Download 3. The Hydrogen Ion Exponent (pH)

Department“ORGANIC CHEMISRTY AND TECHNOLOGY”
1
Chemistry for Engineering Logistics
HYDROGEN ION EXPONENT (pH)
1. Dissociation of water
2. Acids and bases
3. The hydrogen ion exponent (pH)
Assoc. Prof. V. Grozev
1. Dissociation of water
2
The concept of pH was first introduced by Danish Chemist Sorensen
at the Carlsberg Laboratory in 1909 and revised to the modern pH in
1924.
1.1. Water equilibrium
The dissociation of water is an ionization reaction in pure water or
an aqueous solution, in which a water molecule, H2O, loses the nucleus of
one of its hydrogen atoms (H+) to become a hydroxide ion, OH−. The
dissociation is an equilibrium process (water equilibrium):
H 2O
H+ + OH-
1. Dissociation of water
3
1.2. The water equilibrium constant
The water equilibrium constant (Kw) is defined as:
Kw = [H+] [OH-],
where [H+] and [OH-] are concentrations of H+ and OH- ions in the solution.
Experimentally, it has been found that the concentrations:
[H+] = [OH-] = 10-7
Therefore
Kw = [10-7][ 10-7] = 10-14
(1)
(To multiply exponential numbers - simply add the exponents).
The equation (1) shows that the multiplication of the
concentrations [H+] and [OH-] is a constant value and equals to 10-14
at 250C.
The sum of exponents in equation (1) must always be equal to 10-14.
2. Acids and bases
4
2.1. Definition of acid and bases (alkali)
The first modern definition of acids and bases was devised by Svante
Arrhenius (Sweden):
- acid is a substance that dissociates in water to form only hydrogen
ions (H+) and the concentration of H+ ions in aqueous solution increases:
HCl
H+ + Cl- base is a substance that dissociates in water to form hydroxide (OH−)
ions and the concentration of OH− ions in the solution increases:
NaOH
Na+ + OH-
2. Acids and bases
5
2.2. Effect of Acids and Bases on Water Equilibrium
If an acid (H+) is added to the water, the equilibrium shifts to the left
and the OH- ion concentration decreases:
H 2O
H+ + OH-
If base (OH-) is added to water, the equilibrium shifts to the left and
the H+ concentration decreases.
In accordance with water equilibrium principle the multiplication of
H+ and OH- ion concentrations must remain equal to 10-14.
Both H+ and OH- ions are always present in any solution. A solution
is:
- acidic, if the H+ are in excess - [H+] > 10-7, for exsample 10-6, 10-5,
10-4;
- basic, if the OH- ions are in excess ([H+] < 10-7 and [OH-] > 10-7);
- neutral ([H+] = [OH-] = 10-7).
3. The hydrogen ion exponent (pH)
6
The acidity or alkalinity of solutions can be described by means of
hydrogen ion concentrations – 10-12, 10-10, 10-4 and etc. But this way is
unsuitable and pH values have been introduced in 1924.
3.1. Definition
pH is defined as the negative decadic logarithm of the hydrogen
ion concentration. The equation is:
pH = - log [H+],
where - [H+] is hydrogen ion concentration;
- log – decadic (or Brigsian) logarithm with base 10.
The
purpose
positive pH value.
of
the
negative
sign
is
to
give
a
3. The Hydrogen Ion Exponent (pH)
7
3.2. Logarithm
The logarithm of a number is the exponent to which another fixed
value, the base, must be raised to produce that number. For example, the
logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: 1000
= 10 × 10 × 10 = 103. If x = by, then y is the logarithm of x to base b, and is
written y = logb(x), or y = logb(by), so log10(1000) = log10(103) = 3.
Number
Exponent Notation
Logarithm of the Number
1000
103
3
100
102
2
10
101
1
1
100
0
0.1
0.01
0.001
10-1
10-2
10-3
-1
-2
-3
3. The Hydrogen Ion Exponent (pH)
8
3.3. Example
If an acid has an H+ concentration of 0.0001, find the pH.
Solution:
First convert the number to exponential notation, find the logarithm,
then solve the pH equation.
[H+] = 0.0001 = 10-4;
log of 10-4: log (10-4)= - 4;
pH = - log [ H+] = - log (10-4) = - (-4) = +4 = pH
3. The Hydrogen Ion Exponent (pH)
9
3.4. pH scale
The hydrogen ion concentration [H+], pH numbers and hydroxide ion
concentration are represented in the table:
pH values:
Acidic
Neutral
Basic
pH<7
pH= 7
pH >7
3. The Hydrogen Ion Exponent (pH)
10
3.5. Measuring of pH
pH - values can be measured with universal indicator paper.
11
Thank You