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Dissociation constant

From Academic Kids

In chemistry and biochemistry, a dissociation constant or an ionization constant is a specific type of equilibrium constant used for dissociation (ionizationation) reactions. That means that it refers to the extent to which a complex, molecule, or salt separates or splits into smaller molecules, ions, or radicals in a reversible manner. The dissociation constant is represented by the symbol Kd.

Given the reaction

AxBy <=> xA + yB

It is given by the expression

Kd = [A]x×[B]y×[AxBy]-1


Where [A], [B], and [AxBy] indicate the concentrations of A, B, and AxBy, respectively.

Stronger acids, for example sulfuric or phosphoric acids, have larger dissociation constants; weaker acids, like monohydrogen pyrophosphate, have smaller dissociation constants. A molecule can have several dissociation constants. In this regard, that is depending on the number of the protons they can give up, we define monoprotic, diprotic and triprotic acids. The first (e.g. acetic acid or ammonium) have only one dissociable group, the second (carbonic acid, bicarbonate, glycine) have two dissociable groups and the third (e.g. phosphoric acid) have three dissociable groups.

The negative logarithm of the dissociation constant is called pK value. A molecule can have several pK values, if it has several dissociable groups - it can lose or gain protons at more than one site; this is for example true for amino acids. If this is the case, pK values are designated by indices: pK1, pK2, pK3 and so on. For amino acids, the pK1 constant refers to its carboxyl (-COOH) group, pK2 refers to its amino (-NH3) group and the pK3 is the pK value of its side chain.

See also

Template:Chem-stub

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