Does NH4OH really leak

Ammonium hydroxide

Ammonium hydroxide, also Ammonia or Ammonia water called, are aqueous solutions of ammonia (NH3) different concentration. The solutions are colorless, have a pungent odor and have a basic reaction.[1] A concentrated solution is corrosive. The solutions have the CAS number 1336-21-6.

The common name Ammonium hydroxide is misleading, a chemical compound NH4OH doesn't exist.

other names

Ammonia solution, ammonia water (often 10% solution), ammonia lye, caustic ammonia (obsolete, obtained by causticizing), salmiac spirit (popular, obtained from salmia), stag's horn (popular, obtained from stag's horn salt), caustic ammonia, ammonium hydrate, Liquor ammonii caustici (Latin, used in medicine)


Ammonia was already known to the ancient Egyptians and Arabs.[2] For more about the history, see under ammonia.

According to the outdated model by Arrhenius, a base (alkali) was understood to be a compound that can release hydroxide ions in water. To explain the behavior of ammonia, it was assumed that ammonia would form ammonium hydroxide molecules in water:[3][4]

$ \ mathrm {NH_3 + H_2O \ \ longrightarrow NH_4OH} $

These molecules partially dissociate into ammonium ions and hydroxide ions in an equilibrium reaction:

$ \ mathrm {NH_4OH \ rightleftharpoons NH_4 ^ + + OH ^ -} $

This idea could explain the - in comparison to bases such as sodium hydroxide - only weak basic (alkaline) effect. Although this idea did not correspond to reality, it provided the basis for the origin of the name "ammonium hydroxide" for aqueous solution of ammonia.


Ammonia dissolves very well in water, significantly better than other gases such as oxygen or carbon dioxide. The solubility depends on the temperature and the partial pressure of the gaseous ammonia. One liter of water at 0 ° C and a pressure of 1 bar absorbs 880 g (1142 l), at 20 ° C 520 g, at 40 ° C about 340 g and at 100 ° C only 75 g of gaseous ammonia.[2] The enthalpy of solution of ammonia at 25 ° C is −30.64 kJ / mol.[5]

Because of its higher vapor pressure, ammonia evaporates from ammonia water much faster than water, which is why the ammonia concentration in open vessels decreases over time. The typical, pungent-sharp ammonia odor occurs. The vapor pressure of a 25% solution at 20 ° C is 483 hPa.[1] Ammonia can be easily driven off by heating a solution. The boiling point of a 25% solution is only 37.7 ° C, that of a 32% solution is 24.7 ° C[1]

The density and the freezing point of ammonia water decrease with increasing ammonia content, see table.

Content, molarity, density and depression of the freezing point of ammonia water[6]
Mass fraction in% 1 5 10 15 20 26 30
c (mol / L) 0,58 2,87 5,62 8,28 10,84 13,80 15,71
d (g / cm3) 0,996 0,979 0,958 0,941 0,925 0,906 0,894
t (° C) 1,13 6,08 13,55 23,32 36,42 60,77 84,06

Under diluted ammonia a 1 to 2 molar solution (mass fraction 1.75 to 3.5%) and under concentrated ammonia Solutions with the commercial concentrations of 16.5 mol / L (32%) or 13.4 mol / L (25%) are understood.

At low temperatures, ammonia hydrate (NH3·H2O), which melts at −79 ° C. This is crystalline ammonia with attached water.[7]

Solution process and acid-base reaction

In aqueous solutions, the bulk of the ammonia is in molecular form. Hydrogen bonds act between water and ammonia molecules. They are the cause of the high solubility and act on the hydrogen and nitrogen atoms of ammonia:[7]

$ \ mathrm {H_2O \ cdots H-NH_2} $
$ \ mathrm {HO-H \ cdots NH_3} $

In an acid-base reaction between ammonia and water, ammonium (NH4+) and hydroxide ions (OH):

$ \ mathrm {NH_3 + H_2O \ rightleftharpoons NH_4 ^ + + OH ^ -} $.

The equilibrium is clearly to the left of the reaction. The base constantKB.

$ K_ \ mathrm {B} = \ frac {c \ mathrm {[NH_4 ^ +]} \ cdot c \ mathrm {[OH ^ -]}} {c \ mathrm {[NH_3]}} = 1.75 \ cdot 10 ^ {- 5} $

of ammonia is 1.75 · 10−5 (pKB.= 4.75). With that, ammonia is one only medium strengthbase. The degree of dissociation of a 0.1 molar solution is less than 1%, that of a 1 molar solution is 0.4%.


Salmiakgeist is used to clean surfaces in the household and technology. In technology, for example, for cleaning galvanized steel for subsequent painting (ammoniacal wetting agent washing).

In the bleaching and dyeing works, ammonia is used as a cheap basic solution.

Ammonium hydroxide is used as an acidity regulator in food chemistry and is also used to break down milk protein, cocoa products and egg products. The food additive label is E 527.

safety instructions

The labeling data in the table refer to the concentrated and diluted aqueous solutions. For the identification of pure ammonia, see there. From 5% solutions, ammonia water is classified as irritating, from 10% solutions as corrosive.[8]

Ingestion of ammonia water causes severe pain, gastric catarrh, bloody vomiting, lung and voice damage, which are often fatal. It also attacks the eyes.

Individual evidence

  1. 1,01,11,21,3Entry to Ammonia solution in the GESTIS substance database of the IFA, accessed on May 21, 2008 (JavaScript required).
  2. 2,02,1CD Römpp Chemie Lexikon - Version 1.0, Stuttgart, Georg Thieme Verlag, 1995.
  3. ↑ Ammonia In: Meyer's Large Conversation Lexicon, 6th edition 1905–1909 (There is NH4OH Ammonium hydroxide called).
  4. ↑ Ammonium Oxide Hydrate In: Lexicon of all technology, 2nd edition 1904–1920 (There is NH4OH Ammonium oxide hydrate called).
  5. Chemistry Lexicon, Spectrum Academic Publishing House, Heidelberg, 2001.
  6. ↑ Arnold Willmes, Paperback Chemical Substances, Scientific publishing house Harri Deutsch, Frankfurt a. M., 2007. (Limited preview in the Google book search), P. 101.
  7. 7,07,1Arnold F. Holleman, Nils Wiberg: Inorganic Chemistry Textbook. 100th edition, de Gruyter, Berlin 1985.
  8. 8,08,18,2Entry from the CLP regulation too CAS no. 1336-21-6 in the GESTIS substance database of the IFA (JavaScript required)