A logarithmic scale used to measure the completeness of a chemical reaction. The pK is defined as the negative logarithm of the equilibrium constant K (Lewis 1993). The dissociation of a weak electrolyte in water would have a high pK value, e.g., boric acid has a pK=9.24, while the dissociation of a strong acid has a low pK value. In an aqueous solution a compound is 50% dissociated when the pH of the solution reaches the pK value of the compound. Thus the pK value is the optimum pH for buffering.