pH and pH Scale

Introduction to pH: According to ionic product of water, it is found that the concentration of hydrogen ion determines the acidic, basic and neutral character of a solution. So, Russian chemist Sorensen used the pH scale to find the hydrogen ion concentration in convenient way. pH stands for potential of hydrogen in a solution.

The pH of a solution is the negative logarithm to the base 10 of the hydrogen ion concentration in moles per litre.

Mathematically, pH = -log10[10+]

The concentration of OH- ions in aqueous solution of a base can be expressed as:

pOH = -log[OH]

pH Scale: The scale which is used to express the hydrogen ion concentration is called pH scale. The value ranges from 0 to 14. Pure water having pH 7 is taken as reference to for measuring the pH scale of other compounds. The pH scale is computed according to the ionic product value of water at 25ºC.

For example:

For neutral solution,

[H+] = 10-7

pH = -log[10-7]

pH = 7

For acidic solution,

[H+] > 10-7
So, pH = -log[10-6], -log[10-5], -log[10-6] etc.

pH = 6,5,4,3…0

For basic solution,

[H+] < 10-7
So, pH = -log[10-8], -log[10-9], -log[10-10] etc.

pH = 8, 9, 10…..14

Relation between pH or pOH

It is experimentally found that the ionic product of water (Kw) is given by:

Kw = [H+][OH] = 10-14 at 25ºC,

Taking log on both sides,

log{[H+][OH]} = log[10-14]

or, log[H+] + log [OH] = -14

Multiplying both sides by minus sign,

– log[H+] – log [OH] = 14

pH + pOH = 14

The above expression is the relation between pH and pOH.

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