BASIC INTRODUCTION TO SOLUTION, pH
VALUE AND VOLUMETRIC ANALYSIS (MSU
POLYTECHNIC)
CHAPTER-3
BASIC INTRODUCTION TO SOLUTION,
pH VALUE AND VOLUMETRIC
ANALYSIS
SOLUTION:
A solution is a homogeneous mixture of one or more solutes dissolved in
a solvent.
Solute: The substance that dissolves in a solvent to produce a
homogeneous mixture.
Solvent: The substance in which solute dissolves to produce a
homogeneous mixture.
Example: Salt in water, Sugar solution etc.
HYDROGEN ION CONCENTRATION:
The acidic or alkaline property of an aqueous solution depends upon the
concentration of H+ ions present in it. The concentration of H+ ions in a solution
is the number of gram ions present in one litre of it. Water is a weak electrolyte
and its ionisation into H+ ions and OH- ions takes place as below
H2O H+ + OH-
Applying the law of Mass action to the case of water, we find
[ ] [ ]
= Constant
Since concentration of the undissociated molecules of water is very large
in comparison with that of H+ ions and OH- ions, we may assume it to remain
practically constant, hence we write
[𝐻 ]𝑥 [𝑂𝐻 ]= [H2O] * constant = kw
Hence the product concentration of H+ and OH- ions present in water is a
constant that is called as ionic Product of water and is expressed as kw.
-1-
, BASIC INTRODUCTION TO SOLUTION, pH
VALUE AND VOLUMETRIC ANALYSIS (MSU
POLYTECHNIC)
Practically the value of kw has been found as 1 x 10-14 gm ion per litre. Thus
[𝐻 ]𝑥 [𝑂𝐻 ]= 1 x 10-14 gms ion/litre
Pure water being neutral consists of equal concentration of H+ and OH- ions.
Therefore
[𝐻 ] = [𝑂𝐻 ] = 1 x 10-7 gms ion/litre
Thus, the solution having hydrogen ion concentration as 1 x 10-7 gms
ion/litre is neutral. In acid solution concentration of H+ ion predominates i.e. it
is above 1 x 10-7 gms ion/litre. But in alkaline solution concentration of OH- ion
predominates i.e: in such solutions the concentration of H+ ions is less than 1 x
10-7 gms ion/litre. But in all cases, the product of [H]+ and [OH]- is the same.
i) [𝐻 ] > [𝑂𝐻 ] Acidic solution
ii) [𝐻 ] < [𝑂𝐻 ] Basic solution
iii) [𝐻 ] = [𝑂𝐻 ] Neutral solution
pH VALUE:
In order to express smaller concentrations for acid, neutral and alkaline
solutions in a more convenient way, Sorensen in 1909 introduced a new scale
which is known as pH scale. In setting up the pH scale, only the hydrogen ion
concentration is used. It is based on the equation.
pH = -log10[H+]
pH can thus be defined as below:
pH of a solution is the logarithm of hydrogen ion concentration with a negative
sign. The concentration of hydrogen ions in pure water is 0.0000001 per litre or
[10-7]
pH = -log10[10-7]
pH = 7
We know that water is neutral. So, any solution of pH 7 will be neutral.
Thus, by the variations of H+ and OH- ion concentrations the following series of
solutions can be arranged to have a pH scale. It is clear from the scale that total
range of pH value is from 0 to 14. Pure water or all neutral solutions have a pH
of 7. If the concentration of H+ ions is greater than that in pure water i.e., it is
above 10-7, the solution is acidic and pH value lesser than 7 up to zero.
Conversely if the H+ ion concentration is lesser than that in pure water, the
solution is basic and pH is larger number than 7 up to 10.
-2-
VALUE AND VOLUMETRIC ANALYSIS (MSU
POLYTECHNIC)
CHAPTER-3
BASIC INTRODUCTION TO SOLUTION,
pH VALUE AND VOLUMETRIC
ANALYSIS
SOLUTION:
A solution is a homogeneous mixture of one or more solutes dissolved in
a solvent.
Solute: The substance that dissolves in a solvent to produce a
homogeneous mixture.
Solvent: The substance in which solute dissolves to produce a
homogeneous mixture.
Example: Salt in water, Sugar solution etc.
HYDROGEN ION CONCENTRATION:
The acidic or alkaline property of an aqueous solution depends upon the
concentration of H+ ions present in it. The concentration of H+ ions in a solution
is the number of gram ions present in one litre of it. Water is a weak electrolyte
and its ionisation into H+ ions and OH- ions takes place as below
H2O H+ + OH-
Applying the law of Mass action to the case of water, we find
[ ] [ ]
= Constant
Since concentration of the undissociated molecules of water is very large
in comparison with that of H+ ions and OH- ions, we may assume it to remain
practically constant, hence we write
[𝐻 ]𝑥 [𝑂𝐻 ]= [H2O] * constant = kw
Hence the product concentration of H+ and OH- ions present in water is a
constant that is called as ionic Product of water and is expressed as kw.
-1-
, BASIC INTRODUCTION TO SOLUTION, pH
VALUE AND VOLUMETRIC ANALYSIS (MSU
POLYTECHNIC)
Practically the value of kw has been found as 1 x 10-14 gm ion per litre. Thus
[𝐻 ]𝑥 [𝑂𝐻 ]= 1 x 10-14 gms ion/litre
Pure water being neutral consists of equal concentration of H+ and OH- ions.
Therefore
[𝐻 ] = [𝑂𝐻 ] = 1 x 10-7 gms ion/litre
Thus, the solution having hydrogen ion concentration as 1 x 10-7 gms
ion/litre is neutral. In acid solution concentration of H+ ion predominates i.e. it
is above 1 x 10-7 gms ion/litre. But in alkaline solution concentration of OH- ion
predominates i.e: in such solutions the concentration of H+ ions is less than 1 x
10-7 gms ion/litre. But in all cases, the product of [H]+ and [OH]- is the same.
i) [𝐻 ] > [𝑂𝐻 ] Acidic solution
ii) [𝐻 ] < [𝑂𝐻 ] Basic solution
iii) [𝐻 ] = [𝑂𝐻 ] Neutral solution
pH VALUE:
In order to express smaller concentrations for acid, neutral and alkaline
solutions in a more convenient way, Sorensen in 1909 introduced a new scale
which is known as pH scale. In setting up the pH scale, only the hydrogen ion
concentration is used. It is based on the equation.
pH = -log10[H+]
pH can thus be defined as below:
pH of a solution is the logarithm of hydrogen ion concentration with a negative
sign. The concentration of hydrogen ions in pure water is 0.0000001 per litre or
[10-7]
pH = -log10[10-7]
pH = 7
We know that water is neutral. So, any solution of pH 7 will be neutral.
Thus, by the variations of H+ and OH- ion concentrations the following series of
solutions can be arranged to have a pH scale. It is clear from the scale that total
range of pH value is from 0 to 14. Pure water or all neutral solutions have a pH
of 7. If the concentration of H+ ions is greater than that in pure water i.e., it is
above 10-7, the solution is acidic and pH value lesser than 7 up to zero.
Conversely if the H+ ion concentration is lesser than that in pure water, the
solution is basic and pH is larger number than 7 up to 10.
-2-
