|__ Na + __ _ OH -|
white, odorless solid
|External identifiers / databases|
|Molar mass||39.997 g · mol -1|
323 ° C
1390 ° C
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 (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 .
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.
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:
There are three different process technologies for this :
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.
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 .
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  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 . The β-modification is always twinned to approximately equal parts by volume according to . It is derived from the α-form by shifting the layers along . 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.
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.
Sodium hydroxide is packaged airtight in plastic containers in the form of small spheres or cookies.
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 .
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