HYDROGEN ION CONCENTRATION-pH SCALE:

It is clear from the above discussion that nature of the solution (acidic, alkaline or neutral) can be represented in terms of either hydrogen ion concentration or hydroxyl ion concentration but it is convenient to express acidity or alkalinity of a solution by referring to the concentration of hydrogen ions only. Since H⁺ ion concentration can vary within a wide range from 1 mol per litre to about 1.0 × 10^-14mol per litre, a logarithmic notation has been devised by Sorensen, in 1909, to simplify the expression of these quantities. The notation used is termed as the pH scale.

The hydrogen ion concentrations are expressed in terms of the numerical value of negative power to which 10 must be raised. This numerical value of negative power was termed as pH, i.e.,

                [H⁺] = 10^-pH

     or         log [H⁺] = log 10^-pH = -pH log 10 = -pH

     or           pH = -log [H⁺]

     or           pH = log1/[H⁺]

pH of a solution is, thus, defined as the negative logarithm of the concentration (in mol per litre) of hydrogen ions which it contains or pH of the solution is the logarithm of the reciprocal of H⁺ ion concentration.

Just as pH indicates the hydrogen ion concentration, the pOH represents the hydroxyl ion concentration, i.e.,

pOH = -log [OH-] Considering the relationship,

[H⁺][OH^-] = K_w = 1 x 10^-14

 Taking log on both sides, we have

log [H⁺] + log [OH^-] = log K_w = log (1 x 10^-14)

or -log [H⁺] - log [OH^-] = -log K_w = -log (1 x 10^-14)

or    pH + pOH = PK_w^* = 14

i.e., sum of pH and pOH is equal to 14 in any aqueous solution at 25°C. The above discussion can be summarised in the following manner:

       The following table shows the pH range for a few common substances: