Hydrogen Ion Concentration

Aqueous solution of acids and bases are expressed in terms of H and OH ion concentration. The term pH was introduced by Sorensen in 1909 who defined it as the negative logarithm of H+ ions, therefore
pH = - log OH¯
Similarly pOH is expressed as –log OH¯.
Ion product of water at room temperature is 1 x 10^-14,the pH of pure water is 7 because H+ = 10 x 10^-7  and –log(1x10^-7)=7. It follows that pH = pOH=14, that is why when pH is less than 7, solution is acidic and when pH is more than 7, the solution is basic.