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sodium hydroxide

Product name sodium hydroxide
Synonyms caustic soda
soda lye
GOST 2263-79
CAS 1310-73-2

Sodium hydroxide NaOH is a colorless, hygroscopic, deliquescent crystalline solid. It readily dissolves in water with liberation of heat. Solubility is 111g/100ml at 20°C. Being crystallized from water solutions it forms different crystalline hydrates depending on temperature - monohydrate (12.3 - 61.8°C), NaOH*3.5H2O (from -5.4°C to 12.3°C), tetrahydrate (from -17.7°C to -5.4°C), pentahydrate (from -24°C to 17.7°C) and heptahydrate (from -28°C to -24°C). Anhydrous sodium hydroxide crystallizes above 61.8°C. Caustic soda also soluble in methanol and ethanol, but it shows lower solubility in these and other organic solvents than does potassium hydroxide. It is insoluble in acetone, toluene, diethyl ether and liquid ammonia. Sodium hydroxide completely dissociates in water solutions to form sodium cations and hydroxide anions, which make it a strong base.

Solid caustic soda and its water solutions readily absorb carbon dioxide from the air to form sodium carbonate, so it should be stored in air and watertight container.

2NaOH + CO2 --> Na2CO3 + H2O

As a strong alkali sodium hydroxide interacts with inorganic and organic acids to form corresponding salts. In general such processes are called neutralization reactions:

2NaOH + H2SO4 --> Na2SO4 + 2H2O

Interaction of caustic soda with acid oxides such as carbon dioxide, sulfur(III) oxide and phosphorus(V) oxide also results in salts formation. Sodium hydroxide solutions are able to lixiviate silicate glass to form sodium silicate. Flasks, tubes, stopcocks are damaged by long exposure to hot caustic soda, and the glass becomes frosted. Sodium hydroxide does not reacts with cobalt, iron, nickel or copper, but it readily attacks other metals such as aluminium, titanium or zinc:

2Al + 6NaOH --> 3H2 + 2Na3AlO3

Some non-metals also react with caustic soda to form salts. So, interaction with phosphorus results in sodium hypophosphite, while reaction with silicon yields to sodium silicate.

Sodium hydroxide reacts with many organic compounds. It hydrolyses alkyl- and arylhalides to form corresponding alcohols. It reacts with amides resulting in carboxylic acids. Interaction with esters yields to alcohol and sodium salts of carboxylic acid (saponification process):

R1C(O)OR2 + NaOH --> R1COONa + R2OH

Sodium hydroxide, either in pure or dissolved form, can pose several risks. It may cause chemical burns of skin or mucous membranes, permanent injury or scarring, and blindness. It is able to permeate deep into animal tissue damaging adipose, lipids and proteins.


Almost commercial sodium hydroxide is manufactured by electrolysis of aqueous solution of sodium chloride NaCl. This process also produces gaseous chlorine and hydrogen:

2NaCl + 2H2O --> 2NaOH + Cl2 + H2

There are several industrial routes for sodium chloride electrolysis:

  • Mercury cell electrolysis. Titanium anodes are located above a liquid mercury cathode in the electrolysis bath with a solution of sodium chloride. When an electrical current is applied, chlorine is released at the titanium anodes and sodium dissolves into the mercury cathode forming an amalgam. Dissolved sodium is then reacted with water releasing hydrogen to produce NaOH. There have been concerns about mercury releases, although modern plants claim to be safe in this regard. This method consumes vast amounts of energy.
  • Diaphragm cell method. In this route a porous asbestos or polymer diaphragm is deposited on an iron grid cathode preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing. The caustic soda must usually be concentrated to 50% and the salt removed. This is done using an evaporative process. Sodium hydroxide produced by this method is contaminated with sodium chloride (about 2-3%);
  • Membrane cell method. - similar to the diaphragm cell process. The electrolysis cell is divided into two by a ion exchange membrane. This method is nearly as efficient as the diaphragm cell and produces very pure sodium hydroxide but requires very pure sodium chloride solution. Of the three processes, the membrane cell process requires the lowest consumption of energy.


    Sodium hydroxide is applied:

  • in alumina production industry, a strong alkali solution separates pure alumina from bauxite ore;
  • as a chemical reagent and in the manufacture of other chemicals as the principal strong base;
  • for the neutralization of acidic materials;
  • as a neutralizing agent in petroleum and oils refining;
  • to separate lignin from cellulose fibers (Kraft process) and for bleaching the brown pulp;
  • to remove waxes and oils from fibre in order to make the fibre more receptive to bleaching and dying;
  • in the production of viscose rayon;
  • in making soaps and synthetic detergents;
  • for sodium hypochlorite manufacturing;
  • to measure the concentration of acids by titration;
  • in chemical laboratories as a dehydrating agent for many gases and liquids;
  • as an etching agent for aluminium;

  • Manufacturer(s) Bratsk chlorine plant
    JSC Caprolactam
    JSC Dneprazot
    JSC Khimprom
    JSC Middle Volga Chemical Plant
    CJSC Soda-Hlorat
    JSC Ufachemprom
    Chemical structure of sodium hydroxide

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