Figure 2.1: Structure of Water. (A) Chemical structure of water molecule. (B) Hydrogen Bonding between water molecules and (C) Hydrogen bonding pattern observed in Ice.
Ionization of water and pH: The equilibrium constant, Keq for the reversible ionization of water is:
The Concentration of pure water, [H2O]=55.5M and Keq calculated from electrical conductivity measurement, Keq =1.8x10-16M
Ion product is a constant and allows us to calculate [H+], if [OH-] is known or vice-versa. Hence, it is used to develop pH scale to define concentration of H+ or OH- in any aqueous solution. pH is defined as “ negative logarithm of hydrogen ion concentration ” and it ranges from 1-14 (Figure 2.2, A). Solution with pH=7 is neutral whereas solution with pH lower than 7 is acidic and pH above 7 are considered as alkaline. Biological samples of different origin and products of daily use are examples of solution with variable pH (Figure 2.2, B).