Base (chemistry)

Bases (Greek βάση, base - the output, basis, foundation ) in the narrower sense, all compounds in aqueous solution, are able to form hydroxide ions (OH - ), which is the pH of a solution to increase. Hydroxide ions are chemical compounds that can accept protons from an acid to form a water molecule. A base is therefore the equivalent of an acid, and capable of neutralizing it.

In a broader sense various acid-base concepts describe much broader palette of chemical reactions that can go beyond characteristics of hydroxide ions in water. Of particular importance are the concepts by Lewis ( Lewis base and Lewis acid) and the Pearson, who speaks of hard and soft acids and bases ( HSAB concept).

Historical development of the term "Base"

In alchemy, some alkalis such as lime ( CaCO3, CaO and Ca ( OH ) 2), soda, soda ash, potash and ammonia were known. Until the early 18th century, however, was not exactly distinguish between soda and potash. The term " alkali " was little used and recognized no exact relationship between these substances. The base ( alkali ) as a counterpart of the acid was postulated in the Chemiatrie, a medical- theoretical doctrine of Otto Tachenius in the 17th century.

Until the 18th century there was a close link between alkalis and the fire or the "Fire Matter", also because of the well-known exothermic reactions. The term " base " was introduced in the 17th century by chemists such as Georg Ernst Stahl, Robert Boyle and Guillaume François Rouelle because " basic " substances formed the non-volatile basis for fixing volatile acids and can cancel the ( corrosive ) effects of acids. Basic steps in the chemistry reach Antoine Laurent Lavoisier. He thought that acids always were to come from non-metal oxides and water and bases of metal oxides and water. Sir Humphry Davy and Justus von Liebig saw acids as hydrogen compounds which can be converted by metals in salts. 1887 Svante Arrhenius defined bases as substances that dissociate when dissolved in water with the release of hydroxide ions and acids as substances that dissociate with the release of protons. Acids and bases neutralize each other. However, the theory was still insufficient, since compounds without oxygen were not included: ammonia neutralizes an acid.

In 1923, John Nicolaus Brønsted in front of his model. It has been widely accepted and proven in particular in analytical chemistry very much. His theory of interacting base and acid in a proton transfer reaction. It bases accept protons from acids. The by Gilbert Newton Lewis in 1923 also presented model is helpful in viewing the progress of reactions in organic chemistry and in coordination chemistry and goes beyond the usual definitions. Therefore, one speaks of preferred Lewis base and Lewis acid. Many normally designated acid compounds are not acids according to this model. The concept of hard and soft acids and bases ( HSAB concept ) developed 1963 Ralph G. Pearson, expanding ways of looking at reactions in organic and coordination chemistry.

Properties of bases

  • Many bases are soluble in water ( e.g. sodium hydroxide, ammonia), but not all (e.g., aluminum hydroxide)
  • They are corrosive and organic materials have to destructive effect.
  • Of oils and fats to form soaps and glycerol.
  • There are strong and weak bases.
  • Bases can be diluted with water, it is their effect depending on dilution significantly weaker.
  • The basic solutions lead to a reddening of phenolphthalein and color red litmus paper blue.
  • The " opponent " of the bases ( base solution = lye) are the acids ( see illustration). You can neutralize bases. Acids are corrosive and will attack many other substances that react with bases not necessarily.
  • Clothing, skin and eyes on contact in danger. It is to be sure to wear goggles because injuries can always happen.

What are bases?

In close connection with bases are usually and often without explicit mention of the presence and certain properties of water. Pure water is subject to the so-called autoionization, in the form in very low concentrations and same oxonium ions ( H3O ) and hydroxide ions (OH - ):

In this equation shows that the water capacity of a base by formation of OH - ions in water. At the same time H3O ions form in water, a property that distinguishes an acid. However, it refers to water either as a base or as an acid and its behavior is called neutral. This refers to the pH - value, which indicates the concentration of H3O ions in the water. Pure water has a pH of 7, a very small concentration. This reaction is - like all the reactions described in this section - an equilibrium reaction: the formation of ions, as well as their association is to water instead of constantly and with the same frequency.

Many compounds, which are mentioned bases, include hydroxide ions (OH - ) and dissociate in water, and metal hydroxide ions. The solution is commonly referred to as an alkaline solution or water. Thus, in water, the solid sodium hydroxide (NaOH ) is the so-called sodium hydroxide and potassium hydroxide ( KOH), the potassium hydroxide solution.

