Answer Posted / sp goswami

The pH concept was introduced in 1909 by the Danish chemist
S.P.L.Sorenson

pH is defined by the negative logarithm of the hydrogen ion
activity:



where aH = activity of the hydrogen ion

The pH scale derives from the characteristics of the auto-
dissociation of Water. Pure water has a low conductivity
and is only slightly ionised however does Water dissociate
slightly into Hydronium ions and hydroxide ions:

or

The concentration of H+ and OH- ions, which are equal, are
1x 10-7 ions per litre The equilibrium constant (or ion
product ) for the dissociation of water, Kw, is



by taking logs of both side we get:



Using the standard abbreviation p for {-log10} we get:



This equation sets the pH scale to 0-14, which gives a
convenient way to express 14 orders of magnitude of [H+].
Any solution with pH>7 contains excess hydroxyl ions and is
alkaline those with pH<7 are acidic, containing excess
hydrogen ions. Regards

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