10.2 pH Scale

Concentration of hydrogen ion $[{\mathrm{H}}^{+}]$$[\mathrm{H}^{+}]$ in pure water is the basis for the pH scale. Water is a weak electrolyte because it ionizes very slightly into ions in a process called auto- ionization or self- ionization;

As concentration of water $({\mathrm{H}}_2\mathrm{O})$$(\mathrm{H}_{2}\mathrm{O})$ is almost constant. The above equation may be written as

A new equilibrium constant known as ionic product constant of water ${\mathrm{K}}_{\mathrm{w}}^{\mathrm{\prime }}$$\mathrm{K}_{\mathrm{w}}^{\prime}$ is used instead of product of equilibrium constant and $[{\mathrm{H}}_2\mathrm{O}]$$[\mathrm{H}_{2}\mathrm{O}]$ . Therefore,

As we know, one molecule of water produces one ${\mathrm{H}}^{+}$$\mathrm{H}^{+}$ ion and one ${\mathrm{O}\mathrm{H}}^{-}$$\mathrm{OH}^{- }$ ion on dissociation so

As it is difficult to deal with such small figures having negative exponents, so it is convenient to convert these figures into a positive figure using a numerical system. It is taking the common (base- 10) logarithm of the figure and multiplying it with - 1. 'p' before a symbol means' negative logarithm of the symbol. So 'p' before H means negative logarithm of $[{\mathrm{H}}^{+}]$$[\mathrm{H}^{+}]$ . Therefore, pH is the negative logarithm of molar concentration of the hydrogen ions. That is,

With reference to this equation, a scale develops according to the molar concentration of $\mathsf{H}^{+}$ ions that is called pH scale. It ranges from 0 to 14. According to this scale, pH of water is calculated as: