Concept Of pH:

At a given temperature the intensity of the acidic or basic character of a solution is indicated by pH or hydrogen ion activity. Through definition it is the negative logarithm of the hydrogen ion activity a_H⁺.

pH = - log a_H⁺

Within dilute solution the hydrogen ion activity is approximately equivalent to the concentration of hydrogen ion. Pure water is extremely slightly ionized and at equilibrium the ionic product is

[H⁺][(OH^-] = K_w   = - 1.0 × 10^-14 at 25°C

or [H⁺] = [OH^-] = 1.005 × 10^-7

A logarithmic form is

- log [H⁺]  -  log [OH^-] =   -log K_w

or pH + pOH = pK_w

From the above equilibrium it is clear that the pH scale for an aqueous solution lies between 0 and 14. A pH of most raw water sources lies within the range of 6.5-8.5, slightly basic due to the presence of bicarbonates and carbonates of the alkali and alkaline earth metals.

The pH is generally measured with glass electrode and a pH meter. The glass membrane electrode or glass electrode is one of the most common examples of an ion- selective indicator electrode. The whole cell, while the glass electrode is used along with an external reference electrode such as standard calomel electrode (SCE), can be represented by:

Glass membrane | H ⁺ ||SCE