Summary Investment analysis; all you need to know to pass the exam
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Course
Qualitative Analysis
Institution
Qualitative Analysis
19.1 IntroductIon
Qualitative analysis deals with the identification of
inorganic substance or a mixture of substances. Gen-
erally inorganic substances dissociate in water to give
positive and negative ions. The positive ions are also
called as basic radicals and the negative ions are called
...
19.1 IntroductIon Green: Cr2 O3 , Hg 2 I 2 , Cr ( OH )3 ; ferrous salts i.e.,
Qualitative analysis deals with the identification of FeSO 4 ⋅ 7H2 O; FeSO 4 ⋅ ( NH4 )2SO 4 ⋅ 6H2 O; FeCl2 ⋅ 4H2 O
inorganic substance or a mixture of substances. Gen-
nickel salts, CrCl3 ⋅ 6H2 O, CuCl2 ⋅ 2H2 O, CuCO3 , K 2 MnO 4
erally inorganic substances dissociate in water to give
positive and negative ions. The positive ions are also Blue: Anhydrous cobalt salts; hydrated cupric salts;
called as basic radicals and the negative ions are called Prussian blue.
as acid radicals. The tests carried for the identification Brown: PbO2, CdO, Fe3O4, SnS, Fe2O3, and Fe(OH)3
of different ions can be divided into three types (reddish brown)
Black: PbS, CuS, CuO, HgS, FeS, MnO2, Co3O4,
1. Preliminary tests CoS, NiS, Ag2S.
2. Tests for anions The colour of the solution obtained when the
3. Tests for cations substance is dissolved in water or in dilute acids should
be noted. As this may often give valuable information.
The following colours are shown by the ions (the cations
19.2 PrelImInary tests
are usually hydrated) present in the dilute solution
19.2.1 colour Blue: Cupric copper
The colour of the substance gives the indication of the Green: Nickel, Ferrous iron, chromic chromium
presence of certain cations and anions. Generally Yellow: Chromates, ferrocyanides, ferric iron
the transition metals form coloured compound. From Orange-red: Dichromates
the colour of the substance, the presence of certain cati- Purple: Permanganates
ons can be predicted. Some of the commonly occurring Pink: Cobalt, manganese
compounds are listed below.
Red: Pb3 O 4 , As 2 S3 , HgO, Sb2S3 , CrO3 , Cu 2 O
K 3 [Fe ( CN )6 ]; dichromates are orange red; 19.2.2 action of Heat
permanganates and chrome alums are reddish purple. Place a small quantity of the substance in a dry igni-
Pink: Hydrated salts of manganese and of cobalt. tion tube so that none of it remain adhering to the
Yellow: CdS, As2S3, SnS2, PbI2, HgO (precipitated) sides and heat cautiously; the tube should be held in an
K 4 [Fe ( CN )6 ] ⋅ 3H2 O; Normal chromates, ferric chlo- almost horizontal position. The temperature is gradu-
ride and nitrate. ally raised.
,19.2 Objective Chemistry - Vol. II
chlorides are more volatile, the substance is mixed with
Observation Inference
concentrated hydrochloric acid to convert the metal salt
(a) The substance changes colour into metal chloride.
(i) Yellow when hot white ZnO and many Zn Salts
when cold Flame colour Inference
(ii) Yellowish-brown when SnO2 or Bi2O3
Golden Yellow Sodium
hot, yellow when cold
Violet (lilac) Potassium
(iii) Red to black when hot, Fe2O3
Carmine red Lithium
brown when cold
Brick red Calcium
(b) Sublimate is formed
Crimson red Strontium
(i) White sublimate HgCl2, HgBr2, Hg2Cl2
Apple green (Yellow green) Barium
ammonium halides,
Bluish green Copper
As2O3, Sb2O3
Green flashes Zinc or Manganese
(ii) Blue black sublimate Iodine
(c) A gas or vapour is evolved
(i) Water is evolved; Test Compounds with water 19.2.4 charcoal cavity test
with litmus paper of crystallization (often Heat a little substance in a small cavity scooped in a
accompanied by change in charcoal block using a blow pipe. The salt is heated in
colour) ammonium salts, the charcoal cavity directly or after mixing with sodium
acid salts, hydroxides carbonate or fusion mixture (Na2CO3 + K2CO3)
The water is alkaline Ammonium salts The sodium carbonate or fusion mixture converts
The water is acid Readily decomposed salts the metallic salt into a carbonate which decomposes
of strong acids into metal oxides on heating.
