The following table will help the students in the choice of a suitable solvent :
12.9.
QUALITATIVE ANALYSIS
Solvent
1. Cold water
2. Hot water
3. Dil. HCl
Salts which dissolve
+ + +
(a) All NH₄ , Na and K salts.
(b) All nitrites, nitrates and acetates.
(c) Most of the sulphates except that those of Pb, Ba, Ca, Sr.
(d) All chlorides except that of lead.
Lead chloride, lead nitrate.
All carbonates which do not dissolve in water i.e.,
Carbonates of Ca, Ba, Sr, Mg, Zn, Al, Cu, Ni, Mn, Fe etc.
but not of Pb.
The separation of cations into various groups by making use of suitable reagents (known as a group reagents) is based on the
differences in chemical properties of cations. For example, if hydrochloric acid is added to a solution containing all cations,
only the chlorides of lead, silver and mercury (ous) will precipitate, since all other chlorides are soluble. Thus, these cations
form a group of ions which may be precipitated from solution by addition of group reagent HCl. Similarly, H₂S is a group
reagent for group II. The following Table clearly shows the group reagents for different groups and the form in which cations
of the particular group are precipitated out.
Group Group reagent Cations
Form in which cations
are precipitated
Group Zero No
Group I
Group II
Group III
Group IV
Group V
Group VI
Dilute HCl
H₂S in the presence of dilute HCl
NH₄OH in the presence of NH₄Cl
H₂S in the presence of NH₄OH
(NH₄)₂CO₃ in the presence of NH₄OH
No
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Chlorides
Sulphides
Hydroxides
Sulphides
NH₄⁺
Pb ²⁺
Mg ²⁺
Pb²⁺
Cu²⁺, As ³⁺
Fe³⁺, Al ³⁺
Ni ²⁺, Mn ²⁺, Zn²⁺
Co²⁺
Ca²⁺, Ba ²⁺, Sr²⁺ Carbonates
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