Ionization of Water, Weak Acids and Weak Bases

lthough many of the solvent properties of water can be explained in terms of the uncharged H2O molecules,the small degree of ionization of water to hydrogen ions (H) and hydroxide ions (OH ) must also be taken into account. Like all reversible reactions, the ionization of water can be described by an equilibrium constant. When weak acids or weak bases are dissolved in water, they can contribute H by ionizing (if acids) or consume H by being protonated (if bases). These processes are also governed by equilibrium constants. The total hydrogen ion concentration from all sources, which is experimentally measurable, is expressed as the pH of the solution. Water molecules have a slight tendency to undergo reversible ionization to yield a hydrogen ion and a hydroxide ion : H2O l H + + OH ...(3.1) This reversible ionization is crucial to the role of water in cellular functions. Henceforth, we must have a means of expressing the extent of ionization of water in quantitative terms. This is discussed below. The position of equilibrium of any chemical reaction is given by its equilibrium constant. For the general reaction, A + B l C + D ...(3.2)