from Wikipedia, the free encyclopedia

Acid-base ampholytes (composed of Greek αμφίς ( amphis ) = on both sides and λύσις ( lysis ) = resolution) or acid-base Amphoteric or amphiprotic compounds are chemical compounds having both as a Brønsted - acid as a Brønsted base can react. This behavior is also known as acid-base amphoteric . Amphoterics can both accept and release protons.


The water solubility of the ampholytes depends strongly on the pH value. Some ampholytes react with themselves, the best known example of this is water . It reacts with acids to form H 3 O + or with bases to form OH - , this behavior is also shown in pure water as autoprotolysis :

Examples of ampholytes

Compounds prone to autoprotolysis

Examples (autoprotolysis constants pK au nach):

The specified autoprotolysis constants correspond to the negative decadic logarithm (see also pH value ) of the ionic product of the substances. The extent of autoprotolysis usually increases with increasing temperature.

Reaction example : water

Reacts with acid as a base:

Reacts with base as an acid:

Partially deprotonated multi-protonic acids


Reaction example: dihydrogen phosphate

Reacts with acid as a base:

Reacts with base as an acid:

Partially protonated polyvalent bases


Reaction example : basic magnesium chloride

Reacts with acid as a base:

Reacts with base as an acid:

Compounds with acidic and basic functional groups

Compounds with at least one acidic and one basic functional group are also amphoteric substances, for example:

Reaction example : glycine (simplest amino acid)

Reacts with acid as a base:

Reacts with base as an acid:

Calculating the intrinsic pH of ampholytes

If ampholytes (with two functional groups) are dissolved in water, an average pH value is established , which can be calculated using the following approximation formula, also known as the "ampholyte equation" (for not too strong dilutions).

Here, pK are S1 and pK S2 , the acidity constants (pK S values) of the respective Dissoziationsmöglichkeiten of the ampholyte.

Electrically neutral ampholytes, e.g. B. Amino acids also have the lowest solubility at this pH value; if the pH falls or rises, the solubility increases again, since the solvation shell is stabilized with the charge .

See also

Web links

Wiktionary: Ampholyt  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Lothar Kolditz : Inorganic Chemistry. Volume 1. 2nd edition. VEB Deutscher Verlag der Wissenschaften, Berlin 1983, p. 188.
  2. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 , p. 457.