Sodium hydroxide

from Wikipedia, the free encyclopedia
Crystal structure
Structure of sodium hydroxide
__ Na + __ _ OH -00
Surname Sodium hydroxide
other names
  • caustic soda
  • Caustic soda
  • caustic soda
  • Sodium Oxide Hydrate
  • Sodium hydrate
  • Caustic soda (aqueous solution)
  • Soap stone
  • Food additive E 524
Ratio formula NaOH
Brief description

white, odorless solid

External identifiers / databases
CAS number
  • 1310-73-2
  • 12179-02-1 (monohydrate)
  • 23340-32-1 (tetrahydrate)
EC number 215-185-5
ECHA InfoCard 100.013.805
PubChem 14798
ChemSpider 14114
DrugBank DB11151
Wikidata Q102769
Molar mass 39.997 g · mol -1
Physical state


  • 2.13 g cm −3
  • 1.77 g cm −3 (melt at 350 ° C)
Melting point

323 ° C

boiling point

1390 ° C

  • readily soluble in water: (1090 g l −1 at 20 ° C)
  • soluble in water and methanol (monohydrate)
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
05 - Corrosive


H and P phrases H: 290-314
P: 280-301 + 330 + 331-305 + 351 + 338-308 + 310

Switzerland: 2 mg m −3 (measured as inhalable dust )

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Sodium hydroxide in the form of cookies

Sodium hydroxide (also caustic soda , caustic soda ), chemical formula NaOH , is a white, hygroscopic solid. With a world production of 60 million tons in 2010, the compound is one of the most important chemical raw materials and is mainly traded in the form of caustic soda .


Sodium hydroxide can be produced in the laboratory by converting sodium carbonate with calcium hydroxide to form sodium hydroxide and calcium carbonate :

The slightly soluble calcium carbonate is filtered off. The readily soluble sodium hydroxide remains in the filtrate. This causticizing process was previously carried out on an industrial scale and is of interest again today.

Another method is the strongly exothermic reaction of elemental sodium with water to form caustic soda and hydrogen :

This experiment is often shown in schools to demonstrate the reactivity of alkali metals with water.

After the caustic soda has evaporated , solid sodium hydroxide remains:

The Acker process for the production of sodium hydroxide by fused salt electrolysis of sodium chloride was developed by Charles Ernest Acker (1868-1920) in the United States of America.


Sodium hydroxide is produced industrially by the electrolysis of sodium chloride to form caustic soda , hydrogen and chlorine gas :

There are three different process technologies for this :

  1. Amalgam procedure
  2. Diaphragm process
  3. Membrane process

Common to all processes are additional purification and concentration stages in order to obtain anhydrous sodium hydroxide.

Since the demand for chlorine has stagnated since the 1980s , the caustic soda , which is a by-product of chlor-alkali electrolysis , no longer fully meets world demand, which makes causticizing interesting again.


Physico-chemical properties

Sodium hydroxide is a white, hygroscopic solid and is one of the strongest bases. In water it dissolves very well with great heat development due to the negative enthalpy of solution of −44.4 kJ / mol to form a strongly alkaline sodium hydroxide solution (pH 14 at c = 1 mol / l). It is always completely dissociated in aqueous solution. However, at higher concentrations, the interionic forces between the sodium and hydroxide ions affect the free mobility of the ions, so that a normal solution (40 g sodium hydroxide per liter) closes by about 78%, a 0.1 n solution about 90% appears dissociated. It reacts with the carbon dioxide in the air to form sodium hydrogen carbonate and is therefore stored in airtight containers. To prevent the sodium hydroxide from binding water from the air, it can be stored together with a desiccant. As a strong base, the hydroxide ion displaces weaker and volatile bases from their salts .

Crystal structure

Between room temperature and melting point, 318.4 ° C, anhydrous sodium hydroxide occurs in two modifications. Below 299.6 ° C (α-modification) sodium hydroxide crystallizes with an orthorhombic crystal structure with the space group Cmcm (space group no.63 ) , above (β-modification) lower symmetrical with a monoclinic crystal structure with the space group P 2 1 / m ( Room group no.11) . The sodium hydroxide molecule is linear and is arranged in this order parallel to the c-axis. In x, y, sodium and oxygen form extensive double layers similar to the sodium chloride structure , with sodium and oxygen alternating in the directions (xy). The layer thickness is somewhat greater than the distance between sodium and oxygen in the molecule. Layers following one another along c are shifted by 1/2 a. The lattice constants at 24 ° C are a = b = 3.3994 ± 0.001  Å , c = 11.377 ± 0.005 Å, α = β = γ = 90 °. The molecule is angled in the [010] plane. The angle β depends on the temperature. As the temperature rises, the approximation of the type of sodium chloride structure as it is found in sodium fluoride also increases . α-Sodium hydroxide is often twinned according to [110]. The β-modification is always twinned to approximately equal parts by volume according to [001]. It is derived from the α-form by shifting the layers along [100]. The structure of the double layers is retained. In addition, the compound occurs in several hydrate forms . The mono-, di-, 3,5-, tetra-, penta- and heptahydrate are known. The metastable form of the tetrahydrate β-NaOH · 4H 2 O has an orthorhombic crystal structure with the space group P 2 1 2 1 2 1 (space group no. 19) with four formula units per unit cell and the lattice constants a = 6.237, b = 6.288, c = 13.121 Å at −155 ° C. The hydrates of NaOH · 3.5H 2 O and NaOH · 7H 2 O respectively have a crystal structure with space group P 2 1 / c (space group no. 14) with eight formula units (per unit cell lattice constants a = 6.481, b = 12.460, c = 11.681 Å, β = 104.12 ° at −100 ° C) or four formula units per unit cell (a = 7.344, b = 16.356, c = 6.897 Å, β = 92.91 ° ​​at −150 ° C). The monohydrate melts at 64.3 ° C, the 3,5-hydrate at 15.6 ° C. Template: room group / 63 Template: room group / 11 Template: room group / 19 Template: room group / 14


If sodium hydroxide is stored unsealed in air , it reacts with carbon dioxide to form sodium hydrogen carbonate or sodium carbonate , which is why it is kept in airtight containers .

