Bases Ionization Constant and pH
A general equation can be written for the acceptance of H+ ions from water by a molecular base, B, to form its conjugate acid, BH+.
B(aq) + H2O(l) ⇌ BH+(aq) + OH–(aq) Then,
The equilibrium constant Kb is also called the Base Ionization Constant. It refers to the reaction in which a base forms its conjugate acid by removing an H+ ion from water.
The pH of (impure) water is a measure of its acidity. In pure water, about one in ten million molecules dissociate into hydronium ions (H3O+) and hydroxide ions (OH−), according to the following equation:2H2O(l) ⇌ H3O+(aq) + OH–(aq)
A base accepts (removes) hydronium ions (H3O+) from the solution, or donates hydroxide ions (OH–) to the solution. Both actions will lower the concentration of hydronium ions, and thus raise pH. By contrast, an acid donates H3O+ ions to the solution or accepts OH−, thus lowering pH.
For example, if 1 mole of sodium hydroxide (40 g) is dissolved in 1 litre of water, the concentration of hydroxide ions becomes [OH−] = 1 mol/L. Therefore [H+] = 10−14 mol/L, and pH = −log 10−14 = 14.
The basicity constant or pKb is a measure of basicity and related to the pKa by the simple relationship pKa + pKb = 14.