Engineering Chemistry S Datta

194 ENGINEERING CHEMISTRY
+ −4
[NH ] 1.1 10
or =
4 × ×
= 1.8 × 10 –5 ∴ [NH + 4
] = 1.6 × 10 –3 g.ion l –1 .
0.01
Effect of a Salt without a Common Ion on Solubility
For the correct expression for solubility product we should use activity (a) in place of
concentration: i.e., K sp
= a
. a
+ −
Ag Cl
So, true expression is K sp
= γ
+ −
+ −
Ag
Cl
.[ Ag ]. γ [Cl ]
where γ = activity coefficient.
Q
activity (a) = Concentration (c) × activity coefficient (γ)
∴ a = cγ.
For a very dilute solution γ = 1 then a = c.
If we add KNO 3
to a saturated solution of AgCl the γ-term of the expression of K sp
decreases. So, [Ag + ] . [Cl – ] increases in order to keep the true K sp
value constant.
Therefore, solubility of a sparingly soluble salt increases on addition of any salt without
a common ion. This effect is known as salt effect.
Highlights:
• Many proteins, which are sparingly soluble in water, are soluble in salt solutions
due to this salt effect.
• The effect of common ion on solubility is utilised in qualitative inorganic analysis
to separate the ions present in a solution.
Application of Common Ion Effect and Solubility Product in Qualitative Inorganic
Analysis
General Group Separation of Basic Radicals:
Precipitation of a salt takes place when its ionic product exceeds its solubility product. A
salt may not get precipitated even when its ionic product exceeds its solubility product. It may
remain in saturated solution for sometime. That is why, for precipitation in chemical analysis,
the value of solubility product of the salt should always be higher than its ionic product.
Group I. The chlorides of Group I metals (Hg +2 2
, Pb +2 , Ag + ) are insoluble, on addition of
dil. HCl, ionic product (C
+
M
× C Cl
–) easily exceeds the solubility products and the cations are
precipitated. Only PbCl 2
has higher solubility product and for that reason it is not completely
precipitated here. It is also precipitated in Group II.
Group II. The reagent is H 2
S in presence of dil. HCl. The common ion H + suppresses the
ionisation of H 2
S. So concentration of sulphide ion C
=
S
remains low. The metallic ions (Hg +2 ,
Cu +2 , Pb +2 , Bi +3 , As +3 , Sb +3 , Sn +2 ), whose solubility product of sulphides are low, get precipitated
here i.e., despite low S = ion concentration, the ionic products (C
+2
M
× C –2 S
) of metal ions exceed
the solubility products because of low value of solubility products. IIIB metallic ion (Co +2 , Mn +2 ,
Ni +2 , Zn +2 ) sulphides have higher values of solubility products, so their precipitation is arrested
here, CdS and PbS have comparatively higher values of solubility products, so C
=
S
should be
high for them. To increase the ionisation of H 2
S, low acidity or higher pH is necessary; to
reduce acidity of the medium water is added to dilute the medium. Again diluted acid medium
precipitates colloidal As 2
S 3
and SnS which creates difficulty in filtration. So in more or less
concentrated acid medium they (Gr II) are precipitated and filtered and then filtrate is diluted
with water and passed H 2
S to precipitate CdS and PbS.
Group III A. Here reagent is NH 4
Cl/NH 4
OH. The common NH
+
4
suppresses the
dissociation of weak base NH 4
OH thereby C
–
OH
remains low. As a result Fe +3 , Al +3 , Cr +3 get
precipitated as hydroxides because their solubility products are exceeded. The precipitation of