Caustic soda
Sodium hydroxide (caustic soda, lye).
Chemical formula : NaOH
(Na = 57,48 %, O = 40 %, H = 2,52 %)
Molar mass = 39,9971 ± 0,0004 g·mol-1

Historically, sodium hydroxide was produced by treating sodium carbonate with calcium hydroxide in a metathesis reaction. (Sodium hydroxide is soluble while calcium carbonate is not.) This process was called causticizing:

Ca(OH)2(aq) + Na2CO3(s) >>> CaCO3 + 2 NaOH(aq)

This process was superseded by the Solvay process in the late 19th century, which was in turn supplanted by the chloralkali process which we use today.

Sodium hydroxide is also produced by combining pure sodium metal with water. The byproducts are hydrogen gas and heat, often resulting in a flame, making this a common demonstration of the reactivity of alkali metals in academic environments; however, it is not commercially viable, as the isolation of sodium metal is typically performed by reduction or electrolysis of sodium compounds including sodium hydroxide.

Sodium hydroxide is industrially produced as a 50% solution by variations of the electrolytic chloralkali process. Chlorine gas is also produced in this process. Solid sodium hydroxide is obtained from this solution by the evaporation of water.

Process being replaced (2020) by electrolysis membrane.
Also from sodium carbonate (caustication): this technique was that previously used. It is still used in North America where natural deposits of sodium carbonate. This is an addition of lime (Ca [OH] 2) with sodium carbonate [Na2CO3]. The reaction is written:

Na2CO3 + Ca(OH)2 >>> CaCO3 + 2NaOH

It is a white solid and highly caustic metallic base and alkali salt which is available in pellets, flakes, granules, and as prepared solutions at a number of different concentrations.
The dissolution in water of sodium hydroxide is known as lye.
Similar to the hydration of sulfuric acid, dissolution of solid sodium hydroxide in water is a highly exothermic reaction in which a large amount of heat is liberated, posing a threat to safety through the possibility of splashing. The resulting solution is usually colourless and odorless with slippery feeling upon contact in common with other alkalis.

In treatment of water, injection is used in water diluted solid product (with softened water) or commercial solutions (30 and 50% NaOH by weight).

Main Features :

Solid alkali
Solution at 30%
Solution at 50%
% NaOH
99 %
28 to 32 %
49 to 53 %
Volumetric mass density (20 °C)  
2130 kg/m3
1330 kg/m3
1 550 kg/m3
Crystallization temperature
+1 to +4 °C
+9 to +12,5 °C

NaOH is very soluble in water, and also in ethanol.

Solubility in water (soft) as a function of temperature (by weight NaOH %):

The maximum solubility in fresh water (20 °C) is 1 090 g / L.

Quantity of pure sodium hydroxide by the Volumetric mass density [d] of the solutions at 20 °C:
(d = kg/m3 ou g/L)

(% = g as NaOH per 100 g of solution)

(g/l = g as NaOH per liter of solution)

<More precisely> see table >

A formulation for water treatment.
Reaction mechanism of lime with aggressive CO2 (simplified equations):
CO2 + NaOH >>> NaHCO3
.44 .........40 ...........84 or 50 mg/L as CaCO3 (5°F<TAC)

To neutralize 1 mg of CO2, it is necessary to use (40/44)= 0.91 mg as NaOH, which form(84/44)=1.91 mg of sodium bicarbonate NaHCO3, and therefore also the neutralization of 1 mg /L to form CO2 (5/44) = 0.1136 °F/l de TAC (i.e.1.136 mg CaCO3/L of Alkalinity).

Note: if the calculated dose is a reagent for 100% purity, it is necessary to correct this dose to entering the purity of the commercial product.

Standard products used for the production of drinking water: French Official Bulletin - Sodium hydroxide, Caustic soda : NF EN 896.

Effects on the environment.
Caustic soda increases the pH of the stream, thus representing a potential threat to aquatic fauna and flora (stream, river, groundwater).

Sodium hydroxide is a popular strong base used in the industry. Around 56% of sodium hydroxide produced is used by the industry, 25% of which is used in paper industry. Sodium hydroxide is also used in manufacturing of sodium salts and detergents, pH regulation, and organic synthesis.
It is used in the Bayer process of aluminium production. In bulk it is most often handled as an aqueous solution, since solutions are cheaper and easier to handle.


Chemical burns caused by sodium hydroxide solution photographed 44 hours after exposure.
Like other corrosive acids and alkalis, drops of sodium hydroxide solutions can decompose proteins and lipids in skin, eyes or other living tissues via amide hydrolysis and ester hydrolysis, which consequently causes chemical burns and may induce permanent blindness if it contacts eyes. Solid alkali may also express its corrosive nature if there is water, so protective equipment such as rubber gloves, safety clothing and eye protection should always be used when handling the material or its solutions.
Moreover, dissolution of sodium hydroxide is highly exothermic, and the resulting heat may cause heat burns or ignite flammables. It also produces heat when reacted with acids.
The standard first aid measures for alkali spills on the skin is, as for other corrosives, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes.

Sodium hydroxide is also mildly corrosive to glass, which can cause damage to glazing or freezing of ground glass joints. Careful storage is needed.

Sources : personal and Wikipedia, the free encyclopedia.

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