1. pH

2. Ionic product of water
Water undergoes ionization to a very small
extent producing H+ and OH- ions.
So there is an equilibrium between these ions
and the un ionized water molecules.
H2O H+ + OH-

3. The product of the concentrations of H+
and OH- ions in water is called ionic
product of water, Kw
Kw = [H+] [OH-]

4. In pure water [H+] = 10-7 moles per litre at 25oC
And [OH-] = 10-7 moles per litre at 25oC
Kw = [H+] [OH-] = 10-7 X 10-7 moles2/liter2

5. In neutral solution, [H+] = [OH-] = 10-7 moles/litre
If [H+] = 10-7, the solution is neutral.
In acidic solution, [H+] > [OH-]
If [H+] > 10-7, the solution is acidic.
In basic solution, [H+] < [OH-]
If [H+] < 10-7, the solution is basic.

6. pH Scale
pH scale is a method of expressing hydrogen ion
concentration.
It is defined as negative logarithm to the base
10 of hydrogen ion concentration in moles per
litre.
pH = -log10 [H+]

7. pH of pure water
pH of water = -log[H+] = -log 10-7
= -[-7 log 10] = 7
Fundamental equations,
log Xn = n log X, log X-n = -n log X
log (XY)= log X + log Y
log (X/Y) = log X-log Y
log 10 = l
log 1 = 0

8. pH of an alkaline solution
pH of a base < 10-7
Suppose if it is 10-8 [H+] = 10-8
-log [H+] = -log 10-8
= -(-8) log 10 = 8 log 10 = 8 x1 = 8

9. pH of an acidic solution
pH of an acid > 10-7
Suppose if it is 10-2 [H+] = 10-2
-log [H+] = -log 10-2
= -(-2) log 10 = 2 log 10 = 2 x1 = 2

10. Kw = [H+] [OH-]
If [OH-] = 10-3
Kw = [H+] 10-3
10-14 = [H+] 10-3
[H+] = 10-14 = 10-11
10-3
-log [H+] = -log 10-11
= -(-11) log 10 = 11 log 10 = 11 x1 = 11

12. Applications/importance of pH value

13. 1. To find out acidic or basic or neutral nature of a medium
2. Selection of pH can reduce the rate of corrosion.
Solutions with low pH value (acidic Medium) increases the
rate of corrosion.
3. In the production of potable water, coagulation of impurities by
the addition of alum takes place satisfactorily in the slightly
acidic pH range.
4. Effluents from chemical factories are either acidic or alkaline.
If we know the pH of effluents it is very easy to neutralize
properly before discharging them into rivers.
5. In electroplating pH is one of the factor which affects the
character and appearance of the deposit.

14. 6. pH value of the soil is very important in agriculture.
If we know the pH of the soil, it will be helpful to select the
proper cultivation.
It is also important to find out the type of fertilizers to be applied
pH of soil in important.
7. pH of human gastic juice is between 1 and 3. Increase of pH
generally causes vomiting.
8. Acidity in water when used for bleaching cotton will do harm to
the textiles.
9. In food preservation pH is to be known. Because in biological
reactions such as digestion of food depends on pH.
10. pH of human blood = 7.36 to 7.42, a change in pH by 0.2 units
results in death.

15. Important of pH in boilers
Boiler metal is corroded by the acidic nature of
water. In boilers acid is liberated by the
hydrolysis of salts of calcium and magnesium at
high temperature.
CaCl2 + 2 H2O Ca(OH)2 + 2 HCl
MgCl2 + 2 H2O Mg(OH)2 + 2 HCl
The boiler corrosion may leads to explosion.

16. Buffer Solution

17. A solution whose pH value remains constant even
after the addition of small amount of acid or base is
called buffer solution.
It is a solution in which H+ ion concentration remains
practically constant.
i) A weak acid and its salt with a strong base.
Eg: A mixture of acetic acid and sodium acetate.
ii) A weak base and its salt with a strong acid.
Eg: A mixture of ammonium hydroxide and
ammonium chloride.

18. Problems