In the laboratory , ammonia can be easily produced from sodium hydroxide and ammonium chloride using the acid-base reaction .

As a solution , it reacts with aluminum (to form aluminum sodium dioxide ) and many other metals such as iron , copper , cadmium , cobalt and titanium .

Sodium hydroxide reacts with acids to form salts , the heat development being so considerable that with strong acids, e.g. B. when concentrated sulfuric acid is dripped onto powdered sodium hydroxide, an explosion occurs.

Commercial form

Sodium hydroxide is packaged airtight in plastic containers in the form of small spheres or cookies.


Many drain cleaners contain sodium hydroxide

Sodium hydroxide is mainly in the form of caustic soda used and is in the industry one of the most important chemicals . For their use see there.

In addition to aluminum chips, solid sodium hydroxide is an essential component of drain cleaners . Dissolved in water, the strong oxidized base under heat and hydrogen evolution the aluminum and then dissolves fats and proteins in the deposits by saponification .

Burn layers are loosened in saucepans with hot solution . Sodium hydroxide is not suitable for aluminum pots.

Web links

Commons : Sodium Hydroxide  - Collection of Pictures, Videos and Audio Files
Wiktionary: sodium hydroxide  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Entry on SODIUM HYDROXIDE in the CosIng database of the EU Commission, accessed on February 16, 2020.
  2. a b c d e f g Entry on sodium hydroxide in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .
  3. Franz v. Bruchhausen, Siegfried Ebel, Eberhard Hackenthal, Ulrike Holzgrabe: Hager's handbook of pharmaceutical practice . Sequence Volume 5: Substances L-Z . Springer-Verlag, 2013, ISBN 978-3-642-58388-9 , pp. 273 ( limited preview in Google Book search).
  4. Data sheet Sodium hydroxide monohydrate, 99.996% (metals basis) from AlfaAesar, accessed on July 11, 2016 ( PDF )(JavaScript required) .
  5. Entry on sodium hydroxide in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on August 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
  6. Swiss Accident Insurance Fund (Suva): Limit values ​​- current MAK and BAT values , accessed on November 2, 2015.
  7. a b c d e Manfred Baerns, Arno Behr, Axel Brehm, Jürgen Gmehling, Kai-Olaf Hinrichsen, Hanns Hofmann, Ulfert Onken, Regina Palkovits, Albert Renken: Technische Chemie . John Wiley & Sons, 2014, ISBN 978-3-527-67409-1 , pp. 629 ( limited preview in Google Book search).
  8. ^ Winfried R. Pötsch, Annelore Fischer and Wolfgang Müller with the collaboration of Heinz Cassebaum : Lexicon of important chemists . VEB Bibliographisches Institut Leipzig, 1988, p. 9, ISBN 3-323-00185-0 .
  9. ^ George S. Hammond, Y. Osteryoung, TH Crawford, HB Gray: Modeling in Chemistry An Introduction to General Chemistry . Walter de Gruyter, 1979, ISBN 978-3-11-084798-7 , pp. 287 ( limited preview in Google Book search).
  10. ^ A b c Karl A. Hofmann: Inorganic Chemistry . Springer-Verlag, 2013, ISBN 978-3-663-14240-9 , pp. 428 ( limited preview in Google Book search).
  11. Hermann Stehr: New determination of the crystal structures of dimorphic sodium hydroxide, NaOH, at different temperatures with X-ray and neutron diffraction. In: Journal of Crystallography - Crystalline Materials. 125, 1967, doi: 10.1524 / zkri.1967.125.16.332 .
  12. William A. Hart, OF Beumel, Thomas P. Whaley: The Chemistry of Lithium, Sodium, Potassium, Rubidium, Cesium and Francium Pergamon Texts in Inorganic Chemistry . Elsevier, 2013, ISBN 978-1-4831-8757-0 , pp. 426 ( limited preview in Google Book search).
  13. JL Provis, J. S. J. van Deventer: Geopolymers Structures, Processing, Properties and Industrial Applications . Elsevier, 2009, ISBN 978-1-84569-638-2 , pp. 54 ( limited preview in Google Book search).
  14. Dietrich Mootz, Reinhard Seidel: On the sodium hydroxide-water system The crystal structure of the metastable phase β-NaOH · 4H2O. In: Journal of Inorganic and General Chemistry. 582, 1990, p. 162, doi: 10.1002 / zaac.19905820120 .
  15. D. Mootz, H. Rütter, R. Wiskemann: Hydrate weak and strong bases. XI. The crystal structures of NaOH 3,5H2O and NaOH 7H2O. A specification. In: Journal of Inorganic and General Chemistry. 620, 1994, p. 1509, doi: 10.1002 / zaac.19946200903 .
  16. ^ A Manual for the Chemical Analysis of Metals . S. 35 ( limited preview in Google Book search).