Dissociation constants measure how easily a compound splits into ions in a solvent. They determine the strength of acids and bases in aqueous solutions. The acid dissociation constant Ka measures the strength of an acid, while the base dissociation constant Kb measures the strength of a base. The pH scale measures the concentration of hydrogen ions in a solution, with lower pH indicating stronger acidity. Water also has acidic properties due to its weak dissociation into hydrogen and hydroxide ions.
A dissociation constant is a measure of how easily a compound will split into ions when added to a solvent. Compounds that have a high dissociation constant favor the right side of the equilibrium, i.e. ion formation. 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 easily 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 dissociates 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 Ka equals (H+)(A-)/(HA), where the square brackets indicate the concentrations of the respective compounds or ions. Strong acids favor the right side of this equation, and weak acids, such as acetic acid, dissociate only slightly, which means they favor the left side of the equation.
When added to water, hydrochloric acid easily dissociates to form hydrochloric acid. The Ka 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 undissociated molecules in hydrochloric acid. Measuring the concentration of hydrogen ions in a solution is another method of measuring the strength of the acid. This is called the potential hydrogen (pH) and is given by the equation pH=-log10(H+). Strongly acidic solutions with a high concentration of H+ have a low pH.
Water also has acidic properties, although much less so than hydrochloric acid or even acetic acid. The electrical conductivity of a 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 electrode. It follows that completely pure distilled water should be non-conductive, but it is not. This is because water dissociates on its own to form hydrogen and hydroxide ions. This dissociation is very weak and is called the ionic product of water, KW.
There is an analogous relationship constant to describe the strength of the 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 Kb, 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 Kb values.
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