(ii) Oxygen is evolved Nitrates, chlorates, The metal oxides
(rekindles glowing perchlorates, bromates, (i) the metal oxides are left as coloured residues, the
splint). iodates, peroxides, colour of the residue being characteristic of the
permanganates. basic radical present (or)
(iii) Nitrous oxide (rekindles Ammonium nitrate (ii) the metal oxides undergo reduction to metallic
glowing splint) and state by the reducing action of carbon or charcoal
steam are evolved resulting in the formation metallic beads of easily
(iv) Reddish brown fumes Nitrates or nitrites of fusible (or)
heavy metals. (iii) the metal so formed volatilizes in the form of
(v) CO2 is evolved (lime Carbonates, bicarbonates vapours which burn in air to form oxide. These get
water rendered turbid) in the form of the layer or crust around the cavity
(vi) Ammonia is evolved Ammonium salts (incrustation)
(odour; turns red litmus
paper blue, turns Observation Inference
mercurous nitrate paper
(i) White infusible and BaO, SrO, CaO, MgO
black)
incandescent when hot (residue is alkaline to litmus
(vii) SO2 is evolved (odour Normal and acid sulphites
paper)Al2O3, ZnO (residue
smell of burning sulphur,
not alkaline to litmus paper)
turns K2Cr2O7 paper
(ii) Incrustation with out metal
green)
(a) white, yellow when hot ZnO
(viii) H2S is evolved (Rotten Acid sulphides
(b) white, garlic odour As2O3
eggs smell, turns lead
(iii) Incrustation with metal
acetate paper black.)
(a) White incrustation; Sb
brittle metal
19.2.3 Flame colour test (b) Yellow incrustation; Bi
brittle metal
Place a small quantity of the substance on a watch glass
(c) Yellow incrustation, Pb
moisten with a little concentrated hydrochloric acid and
malleable metal,
introduce a little of the substance on a clean platinum wire
marks paper
into the base of the non luminous Bunsen flame. Since
, Qualitative Analysis 19.3
In certain cases the bead becomes even opaque in
Observation Inference
the reducing flame. This happens when too much of
(iv) Metal without the metal borate is present and its reduction to metallic
incrustation state is possible.
(a) Grey metallic particles Fe, Ni, Co
attracted by magnet 2 CuBO2 + C ⎯⎯
→ 2 Cu + B2 O3 + CO
(b) Malleable beads Ag and Sn (white), Cu (red Re ddish brown when hot.
flakes) The colours produced by different cations are
shown in Table 19.1.
Cobalt nitrate Test: This test is performed if white
infusible residue is obtained in charcoal cavity test. The table 19.1 Colour of the borax bead with different
salts of aluminium, magnesium, zinc, calcium etcyields cations
oxides which are white in colour. This residue is heated
with cobalt nitrate solution in a reducing flame. Cobalt Colour of bead in Colour of bead in Metal
oxide formed as a result of decomposition of cobalt oxidizing flame reducing flame
nitrate combines with metallic oxides to form coloured Hot Cold Hot Cold
compounds which are characteristic cations. (i) Green Blue Colourless Opaque red Copper
2 Co ( NO3 ) ⎯⎯
→ 2 CoO + 4 NO2 + O2 (ii) Yellowish- Yellow Green Green Iron
2
brown or
CoO + ZnO ⎯⎯
→ Co ZnO2 red
Green residue ( Rinmann ’s green ) (iii) Dark Green Green Green Chromium
yellow
→ Co ( AlO2 )
CoO + Al2 O3 ⎯⎯
2 (iv) Violet Violet Colourless Colourless Manganese
Blue residue ( Thenard Blue ) (amethyst) (amethyst)
(v) Blue Blue Blue Blue Cobalt
CoO + MgO ⎯⎯
→ CoMgO2
(vi) Brown Brown Grey or Opaque Nickel
Pink residue
Black
19.2.5 Borax Bead test
When borax is heated, it first loses its water of crystal- 19.2.6 microcosmic salt Bead test
lization and then decomposes to give a clear and trans- The bead is produced similarly to the borax bead except
parent bead consisting of boric anhydride and sodium that microcosmic salt is used. The colourless transpar-
metaborate. ent bead contains sodium metaphosphate
Na 2 B 4 O7 ⋅10H2 O ⎯Δ⎯
→ Na 2B 4O7 + 10H2 O
Na( NH 4 )HPO3 ↽
NaPO3 + H2 O + NH3
⇀
Na 2 B 4 O7 ⎯Δ⎯
→ 2 NaBO2 + B2 O3
This combines with metallic oxides forming
Boric anhydride being less volatile displaces more
orthophosphates which are often coloured.
volatile acidic anhydrides from their salts to form
The colours of the metal phosphate beads are same
metaborates which possess characteristic colours
as in the borax bead test given in Table 19.1.
→ Cu ( BO2 ) + SO3
B2 O3 + CuSO 4 ⎯⎯
2
In some cases meta borates of two different colours 19.3 tests For anIons
are produced in oxidizing and reducing flames The tests for anions can be grouped into three types
→ Cu ( BO2 ) + SO3
B2 O3 + CuSO 4 ⎯⎯ 1. The anions which respond to dilute acid.
