The speciation plot for any conjugate acid-base pair is determined using the expression for the acid’s ionization constant, K_a. We will use acetic acid (CH­₃COOH) as an example. Acetic acid reacts with water in the following acid-base reaction:

The expression for acetic acid’s ionization constant is:

This equation can be rearranged to give:

Using this equation and the K_a value for acetic acid, the ratio of acetic acid to acetate ions (CH₃COO^-) can be calculated for any concentration of hydronium ions and, subsequently, for any pH. For instance, if [H₃O⁺]=1.8×10^-2 mol L^-1 and pH=1.74, the [CH₃COOH]/[CH₃COO^-] ratio is:

To make a speciation plot, the ratio of [CH₃COOH]/[CH₃COO^-] is experimentally determined at many different pH values. However, speciation plots can also be calculated from equilibrium constants. The relative concentrations of CH₃COOH and CH₃COO^- are expressed as percentages of the total species present in the solution at each pH, and this is plotted on a speciation curve similar to the one shown on the previous page.