Degree of ionization

The dimensionless degree of dissociation α (also called degree of dissociation ) is the ratio of the dissociated acid or base particles to formal initial concentration of the undissociated acid or base in an aqueous solution. The degree of dissociation of an acid or base depends on its acidity constant (or base constant ), their concentration or the present pH value of a solution. Located in a solution only an acid or base, you can detect the degree of dissociation of the electrolytic conductivity of the solution determined experimentally. If the pH of a solution is known, the degree of dissociation can be estimated by calculation.

The degree of dissociation can take values ​​between 0 and 1.

Monoprotic acids HA

For a monoprotic acid HA with the formal initial concentration c0, which dissociates according to the equation


There is the relative amount of dissociated acid at, c0 the initial concentration of the undissociated acid.

The definition of a degree of dissociation of the bases is not specially required, since it is already included.

The degree of association of the relative proportion of undissociated acid results from:

The combination of the mass action law for the protolytic equilibrium

(KS is the acid dissociation constant, which is a measure of the acid strength ) the conservation of mass of the acid in the solution

Yields the following expression for the dissociation:

The equation shows that the degree of dissociation of a particular acid is dependent only with a known pKa value of the pH of the solution. You can convert to:

That is, the pH of such a solution is itself a function of the total concentration c 0 of the acid.

Dibasic acids H2A

With two or polybasic acids, it is expedient to dispense with the distinction between dissociation and association degree. Instead, one generally defines the share of the formal initial concentration c0 of the acid, which is attributable to one of the species present in solution. The distribution to the various forms depends on the pH value of the solution.

A dibasic acid dissociates in aqueous solution to two equilibrium reactions:

The equilibrium described by the two acid dissociation constant:

The at a given pH value attributable to the different acid species shares are then calculated according to:

With the auxiliary variables

Polybasic acids HnA

A polyprotic acid HnA subject in solution n coupled Protolysegleichgewichten, described by the acid dissociation constant. At a given pH, the proportion attributable to the species calculated according to:

In all cases, we always have:

Conductometric determination

For acids with a hydrogen atom can be the Dissoziationgrad on conductometric measurements determined according to the formula:


  • The molar conductivity
  • The molar conductivity boundary

A similar provision is also of acids with more hydrogen atoms or with salts possible (Activity (chemistry) ).