2
green when hot and blue CO32 −, SO32 −, S2 −, CH3COO −
when cold in oxidising flame. 2. The anions which respond to conc. H2SO4
Cl −, Br −, I −, NO3−
2 Cu ( BO2 ) + C ⎯⎯
→ 2 CuBO2 + B2 O3 + CO
2 3. The anions which does not respond to dil or conc
2–
Colourless H2SO4, SO4
, 19.4 Objective Chemistry - Vol. II
19.3.1 anions which respond to dilute acid BaCl2 + Na 2 CO3 ⎯⎯
→ BaCO3 + 2 NaCl
Treat a small amount of the substance in a small test- CaCl2 + Na 2 CO3 ⎯⎯
→ CaCO3 + 2NaCl
tube with 2 mL of 2N hydrochloric acid or sulphuric
acid and note the reaction taking place BaCO3 + 2HCl ⎯⎯
→ BaCl2 + H2 O + CO2
CaCO3 + 2HCl ⎯⎯
→ CaCl2 + H2 O + CO2
Observation Inference
1. Colourless gas is evolved with CO2 from carbonate or 4. Silver Nitrate Solution: White precipitate of silver
effervescence. Gas is odourless bicarbonate carbonate Ag2CO3 with solution of normal carbon-
and produces turbidity when ates soluble in ammonia solution and in nitric acid. The
passed into lime water precipitate become yellow upon addition of excess of
the reagent and is partly decomposed on boiling with
2. Colourless gas is evolved, with SO2 from sulphite water into brown silver oxide Ag2O and carbon dioxide.
suffocating odour; turns filter 5. Test for Bicarbonate: If sodium bicarbonate solu-
paper moistened with acidified tion is added to magnesium sulphate, no precipitate is
K2Cr2O7 solution to green formed because magnesium bicarbonate is soluble in
3. Colourless gas with rotten H2S gas from sulphides water. But on boiling the solution a white precipitate
eggs smell; gives above test and appears due to the formation of magnesium carbonate
turns the filter paper dipped in MgCO3. Solutions of carbonates give white precipitate
lead acetate to black of magnesium carbonate in cold condition with magne-
4. Smell of vinegar on warming Acetic acid from acetates sium sulphate solution.
reactIons oF anIons WHIcH Δ
Mg ( HCO3 ) ⎯⎯→ Mg CO3 + H2 O + CO2
resPond WItH dIlute acId 2
Mg SO 4 + Na 2 CO3 ⎯Cold
⎯⎯ → Mg CO3 + Na 2SO 4
19.4.1 reactions of carbonate Ion
1. All normal carbonates except those of alkali metal 6. Test with Phenolphthalein: Carbonate solutions turns
carbonates and ammonium carbonate are insoluble in the phenolphthalein to pink while bicarbonate solutions
water. The bicarbonates of Ca, Sr, Ba, Mg and possi- cannot.
bly of iron exist only in solution. The bicarbonates of
the alkali metals are soluble in water but are less soluble
than the corresponding normal carbonates. 19.4.2 reactions of sulphite Ion so32–
2. With dilute hydrochloric acid: When dil HCl or 1. Solubility: Only the sulphites of the alkali metals and
dil H2SO4 is added to a salt containing carbonate or ammonia are soluble in water; the sulphites of other
bicarbonate brisk effervescence will be evolved due to metals are either difficultly soluble or insoluble in water.
evolutions of CO2 by the decomposition of carbonate The bisulphites of the alkali metals are soluble in water.
or bicarbonate. The gas liberated when passed into lime The bisulphites of the alkaline earth metals exist
water turns it milky due to the formation of calcium car- only in solution.
bonate. But passing more, the white precipitate formed 2. Dilute Hydrochloric or Dilute Sulphuric Acid: Decom-
will be dissolved again. poses more rapidly on warming with evolution of sul-
→ Na 2SO 4 + H2 O + CO2 ↑
Na 2 CO3 + H2SO 4 ⎯⎯ phur dioxide which may be identified
NaHCO3 + H2SO 4 ⎯⎯
→ NaHSO 4 + H2 O + CO2 ↑ (i) By its suffocating odour of burning sulphur
(ii) It turns the filter paper dipped in acidified potas-
Ca ( OH )2 + CO2 ⎯⎯
→ CaCO3 ↓ + H2 O sium dichromate to green
Lime water milky
Na 2SO3 + 2HCl ⎯⎯
→ 2 NaCl + H2 O + SO2
CaCO3 + CO2 + H2 O ⎯⎯ a ( HCO3 )
→ Ca
2 → K 2 SO 4 + Cr2 ( SO 4 ) + H2 O
K 2 Cr2 O7 + H2 SO 4 + 3SO2 ⎯⎯
3
3. Barium Chloride or Calcium Chloride Solutions:
White precipitate of barium carbonate BaCO3 or calci- 3. Barium Chloride or Calcium Chloride Solution: White
um carbonate CaCO3 with solutions of normal carbon- precipitate of the sulphite BaSO3 or CaSO3 readily soluble
ates. The precipitate is soluble in dil HCl or dil HNO3 in dilute hydrochloric acid. On standing, the precipitate is
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