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The ionization constant, also called acid or base ionization constant, is a mathematical constant used in chemistry to measure the acidity or basicity of a solution. Acids and bases are chemical compounds that dissociate completely or partially into their components when dissolved in water. They ionize, or break into positively and negatively charged atoms or molecules called ions. The more completely an acid or base breaks apart, the more acidic or basic its solution will be, and the higher its acid or base ionization constant is.
Acids and bases can be classified into two categories: strong acids and bases, and weak acids and bases. Strong acids and bases dissociate completely into their constituent ions in a solution, but weak acids and bases do not. A weak acid, such as acetic acid, only partially dissociates into ions in water and can reform into molecules in solution.
As it reacts with water, acetic acid dissolves and reforms constantly, so that there is no net change in the concentration of any substance in the solution. A hydrogen atom breaks off the acetic acid molecule to bond with water and form a hydronium ion, leaving behind an acetate ion. Since this reaction is reversible and can go in either direction, the respective concentrations of hydronium, acetate, and acetic acid do not change over time. The system is said to be in dynamic equilibrium.
The ionization constant is a special type of equilibrium constant, used to describe a situation where a weak acid or base has achieved equilibrium. Like other types of equilibrium constants, it is calculated using the concentration of each chemical component in the solution at equilibrium. Concentrations are given in number of moles, a unit used in chemistry to quantify the amount of a chemical substance.
In determining the ionization constant of a weak acid, the concentrations of the two ionic components of the acid are multiplied, and the product is then divided by the concentration of the molecular acid in solution. For example, with acetic acid, the concentration of hydronium ions would be multiplied by the concentration of acetate ions. The product would then be divided by the concentration of complete acetic acid molecules. This final number is the ionization constant.
Ionization constants can vary quite widely, so scientists often prefer to use a logarithmic scale, a mathematical conversion based on factors of ten, to measure the value. The modified ionization constant number is equal to the negative logarithm of the original value. The advantage to using factors of ten is that it converts the measurement to a smaller scale, so that, for example, the modified ionization constants of acids vary only from about -2 to 12 when measured in this way, instead of encompassing a much wider spectrum of numerical values.