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Experiment # 9 : Cation Analysis: Group ( I )
I. PURPOSE OF THE EXPERIMENT
The purpose of inorganic qualitative analysis is to enable
 the acquisition of powers of observation and deduction
 the learning of reactions of inorganic chemistry
 the development of practical chemical skills
II. INTRODUCTION
In many real situation, a chemist must gather observation to decide the chemical
identity of an unknown sample. Given a totally “ unknown” sample, how does one
go about determining what is actually present? This process is called “ qualitative
analysis”.
Cations are classified into five groups on the basis of their behavior against some
reagents by using group reagents; we can decide presence or absence of groups of
cations and can also separate these groups for further examination.
The group reagents used for the classification of most common cations are
hydrochloric
acid,
hydrogen
sulphide,
ammonium
sulphide
and
ammonium carbonate.
Classification is based on whether a cation reacts with these reagents by
the formation of precipitates or not. Therefore, it can be said that
classification of most common cations is based on the differences of
solubilities of their chlorides, sulphides, and carbonates.
The five groups are as follows: Group I…Cations form precipitates with dilute hydrochloric acid.
Ions of this group are…
Lead, Mercury (I), and Silver
Group II.Cations do not react with hydrochloric acid, but form precipitates
with hydrogen sulphide in dilute mineral acid medium.
Ions of this group are: II A. Mercury (II), Copper, Bismuth, Cadmium
Sulphides of II A are insoluble in (NH4) 2 S
IIB.Arsenic (III), Arsenic (V), Antimony (III), Antimony (V), Tin (II) and
Tin (IV). Sulphides of II B are soluble in (NH4) 2 S.
GROUP III. Cations do not react either with dilute hydrochloric acid, or
with hydrogen sulphide in dilute mineral acid medium.
They form precipitates with ammonium sulphide in neutral or ammonical
media.
They are Cobalt (II), Nickel (II), Iron (II), Iron (III), Chromium (III),
Aluminium, Zinc, and Manganese (II).
GROUP IV. Cations do not react with the reagents of group I, II and III.
They form precipitates with Ammonium carbonate in the presence of
Ammonium chloride in neutral or slightly acidic media.
GROUP V. Cations do not react with reagents of the previous groups.
They are Magnesium, Sodium, Potassium, Ammonium, Lithium, and
Hydrogen ions.
III. EXPERIMENTAL
3.1 Chemicals
3.2 Equipment
3.3 Procedure
Take 5ml known solution sample in the test tube and follow the instructions
accordingly, and report your results in the table given below:
Group I
No.
Reagent
1.
Add 3M HCl
(a)
+ NH3 soln (or)
NH4OH soln.
(b)
+ boil in hot water
2.
NaOH soln.
3.
KIsoln
(Potassium.Iodide)
4.
K2CrO4soln.
(Potassium
Chromate)
5.
Na2CO3 soln.
Pb 2+
Hg2 2+
Ag +
Flow Chart for Cation Group I
Salt solution mixture
1. Add Dil.HCl and stir well
2. Filter
Filterate I
(may contain ions of Group II to V)
Residue I
AgCl (white ppt.)
PbCl2 ( white ppt.)
Hg2Cl2 (white ppt.)
1.Wash with a small
amount of D. H20
2.Boil with water
3.Filter while hot
Filterate II (PbCl2)
Divide into two portions
Residue II (AgCl & Hg2Cl2)
Add NH4OH soln.
& filter
Residue III
HgNH2Cl
+ Hg (black)
Filtrate III
Ag (NH3)2Cl
Add K2CrO4 soln.
and then add NaOH
soln. in excess
Add KI soln.,
boil and cooled
down
Acidify with
Dil. HCl
1.Dissolve in
Aqua Regia
2.Dilute the
soln. with H2O
3.Add SnCl2 in
excess
Greyish black ppt.
“Hg”
white ppt. Ag Cl
Yellow ppt. PbCrO4
dissolve in excess
NaOH to give
yellowish green soln.
Golden yellow
PbI2 crystals
confirmed
presence of Pb2+
ion.
Equations for Cation Analysis : Group ( I )
1. Ag + ion (Silver Nitrate salt solution)
AgNO3(aq) + HCl(aq)
AgCl (s) + HNO3(aq)
AgCl(s)
Ag (NH3)2Cl(aq) + 2H2O (l)
+ 2NH4OH(aq)
Ag(NH3)2Cl + 2HCl(aq)
AgCl(s)
+ 2NH4Cl(aq)
Ag (NH3)2Cl ……………. Silver Ammonium Chloride
2. Hg22+ ion [Mercury (I) Nitrate salt solution]
Hg2(NO3)2(aq) + 2HCl(aq)
Hg2Cl2(s)
Hg2Cl2 (s) + 2HNO3 (aq)
+ 2NH4OH(aq)
HgNH2Cl(aq) + Hg(s)
+ NH4Cl(aq) + 2H2O(l)
HgNH2Cl(aq) + Aqua Regia
HgCl2(aq) (oxidize from +1 to +2)
2HgCl2(aq)
+ SnCl2(aq)
Hg2Cl2(s) + SnCl4(aq)
Hg2Cl2(aq)
+ SnCl2(aq)
Hg(s)
+ SnCl4 (aq)
(excess)
Hg2(NO3)2
………Mercury (I) Nitrate
HgNH2Cl
……….Mercury (II) amino Chloride
Hg2Cl2
………Mercury (I) Chloride
3. Pb2+ ion [Lead (II) nitrate salt solution]
Pb(NO3)2(aq) + 2HCl(aq)
PbCl2(s) + 2HNO3(aq)
PbCl2(s) + K2CrO4(aq)
PbCrO4(s) + 2KCl(aq)
PbCrO4 (s) + 4NaOH(aq)
Na2CrO4(aq) + Na2Pb(OH)4(aq)
PbCl2(s) + 2KI (aq)
PbI2(s)
+ 2KCl(aq)
Golden yellow crystal
K2CrO4
…………..Potassium Chromate
PbCrO4
………….Lead Chromate
Na2CrO4
………….Sodium Chromate
Na2Pb(OH)4
…………..Tetrahydroxy Sodium Plumbate
(Disodium Lead (II) tetrahydroxide)
IV.
RESULTS and DISCUSSIONS
The given unknown salt mixture solution is found to be Cation Group (I)
(i)
Silver ion is confirmed
(ii)
Mercury (I) ion is confirmed
(iii)
Lead (II) ion is confirmed
Discuss (a) What is the group reagent for Cation Group (I) ?
(b) How can you separate Lead ion from Mercury(I) ion and Silver ion?
( c) What is an Aqua Regia?
V.
CONCLUSION
The given unknown salt mixture solution is found to be Silver ion,
Mercury (I) ion and Lead (II) ion from Cation Group (I).