Other compounds themselves have no OH - ions, but they form in a reaction with water. They react alkaline by the acquisition of a proton H from an H2O molecule and thus allow an OH - ion back. For example, forming the salt trisodium phosphate ( Na3PO4 ) or the salt sodium carbonate ( Na2CO3) in hydroxide aqueous solution. Responding to the same path and organic compounds such as salts of carboxylic acids and amines as derivatives of ammonia. The corrosive effect of all of these bases is mainly due to the formation of OH - ions due.

Basic reactions

For weak and medium strong bases lie in the equilibrium reactions, all components involved in the reaction present in the solution. Each two of the reactants only differ in the presence or absence of a proton (H ). They form a corresponding acid-base pair. Particles, which have a suitable proton, proton donors and proton particles will have the ability to take, called proton acceptors. The entire reaction is called protolysis. The strength of a base is described by the position of the equilibrium ( the base constant ).

In case of strong and very strong bases, the equilibrium reactions are completely on the side of the OH - ions. This is for example the reaction of alkali metal with water in the case:

The cation Na is irrelevant. The hydroxide ion is here the actual base and water proton donor:

Because of this balance can be strong to very strong bases ( such as sodium ethoxide ) no longer differ in aqueous solution due to their alkalinity. This is called a leveling effect ( v. French: niveler = same make ) of water. In order to distinguish even very strong bases with respect to the strength, is determined equilibrium constants in non-aqueous solutions and transfers it approximately to the solvent water.

Water plays an important role in acid-base reactions. In addition to the above-described protolysis water is capable of the so-called autoionization. It can release protons and OH - form, or pick up protons and form H3O . This is a reaction time as the base and the other a reaction as the acid. We call water so as ampholyte.


Specific compounds are referred to as bases because of their particular chemical nature. The wide range of these chemicals easily categorized into groups according to various characteristics. We can separate bases according to their ionic charge in neutral, anionic or cationic bases. Ammonia ( NH3) bears no ionic charge and is thus a neutral base. Anionic base can be sodium denote, as in solution, the anion HCO3- is present. Also, the hydroxide anion ( OH ) itself can be described as anionic base.

Another way of classification is the division into monovalent or divalent bases. Sodium hydroxide ( NaOH) is formed in a solution of Na per OH and is monovalent, calcium hydroxide (Ca ( OH) 2) forming two Ca per OH and is therefore divalent.

As a base generator can be described compounds in which the basic reaction before further chemical reaction is still upstream. As a base generator can be the metal oxides described, forming the corresponding hydroxides when dissolved in water. As calcium oxide ( CaO) with water, the base is Ca (OH ) 2 Base metals such as the alkali metals can be previously oxidized by the action of water. The violent reaction of sodium hydroxide solution developed in addition to the hydrogen.


The basis of neutralization based on the fact that the effects of an acid when mixed with a base does not add up, but cancel. As a base may be neutralized with an appropriate amount of an acid. In this case, bases and acids react to form water.

Reaction of sodium hydroxide in water and with sodium hydroxide to:

Reaction of hydrogen chloride in hydrochloric acid and with water to form:

Reaction of sodium hydroxide with hydrochloric acid ( neutralization):

The determining process is the reaction between the hydroxide and the oxonium ion:

Acid-base balance

The strength of a base is referred to as their basicity and describes it by the base constant. The base constant ( Kb ) describes the position of the equilibrium in the reaction between an acid-base pair in aqueous solutions. Often the negative decadic logarithm of Kb, the so-called pKb value is specified.

In the reaction

If the base constant Kb defined as follows:

The pKb value is accordingly:

Acid -base reactions without water

Similar to the acid -base reactions that occur in aqueous solutions and with the participation of water, reactions exist in other media. In anhydrous ethanol place with hydrogen chloride instead of a reaction in the ethanol, the role of a base accepts:

In the gas phase, the gas, ammonia gas and the hydrogen chloride reacts to form the salt ammonium chloride.

In acid -base reactions in addition to water, other sufficiently polar solvents can act as reactants. A good example is the autoionization of liquid ammonia:

  • Chemicals group
  • Acidity and basicity