Caustic Soda solution

NaOH

IUPAC name: Sodium hydroxide
Other names: sodium hydrate; sodic hydroxide; caustic soda; lye.
CAS number: 1310-73-2

Sodium hydroxide, known commercially as Caustic Soda (sometimes named incorrectly as sodium hydrate) is a strong mineral base; it is a solid at room temperature, extremely hygroscopic and deliquescent.
It is very soluble in water (over one kg per litre at 20 °C) and quite soluble in ethanol (139 g/l). Its dissociation is accompanied by a steady release of heat; in the case of ethanol and other organic solvents, this heat can even ignite the solvent vapours.

Manufacture

Sodium hydroxide is among the most widely produced chemical compounds in the world. Most of it is made by the electrolysis of aqueous solutions of sodium chloride; smaller quantities are made by the so-called “caustification” reaction between sodium carbonate and calcium hydroxide.

Diaphragm cells
These cells have an iron cathode and a graphite anode immersed in a concentrated sodium chloride solution. The main drawback of the diaphragm process is its low yield; in fact, the sodium hydroxide reaches a concentration of about 10-12%. The residual chloride separates almost completely during the subsequent concentration under vacuum, as the hydroxide becomes concentrated. The final solution, containing about 50% of NaOH and 1% of NaCl, is used as it is, or is evaporated with direct heat in iron containers until molten sodium hydroxide is obtained.

Amalgam cells (Mercury)
The electrolysis of a sodium chloride solution with a mercury cathode generates a sodium amalgam which upon successive oxidation with water, yields a solution of pure sodium hydroxide. This method has the advantage of directly providing concentrated sodium hydroxide free of chlorides and other impurities. The main disadvantage is the toxicity of mercury; this is why installations based on this process are no longer being built.

Membrane cells
The membrane process is the most recent one (1970) and it is being increasingly utilised because it is more environmentally friendly and safer than the others. The electrolytic cell is divided by a membrane semi-permeable to the chloride ions that separates the anode immersed in the brine from the cathode immersed in distilled water. This process has the further advantage of generating high purity sodium hydroxide.

Caustification
This was the earliest method of preparation of caustic soda and it is still used sometimes, even though it provides amounts of NaOH greatly lower compared to the electrochemical methods. This process utilises the exchange reaction between calcium hydroxide (slaked lime) and sodium carbonate. The resulting sodium hydroxide has a low concentration (8-10%) and contains impurities.

The membrane cell process is more sustainable and has inherent environmental advantages relative to the two older processes because it does not utilise either mercury or asbestos and it reduces energy consumption at the same time. For this reason, all companies in Europe that utilise the mercury cathode technology are committed to switching to the membrane technology by 2020, despite the very high conversion cost, forecast at 3 billion Euros.

Principal uses

It is a widely used reagent in the chemical industry; it is utilised in paper manufacturing, in the metallurgical Sector and in water treatment.

Papermaking
Caustic soda plays a specific role in the process of bleaching the pulp used in papermaking; it activates the hydrogen peroxide action in the reaction H2O2 + OH- → OOH- + H2O where the resulting ion is a weak oxidant that allows pulp bleaching while avoiding fibre degradation.

Metallurgy
Treatment of Bauxite from which alumina (the source of aluminium) is extracted.

Water treatment

  • lowering water hardness;
  • pH adjustment;
  • neutralisation of wastewater;
  • regeneration of ion exchange resins utilised for water softening.

Food industry

  • washing and cleaning of bottles and equipment;
  • peeling and shelling of fruits and vegetables;
  • manufacture of modified starch;
  • preparation of salts such as sodium citrate and acetate;
  • production of carboxymethylcellulose.

Caustic soda is also utilised:

  • to manufacture detergents and soaps;
  • to manufacture sodium hypochlorite (common bleach) and other sodium salts, such as phosphate and sulphide;
  • to synthesise dyes;
  • as a catalyst;
  • as an engraving/cleaning agent;
  • in the treatment of cotton fibres;
  • in the oenological sector to remove the tartrate residues (“barrel tartrate”) from the inside of the vats, utilising appropriate pumps.