pH value – pH scale: Acidic character and Basic character

Sorensen in 1909, introduced the term pH – value to express H⁺ ion concentration. The scale on which pH values are computed is called the pH scale. As hydrogen ion concentration can vary within wide limits usually from 1 mole/liter (as in 1 M-HCl) to 10-14 mole/liter (as in 1M-NaOH) in aqueous solutions, therefore, the pH values vary from 0-14. The complete pH scale is:

mathematically,

pH = -log₁₀[H⁺]

It may be noted that solutions having a p^H between 0 to 2 are strongly acidic, solutions having p^H of 4 are moderately acidic; and solutions having p^H between 4 to 7 are weakly acidic. Similarly, solutions having pH values between 7 to 10 are weakly basic.

Solutions having pH between 10 to 12 are moderately basic, and between 12 to 14 are strongly basic, solution having p^H value of 7, are neutral in nature.

pH value of the solution

The pure water dissociates to a very small extent so that we have the equilibrium,

For this equilibrium, the equilibrium constant is given by the expression

where k_c is the ionization constant.

Since water is found to be poorly ionized, the concentration of undissociated molecule H2O can be regarded as constant. Therefore, from the above expression (2) we have;

Where Kw is called the ionic product of water. When the concentration of [H⁺] and [OH^–] ions in water are expressed in mole per liter the value of Kw found experimentally is 1.0 × 10^-14 at 25^oC. Therefore,

In an acidic solution, however, the concentration of H⁺ ions must be greater than 1.0 × 10^-7 mole per liter. Similarly, in a basic solution, the concentration of OH^– ions must be greater than 1.0 × 10^-7 mole per liter.

Hence;

For neutral solution = [H⁺] = [OH^–] = 10-7

For acidic solution = [H⁺] > [OH^–], i.e, [H+] > 10-7

For basic solution = [H⁺] < [OH^–], i.e, [H⁺] < 10-7

Thus, acidity or basicity of a solution can be expressed in terms of its hydrogen ion concentration, [H⁺].

[H⁺] = 10^-pH

where [H⁺] is the concentration of hydrogen ion in mole per liter. The pH of a solution may be defined as the negative power to which 10 must be raised in order to express the hydrogen ion concentration of the solution.

Just as pH is used to express hydrogen ion concentration, in the same manner, pOH is used to indicate hydroxyl ion concentration.Thus,

pOH = -log10[OH^–]

Relation Between pH and pOH

we know that,

Kw = [H⁺] [OH^–]

Taking the logarithm of both sides, we get

log10 K_w = log10[H⁺] + log10[OH^–]

or, -log10K_w = -log10[H⁺] – log10[OH^–]

or, P^Kw = pH + pOH

or, 14 = pH + pOH

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