOHg(NO3 )2 + 2HCl ¾¾® HgCl 2 + 2HNO3

Pb2+ (lead)

  • PbCl 2 is soluble in hot water and on cooling white crystals are again

 

  • The solution of

PbCl 2

gives a yellow precipitate with potassium chromate solution which is insoluble in

 

acetic acid but soluble in sodium hydroxide.

PbCl 2  + K2 CrO4  ¾¾® PbCrO4 + 2KCl ;

yellow ppt.

PbCrO4 + 4 NaOH ¾¾® Na2 PbO2 + Na2 CrO4  + 2H 2 O

 

  • The solution of

PbCl 2 forms a yellow precipitate with potassium iodide solution.

 

 

 

PbCl 2 + 2KI ¾¾®

PbI 2

Yellow ppt.

  • 2KCl

 

  • White precipitate of lead sulphate is formed with dilute

H 2 SO4 .

The precipitate is soluble in ammonium

 

acetate,

PbCl 2 + H 2 SO4  ¾¾® PbSO4  + 2HCl ;

PbSO4  + 2CH 3 COONH4  ¾¾® Pb(CH 3 COO)2  + (NH 4 )2 SO4

 

Ag+(silver)

  • AgCl dissolves in ammonium hydroxide,

AgCl + 2NH 4 OH ¾¾® Ag(NH 3 )2 Cl + 2H 2 O

Diammine silver (I)

chloride

 

  • On adding dilute

HNO3 to the above solution, white precipitate is again obtained

 

 

Ag(NH 3 )2 Cl + 2HNO3 ¾¾® AgCl + 2NH 4 NO3

White ppt.

  • On adding KI to the complex solution, yellow precipitate is

Ag(NH 3 )2 Cl + KI ¾¾® AgI + KCl + 2NH 3

 

2

Hg 2+

(mercurous)

 

 

 

(i)

Hg 2 Cl2 turns black with

NH 4 OH , Hg 2 Cl2  + 2NH 4 OH ¾¾®Hg–+–Hg(NH–2–)Cl + NH 4 Cl + 2H 2 O

Black

 

  • The black residue dissolves in aqua-regia forming mercuric

 

3HCl + HNO3 ¾¾® NOCl + 2H 2 O + 2Cl ;

Hg + 2Cl ¾¾® HgCl 2

2Hg(NH 2 )Cl + 6Cl ¾¾® 2HgCl 2 + 4 HCl + N 2

 

  • The solution of

HgCl 2` forms white or slate-coloured precipitate with stannous chloride.

 

2HgCl2  + SnCl2  ¾¾® Hg 2 Cl2 + SnCl4 ;

white ppt.

Hg 2Cl2 + SnCl2 ¾¾®

2Hg + SnCl4 Grey ppt.

 

  • The solution of HgCl 2 with copper turning forms a grey

 

 

HgCl2 + Cu ¾¾®

Hg

Grey ppt.

  • CuCl2

 

 

 

Group II : When hydrogen sulphide is passed in acidified solution, the radicals of second group are precipitated as sulphides. The precipitate is treated with yellow ammonium sulphide. The sulphides of IIB are first oxidised to higher sulphides which then dissolve to form thio-compounds.

Ag 2 S3  + 2(NH 4 )2 S2  ¾¾® 2(NH 4 )2 S + As 2 S5 ; Sb2 S3  + 2(NH 4 )2 S2  ¾¾® 2(NH 4 )2 S + Sb2 S5 SnS + (NH 4 )2 S2  ¾¾®(NH 4 )2 S + SnS2

As 2 S5  + 3(NH4 )S ¾¾® 2(NH4 )3 AsS4 ; Sb2 S5  + 3(NH4 )2 S ¾¾® 2(NH 4 )2 SbS4 ; SnS2  + (NH 4 )2 S ¾¾®(NH 4 )2 SnS3

 

Ammonium thioarsenate

All the three are soluble.

Ammonium thioantimonate

Ammonium thiostannate

 

In case, the precipitate does not dissolve in yellow ammonium sulphide, it may be either HgS or PbS or

 

Bi2 S3

or CuS or

CdS.

The precipitate is heated with dilute

HNO3 .

Except HgS , all other sulphides of IIA are

 

soluble.

3PbS + 8HNO3 ® 3Pb(NO3 )2 + 2NO + 3S + 4 H 2 O ;

3CuS + 8HNO3  ® 3Cu(NO3 )2  + 2NO + 3S + 4 H 2 O ;

3CdS + 8HNO3 ® 3Cd(NO3 )2  + 2NO + 3S + 4 H 2 O

Hg2+ (mercuric)

Bi2 S3 + 8HNO3  ® 2Bi(NO3 )3  + 2NO + 3S + 4 H 2 O

 

HgS is dissolved in aqua-regia,

3HgS + 2HNO3  + 6HCl ® 3HgCl 2  + 3S + 2NO + 4 H 2 O

 

The solution is divided into two parts:

Part I : Stannous chloride solution reduces

HgCl 2

first into white

Hg 2 Cl2 and then to grey metallic mercury.

 

Part II : Copper displaces Hg from

Pb2+ (lead)

HgCl 2

which gets coated on copper turnings as a shining deposit.

 

In case the sulphide dissolves in dilute

HNO3 , a small part of the solution is taken. Dilute

HSO4

is added. If

 

lead is present, a white precipitate of lead sulphate appears,

Pb(NO3 )2  + H 2 SO4

® PbSO4 + 2HNO3

(White ppt.)

 

In absence of lead, the remaining solution is made alkaline by the addition of excess of

NH 4 OH. Bismuth

 

forms a white precipitat of soluble complex,

Bi(OH)3 ,

copper forms a deep blue coloured solution while cadmium forms a colourless

 

Bi(NO3 )3  + 3NH 4 OH ¾¾® Bi(OH)3  + 3NH 4 NO3 ;

White ppt.

Cu(NO3 )2  + 4 NHOH ¾¾®[Cu(NH 3 )4 ](NO3 )2  + 4 HO; Cd(NO3 )2  + 4 NH OH ¾¾®[Cd (NH 3 )4 ](NO3 )2  + 4 H O

 

Tetrammine cupric nitrate (deep blue solution)

Bi3+ (bismuth) : The precipitate dissolves in dilute HCl,

Tetrammine cadmium nitrate (colourless solution)

Bi (OH)3 + 3HCl ¾¾® BiCl3 + 3H 2 O

 

Part I : Addition of excess of water to

BiCl3

solution gives a white precipitate due to hydrolysis.

 

 

 

BiCl3  + H 2 O ¾¾® BiOCl + 2HCl

Bismuth Oxychloride (White ppt.)

 

Part II : The solution of

BiCl3

is treated with sodium stannite when a black precipitate of metallic bismuth is

 

formed, 2BiCl3  + 3Na2 SnO2 + 6 NaOH ¾¾® 3Na2 SnO3 + 2Bi + 6 NaCl + 3H 2 O

Sod. stannite                                            Sod. stannate

Cu2+ (copper) : Blue coloured solution is acidified with acetic acid. When potassium ferrocyanide is added a chocolate coloured precipitate is formed, Cu(NH 3 )4 (NO3 )2  + 4CH 3 COOH ¾¾® Cu(NO3 )2  + 4CH 4 COONH4

2Cu(NO3 )2 + K4 [Fe(CN)6 ] ¾¾® Cu2 [Fe(CN)6 + 4 KNO3

Chocolate ppt.

 

Cu2+ (cadmium) :

HS

is passed through colourless solution. The appearance of yellow precipitate

 

confirms the presence of cadmium, Cd (NH 3 )4 (NO3 )2  + H 2 S ¾¾® CdS + 2NH 4 NO3 + NH 3

Yellow ppt.

 

Group IIB : In case the precipitate dissolves in yellow ammonium sulphide, the tests of the radicals arsenic, antimony and tin are performed. The sulphide is treated with concentrated hydrochloric acid. Antimony and tin sulphide dissolve while arsenic sulphide remains insoluble.

As3+ (arsenic) : The insoluble sulphide is treated with concentrated nitric acid which is then heated with ammonium molybdate. Yellow precipitate of ammonium arsenomolybdate is formed.

As 2 S5  + 10HNO3  ¾¾® 2H3 AsO4  + 10 NO2  + 2H 2 O + 5S

Arsenic acid

 

H 3 AsO4  + 12(NH 4 )2 MoO4  + 21HNO3  ¾¾®(NH 4 )3 AsO4 . 12MoO3  + 21NH 4 NO3  + 12H 2 O

Yellow ppt.

Sn2+ or Sn4+ (tin) : Solution of sulphide in concentrated HCl is reduced with iron fillings or granulated zinc.

 

SnS2 + 4 HCl ¾¾® SnCl4 2H 2 S ;

White ppt.

SnCl4  + Fe ¾¾® SnCl2  + FeCl 4

Grey

 

HgCl 2 solution is added to above solution which gives first a white precipitate that turns to grey.

 

 

2HgCl 2 + SnCl2 ¾¾® HgCl 2 + SnCl4 ;

White ppt.

Hg 2 Cl2  + SnCl 2  ¾¾® 2Hg + SnCl4

Grey

 

Sb2+ (antimony) : Filtrate of sulphide in concentrated HCl is divided into two parts.

Part I : On dilution with excess of water, a white precipitate of antimony oxychloride is obtained.

SbCl3 + HO ¾¾® SbOCl + 2HCl

White ppt.

