Sodium hydroxide NaOH (also known as caustic soda) is a basic substance, deliquescent in air, from which it absorbs carbon dioxide, turning into NaHCO3 and Na2CO3. At ambient temperature, it corrodes neither rubber nor synthetic resins, but it attacks metals such as aluminum, tin, zinc and lead. It destroys organic, animal and plant tissues.
Caustic soda is obtained in two ways:
1)By the reaction of sodium carbonate with calcium hydroxide
2)By electrolysis of aqueous solutions of sodium chloride
Reaction of sodium carbonate with calcium hydroxide.
This equilibrium reaction is called carbonate causation and is as follows:
Na2CO3 + Ca (OH)2 ⇌ 2 NaOH + CaCO3
Na2CO3 + Ca (OH) 2 ⇌ 2 NaOH + CaCO3
The reaction occurs with high yields when the concentration of sodium carbonate is low: with 10% solutions of Na2CO3, the yield is 97%, while for the 16% solutions, the yield drops to 91%. The reaction is carried out by introducing a solution of sodium carbonate and calcium oxide into boilers in cascade arrangement equipped with an agitator and a jacket for steam heating. Calcium oxide reacts with water according to the exothermic reaction:
CaO + H2O = Ca (OH) 2
The solution passes into decanters where the forming insoluble calcium carbonate is separated by decantation and then by filtration. At this stage, a 10% solution is obtained. Evaporators are used to obtain higher concentration in a way a part of water evaporates leading to a concentration of up to 50%. In order to obtain 98% caustic soda, concentration is carried out in special cast iron boilers and the temperature is raised to 300 ° C is removed and the melt flows into molds in which it solidifies. However, the product obtained is impure and, in the case where a product of high purity is required, the caustic soda is dissolved in ethyl alcohol, a solvent in which the impurities typically contained (sodium carbonate, iron oxides, silica, etc.) are insoluble. From the solution, the alcohol is evaporated, the residue is melted and poured into pure caustic soda.
Electrolysis of aqueous solutions of sodium chloride.
In the electrolytic process, the electric current passes through a solution of sodium chloride into special electrolysis cells. The sodium chloride decomposed by the stream forms a 10-12% NaOH solution with the development of H2 at the cathode of iron and Cl2 at the graphite-anode according to the reaction:
2 NaCl + 2 H2O → H2 + Cl2 + 2 NaOH
The reaction 2 H2O → 2 OH– + H2 takes place at the cathode
As the concentration of OH- ions increases around the cathode, they move towards the anode where they react with Cl2, generating hypochlorites:
Cl2 + 2 OH– → ClO– + Cl– + H2O
Because of the loss of chlorine and caustic soda, the diffusion of the substances produced is avoided by using porous or bell-shaped cement diaphragms in which the separation of the two liquids is obtained from the difference in density between the densest cathodic solution (NaOH) and the anodic solution. less dense (NaCl).
In addition to this technique, an NaCl solution can be electrolyzed between a chlorine-resistant anode such as graphite and a mercury cathode: this process is known as the Castner-Kellner process. The reaction that occurs at the anode level is:
2 Cl– → Cl2 + 2e–
The reaction that occurs at the cathode of mercury is as follows:
2 Na + + 2 et – → 2 Na
Sodium metal forms an amalgam with the mercury giving rise to the reaction
2 Na (amalgame) → 2 Na + + 2 e–
The amalgam reacts with water to form sodium hydroxide and water.
Chemical formula: NaOH
Formula weight (U): 39,971
Appearance: white crystalline solid
CAS number: 1310-73-2
EINECS number: 215-185-5
Density (g / cm3, c.s.): 2.13
Basic dissociation constant at 398 K: 2.6915 x 102
Solubility in water: 1090 g / L à 293 K
Melting temperature: 323 ° C (596K)
Boiling temperature: 1390 ° C (1663K)
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