What Is a Dissociation Constant?

A dissociation constant is a measure of how readily a compound will split into ions when added to a solvent. Compounds that have a high dissociation constant favor the right-hand side of the equilibrium, that is, the formation of ions. The most common use of dissociation constants is to determine the strength of acids and bases in aqueous solutions.

Johannes Nicolaus Brønsted and Thomas Martin Lowry defined acids as proton donors and bases as proton acceptors. When added to water, an acidic compound will readily ionize, donating a positively charged hydrogen atom, or proton. The strength of the acid that is formed is related to the stability of the ion formed when the hydrogen atom is donated.

An acid of the general formula HA will dissociate in water as described by the equation HA + H₂O -> H⁺_(aq) + A^-_(aq), where H₂O is water, H⁺ is a positively charged hydrogen ion, also called a proton, and A^- is the negative ion formed when the proton is donated. The acid dissociation constant K_a is equal to [H⁺][A^-]/[HA], where the square brackets denote the concentrations of the respective compounds or ions. Strong acids favor the right of this equation, and weak acids, such as acetic acid, will dissociate only slightly, meaning that they favor the left side of the equation.

When added to water, hydrogen chloride readily dissociates to form hydrochloric acid. The K_a value for hydrochloric acid is between 10⁶ moles per liter and 10⁷ moles per liter. This means that there will be hundreds of thousands more dissociated ions than non-dissociated molecules in hydrochloric acid. The measurement of hydrogen ion concentration in a solution is another method of gauging acid strength. This is called potenz hydrogen (pH) and is given by the equation pH=-log₁₀[H⁺]. Strongly acidic solutions with a high H⁺ concentration have a low pH.

Water also has acidic properties, though much less so than of hydrochloric acid or even acetic acid. The electrical conductivity of an solution depends on the presence of ions within that solution, so the strength of an acid can be determined by measuring its conductivity using an electrodes. It follows that completely pure, distilled water ought to be non-conductive, but this is not the case. This is because water will self-dissociate to form hydrogen and hydroxide ions. This dissociation is very weak and is called the ionic product of water, K_W.

A analogous relationship constant exists to describe the strength of bases. A general base, B, will cause water to dissociate to form BH⁺ and OH^- hydroxide ions. The strength of this base is given by the base dissociation constant K_b, which is equal to [BH⁺][OH^-]/[B]. The strength of the base depends on the stability of the BH⁺ ion. Sodium hydroxide or potassium hydroxide form highly stable bases and therefore have high values for K_b.