 

Part II :

H 2 S is circulated. Orange precipitate is formed,

2SbCl3  + 3H 2 S ¾¾® Sb2 S3  + 6HCl

Orange ppt.

 

Group III : Hydroxides are precipitated on addition of excess of ammonium hydroxide in presence of ammonium chloride.

 

 

 

 

AlCl3 + 3NH 4 OH ¾¾®

Al(OH)3 + 3NH 4 Cl ;

Gelatinous ppt.

CrCl3  + 3NH 4 OH ¾¾® Cr(OH)3 + 3NH 4 Cl

Green ppt.

 

 

 

FeCl3 + 3NH 4 OH ¾¾®

Fe(OH)3

Brownish red ppt.

  • 3NH 4 Cl

 

Fe3+ (iron) : The brownish red precipitate dissolves in dilute HCl. The solution is divided into two parts.

Part I : K4 [Fe(CN)6 ] solution is added which forms deep blue solution or precipitate.

 

 

Fe(OH)3 + 3HCl ¾¾® FeCl 3 + 3HO ;

4 FeCl3  + 3K4 [Fe(CN)6 ] ¾¾® Fe4 [Fe(CN)6 ]3 + 12KCl

Prussian blue

 

Part II : Addition of potassium thiocyanate solution gives a blood red colouration.

FeCl3  + 3KCNS ¾¾® Fe(CNS)3 + 3KCl

Blood red colour

 

Cr3+(chromium) : The green precipitate is fused with fusion mixture (Na2 CO3 + KNO3 ). The fused product is extracted with water or the precipitate is heated with NaOH and bromine water.

2Cr(OH)3  + 3KNO + 2Na2 CO3  ¾¾® 2Na2 CrO4  + 3KNO2  + 2CO2  + 3H 2 O

 

or 2NaOH + Br2  ¾¾® NaBrO4 + NaBr + H 2 O ;

NaBrO ¾¾® NaBr + [O]

 

2Cr(OH)3 + 4 NaOH + 3[O] ¾¾® 2NaCrO4 + 5HO

The solution thus obtained contains sodium chromate. The solution is acidified with acetic acid and treated with lead acetate solution. A yellow precipitate appears.

Na2 CrO4  + Pb(CH3 COO)2  ¾¾® PbCrO4 + 2CH3 COONa

Yellow ppt.

 

 

Al3+(aluminium) : The gelatinous precipitate dissolves in NaOH ,

Al(OH)3

  • NaOH ¾¾® NaAlO2

Soluble

  • 2H 2 O

 

The solution is boiled with ammonium chloride when

Al(OH)3 is again formed.

 

 

NaAl 2  + NH 4 Cl + H 2 O ¾¾® Al(OH)3  + NaCl + NH 3

 

Group  IV  :  On passing

HS

through the filtrate of the third group, sulphides of fourth group are

 

precipitated. NiS and CoS are black and insoluble in concentrated HCl while MnS (buff coloured), ZnS (colourless) are soluble in conc. HCl.

 

Zn2+ (zinc) : The sulphide dissolves in HCl.

ZnS + 2HCl ¾¾® ZnCl 2 + HS

 

When the solution is treated with NaOH, first a white precipitate appears which dissolves in excess of NaOH

 

 

ZnCl 2  + 2NaOH ¾¾® Zn(OH)2 + 2NaCl ;

White ppt.

Zn(OH)2  + 2NaOH ¾¾® Na2 ZnO2 + 2H 2 O

(Soluble)

 

 

 

On passing

H 2 S , white precipitate of zinc sulphide is formed

Na2 ZnO2  + H 2 S ¾¾®

ZnS

White ppt.

  • 2NaOH

 

 

 

Mn2+ (manganese) : Manganese sulphide dissolves in HCl

MnS + 2HCl ¾¾® MnCl2 + H 2 S

 

On heating the solution with NaOH and

Br2 -water, manganese dissolne gets precipitated.

 

 

 

MnCl2  + 2NaOH ¾¾® Mn(OH)2  + 2NaCl ;  Mn(OH)2  + O ¾¾® MnO2 H 2 O

 

The precipitate is treated with excess of nitric acid and

PbO2  or

PbO4

(red lead). The contents are heated.

 

The formation of permanganic acid imparts pink colour to the supernatant liquid.

2MnO2 + 4 HNO3 ¾¾® 2Mn(NO3 )2 + 2H 2 O + O2

2Mn(NO3 )2  + 5Pb3 O4  + 26HNO3  ¾¾® 2HMnO4  + 15 Pb (NO3 )2  + 12 H 2 O

Permanganic acid (pink)

Note : ®  The above test fails in presence of HCl.

Ni2+ (nickel) and Co2+ (cobalt)

The black precipitate is dissolved in aqua- regia.

