Amphoterism
Ampholytes (composed of Greek αμφίς ( amphis ) = on both sides and λύσις ( lysis ) = resolution ) or amphoteric or amphiprotic compounds are chemical compounds that can react both as a Brønsted acid and a Brønsted base. This behavior is also called acid-base Amphotery. Amphoteric can record both protons also make protons.
Properties
The water solubility of the ampholytes is strongly dependent on pH. Some ampholytes react with themselves, the most famous example of this is water. It reacts with acids to form H3O or with bases to OH, this behavior is also reflected in pure water as autoprotolysis:
Examples of ampholytes
Compounds which tend to autoprotolysis
Examples ( Autoprotolysekonstanten pKau after ):
- Water H2O ( pKau = 14)
- Ammonia NH3 ( pKau = 29 at -50 ° C)
- Sulfuric acid H2SO4 ( pKau = 3.85)
- Acetic acid CH3COOH ( pKau = 14,45 )
- Formic acid HCOOH ( pKau = 6.2 )
- Methanol CH3OH ( pKau = 16.9 )
- Ethanol CH3CH2OH ( pKau = 19.5 )
- Hydrogen fluoride HF ( pKau = 10.7 at 0 ° C)
The Autoprotolysekonstanten given correspond to the negative decadic logarithm (see pH) of the ion product of the substances. With increasing temperature, the extent of the autoionization increases usually.
Reaction Example: water
Reacts with acid as the base:
Reacts with base as an acid:
Partially deprotonated polyprotic acids
Examples:
- Monohydrogenphosphate HPO42 -
- Dihydrogen phosphate H2PO4 -
- Bisulfate HSO4 -
Reaction Example: dihydrogen
Reacts with acid as the base:
Reacts with base as an acid:
Partially protonated multivalent bases
Examples:
- Basic magnesium chloride, Mg ( OH ) Cl and Mg (OH) Cl
- Hydrazine monohydrochloride H2N - NH2 · HCl or H 2 N- NH 3 Cl -
Reaction Example: basic magnesium chloride
Reacts with acid as the base:
Reacts with base as an acid:
- Amino acids having acidic carboxyl groups and their basic amino groups (and hence the peptides and many proteins)
- Zwitterions
Reaction Example: glycine ( the simplest amino acid )
Reacts with acid as the base:
Reacts with base as an acid:
Solving ampholytes (with two functional groups ) in water so there is set a medium pH, which can be calculated with the following (concentration- independent ) approximation formula:
Here, pKS1 pKS2 and the acid constants ( pKa values ) of the respective Dissoziationsmöglichkeiten of the ampholyte.
At this pH ampholytes have the lowest solubility. The solubility increases with both increasing and with decreasing pH. In addition, the ampholyte will " electrically neutral " at this pH value, which is exploited in the isoelectric focusing.