Sunday 2 September 2007

THE MEANING OF pH SCALE

In acid base titrations the concentration of H+ and OH- in aqueous solution can vary in wide ranges, to construct a plot of H+ concentration against some variable would be difficult if the concentration change from, say 10exp-1 to 10exp-13 M. In order of that, it is more convenient to compress the acidity scale by placing a logarithm basis. The pH of a solution was defined as :

pH = - log [H+]

a similar definition is made for the hydroxyl ion concentration:

pOH = - log [OH-]

The minus sign is used because most of the concentration encountered are less than 1 M, and this designation gives a positif number.

The product of the hydrogen ion concentration and the hydroxyl ion concentration in aqueous solution is always equal to 1.0 x 10exp14 at room temperature:

Kw = [H+][OH-] = 1.0 x 10-14

Kw is the molar equilibrium constant. The above equation can be used to calculate the hydrogen ion concentration if the hydroxyl ion concentration is known, and vice versa. The equation in logarithm form can be written as :

- log Kw = -log[H+][OH-] = -log [H+] – log [OH-]
                                                    
Kw = pH + pOH

14= pH + pOH








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