Q. No.3. What is auto-ion ization of water? How is it used to establish the pH of water?
Answer. Concentration of hydrogen ion [H+] in pure water is the basis for the pH scale. Water is a weak electrolyte because it ionizes very slightly into ions. This process is called auto-ionization or self ionization;
H2O H+ + OH-
The equilibrium expression of this reaction may be written as-
Kc = [H+] [OH-]
As concentration of water H2O is almost constant. The above equation may be written as
Kc [H20] = [H+] [OH]
Taking Kc [H2O] = Kw
Kw is ionic product constant of water
One molecule of water produces one H+ ion and one OH- ion on dissociation so
[H+] = [OH-]
Kw = H+] H+] = [H+]2
Kw at 25°C is = 1.0 x 10-14 so 1.0 x 10-14= [H+]2
or [H+]= 1.0 x 10-7 M at 25°C
[H+] = [OH-] = 1.0 x 10-7 M
Values of [H+] -and [OH-] are very small. So, new terms pH and pOH are used to change very small values into simple numbers. pH is the negative logarithm of molar concentration of the hydrogen ions. i.e.
So, according to this scale, pH of water is = -log-(l.0 x 10-7) =7
Similarly pOH = -log (1.0 x 10-7) = 7
pH value normally varies from 0 to 14.
Therefore; pH + pOH = 14
So, the sum of the pH and pOH of the solution is always 14 at 25’C.