# 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 = -log_{10}[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 (K_{w}) is given by:

K_{w} = [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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