3Nis + 6HCl + 2HNO3  ¾¾® 2NiCl2  + 2NO + 3S + 2H 2 O

3CoS + 6HCl + 2HNO3  ¾¾® 3CoCl2  + 2NO + 3S + 4 H 2 O

The solution is evaporated to dryness and residue extracted with dilute HCl. It is divided into three parts.

 

Part I : Add

NH 4 OH

(excess) and dimethyl glyoxime. A rosy red precipitate appears, if nickel is present,

 

 

 

 

NiCl2 + 2

 

CH3C = NOH

|

CH3C = NOH

 

 

+ 2NH 4 OH ¾¾®

OH

|

CH3C = N

|

CH3C = N

¯

O

O

­

N   = C CH3

Ni                 |

N   = C CH3

|

OH

 

 

+ 2NH 4 Cl + 2H 2O

 

Part II : Add cobalt.

CH 3 COOH

in excess and

KNO2 . The appearance of yellow precipite confirms the presence of

 

KNO2 + CH 3 COOH ¾¾® CH 3 COOK + HNO2 ; CoCl2  + 2KNO2  ¾¾® Co(NO2 )2  + 2KCl

Co(NO2 )2 + 2HNO2 ¾¾® Co(NO2 )3 + NO + H 2 O ; Co(NO2 )3 + 3KNO2 ¾¾® K3 [Co(NO2 )6 ]

 

Part III : Solution containing either nickel or cobalt is treated with

NaHCO3

and bromine water. Appearance

 

of apple green colour is observed, the solution is heated when black precipited is formed, which shows the presence of nickel, CoCl2  + 2NaHCO3  ¾¾® Co(HCO3 )2  + 2NaCl

 

Co(HCO3 )2  + 4 NaHCO3  ¾¾® Na4 Co(CO3 )3  + 3H 2 O + 3CO2 ;

2Na4 Co(CO3 )3  + H 2 O + O ¾¾® 2Na3 Co(CO3 )3 + 2NaOH

sod. cobalti carbonate (Green colouration)

Br2 + H 2O ¾¾® 2HBr + O

 

 

 

NiCl2  + 2NaHCO3  ¾¾® NiCO3  + 2NaCl + H 2 O + CO2 ;

2NiCO3 + O ¾¾® Ni2 O3 + 2CO2

(Black)

 

Group V : Ammonium carbonate precipitates V group radicals in the form of carbonates are soluble in acetic

acid.

 

 

 

BaCO3 + 2CHCOOH ¾¾®(CH 3 COO)2 Ba + CO2 + H 2 O

SrCO3 + 2CH 3 COOH ¾¾®(CH 3 COO)2 Sr + CO2 + H 2 O

CaCO3 + 2CH 3 COOH ¾¾®(CH 3 COO)2 Ca + CO2 + H 2 O

Ba2+ (barium) : Barium chromate is insoluble and precipitated by the addition of potassium chromate

 

solution,

Ba(CH 3 COO)2  + K 2 CrO4  ¾¾® BaCrO4  + 2CH 3 COOK

 

Sr2+ (Strontium) : Strontium sulphate is insoluble and precipitated by the addition of ammonium sulphate solution, Sr(CH3 COO)2  + (NH 4 )2 SO4  ¾¾® SrSO4 + 2CH3 COONH4

White ppt.

 

Ca2+ (calcium) : Calcium oxalate is insoluble and precipitated by the addition of ammonium oxalate.

Ca(CH3 COO)2  + (NH 4 )2 C2 O4  ¾¾® CaC2 O4 + 2CH3 COONH4

White ppt.

 

Group VI : In the filtrate of V group, some quantity of ammonium oxalate is added as to remove

Ba, Ca

and

 

Sr completely from the solution. The clear solution is concentrated and made alkaline with hydrogen phosphate is now added, a white precipitate is formed.

MgCl2  + Na2 HPO4  + NH 4 OH ¾¾® Mg(NH 4 )PO4  + 2NaCl + H 2 O

Megnesium ammonium phosphate (White ppt.)

NHOH.

Disodium

 

4

NH +

(ammonium) : The substance (salt or mixture) when heated with NaOH solution evolves ammonia.

NHCl + NaOH ¾¾® NaCl + NH 3 + HO

 

When a rod dipped in HCl is brought on the mouth of the test tube, white fumes of ammonia chloride are

 

formed,

NH 3 + HCl ¾¾® NH 4 Cl

White fumes

 

To the aqueous solution of ammonium salt when Nessler’s reagents is added, brown coloured precipitate is formed.

 

 

Hg

2KHgI 4 + NHCl + 4 KOH ¾¾®

Hg

NH 2

O I

 

  • 7KI + KCl + 3H 2O

 

Iodide of Millon’s base

(Brown ppt.)

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