pH

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-+

The product of the concentrations of H+ and OH- ions in water is called ionic product of water, Kw

Kw [H+] [OH-]=

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

In neutral solution, [H+] = [OH-] = 10-7 moles/litre

In acidic solution, [H+] > [OH-]

In basic solution, [H+] < [OH-]

If [H+] = 10-7, the solution is neutral.

If [H+] > 10-7, the solution is acidic.

If [H+] < 10-7, the solution is basic.

pH ScalepH 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+]

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 Xlog (XY)= log X + log Ylog (X/Y) = log X-log Ylog 10 = llog 1 = 0

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

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

Kw [H+] [OH-]=

If [OH-] = 10-3

Kw [H+] 10-3=

10-14 [H+] 10-3=

[H+] = 10-14

10-3

= 10-11

-log [H+] = -log 10-11

= -(-11) log 10 = 11 log 10 = 11 x1 = 11

Applications/importance of pH value

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.

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.

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.

Buffer Solution

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.

Problems