Ionic Product of Water:
Pure water is a very weak electrolyte and ionises according to the equation
H2O ↔ H+ + OH-
Applying law of mass action at equilibrium, the value of dissociation constant, K comes to
K = [H+] [OH-]/[H2O]
or [H+][OH-] = K[H20]
Since dissociation takes place to a very small extent, the concentration of undissociated water molecules, [H20], may be regarded as constant. Thus, the product #[H20] gives another constant which is designated as Kw. So,
[H+][OH-] = Kw
The constant, Kw, is termed as ionic product of water.
The product of concentrations of H1 and OH ions in water at a particular temperature is known as ionic product of water. The value of Kw increases with the increase of temperature, i.e., the concentration of H+ and OH- ions increases with increase in temperature.
Temperature (°C) Value of Kw
0 0.11 x 10-14
10 0.31 x 10-14
25 1.00 x 10-14
100 7.50 x 10-14
The value of Kw at 25°C is 1 x 10-14. Since pure water is neutral in nature, H+ ion concentration must be equal to OH- ion concentration.
[H+] = [OH˜] = x
or [H+][OH-]=x2= 1 x 10-14
or x = 1 x 10-7 M
or [H+] = [OH-] = 1 × 10-7 mol litre-1
This shows that at 25°C, in 1 litre only 10-7 mole of water is in ionic form out of a total of approximately 55.5 moles.
When an acid or a base is added to water, the ionic concentration product, [H+][OH-], remains constant, i.e., equal to Kw but concentrations of H+ and OH- ions do not remain equal. The addition of acid increases the hydrogen ion concentration while that of hydroxyl ion concentration decreases, i.e.,
[H+] > [OH-]; (Acidic solution)
Similarly, when a base is added, the OH- ion concentration increases while H+ ion concentration decreases,
i.e., [OH-] > [H+]; (Alkaline or basic solution)
In neutral solution, [H+] = [OH-] = 1 x 10-7 M
In acidic solution, [H+] > [OH-]
or [H+] > 1 x 10-7 M
and [OH-] < 1 x 10-7 M
In alkaline solution, [OH-] > [H+]
or [OH-] > 1 × 10-7 M
and [H+] < 1 x 10-7 M
Thus, if the hydrogen ion concentration is more than 1 x 10-7 M, the solution will be acidic in nature and if less than 1 x 10-7 M, the solution will be alkaline.
[H+] = 10-0 10-1 10-2 10-3 10-4 10-5 10˜6 (Acidic)
[H+] = 10-7 (Neutral)
[H+] = 10-14 10-13 10-12 10-11 10-10 10-9 10-8 (Alkaline)
We shall have the following table if OH- ion concentration is taken into account.
[OH-] = 10-14 10-13 10-12 10-11 10-10 10-9 10-8 (Acidic)
[OH-] = 10-7 (Neutral)
[OH-] = 10-0 10-1 10-2 10-3 10-4 10-5 10-6 (Alkaline)
It is, thus, concluded that every aqueous solution, whether acidic, neutral or alkaline contains both H+ and OH- ions. The product of their concentrations is always constant, equal to 1 × 10-14 at 25°C. If one increases, the other decrease accordingly so that the product remains 1×10-14 at 25o C.
If [H+] = 10-2 M, then [OH-] = 10-12 M; the product, [H+][OH-] = 10-2 × 10-12 = 10-14; the solution is acidic.
If [H+] = 10-10 M, then [OH-] = 10-4 M; the product, [H+][OH-] = 10-10 × 10-4 = 10-14; the solution is alkaline.