2. The industrial electrolysis of sodium chloride solution (brine)

The chlor-alkali industry process, the starting point for many useful products for industrial and domestic use

THE ELECTROLYSIS CELL

Summary of the ions involved and what happens to them at the two electrodes.

hydrogen gas formed H_2(g) <==

hydrogen ion H⁺_(aq) changes to <==

chloride ion Cl^–_(aq) changes to ==>

==> chlorine gas formed Cl_2(g)

ion left in solution ==>

<== ion left in solution

• sodium chloride + water ==> sodium hydroxide + hydrogen + chlorine
• 2NaCl _(aq) + 2H₂O _(l) + elec. energy ==> 2NaOH _(aq) + H_{2 (g)} + Cl_{2 (g)} 

• The industrial electrodes must be made of an inert material like titanium which is not attacked by chlorine or alkali.
• However a simple cell using carbon electrodes can be used to demonstrate the industrial process in the laboratory.
• The (–) cathode gas gives a squeaky pop with a lit splint – hydrogen. The (+) anode gas turns damp blue litmus red and then bleaches it white – chlorine.

• The (–) cathode attracts the Na⁺ and H⁺ ions. The hydrogen ions are reduced by electron (e^–) gain to form hydrogen molecules:

  • 2H⁺_(aq) + 2e^– ==> H_2(g)

• The (+) anode attracts the OH^– and Cl^– ions. The chloride ions are oxidised by electron loss to give chlorine molecules:

  • 2Cl^–_(aq) ==> Cl_2(g) + 2e^–

• The ions remaining in solution (Na⁺ plus OH^–) form the 3rd product, sodium hydroxide solution.

• For more details laboratory experiments and theoretical explanation see Electrolysis of sodium chloride solution (brine) and molten sodium chloride

Large scale electrolysis uses large amounts of electrical energy, much of which is derived from fossil fuelled power stations.

Fossil fuels are a finite resource and their use releases carbon dioxide into the atmosphere potentially leading to global warming - climate change, increased acidity in the oceans with threats to ecological systems.

At one time, (hopefully not now?) some kinds of electrolysis cells used large quantities of the toxic metal mercury. If mercury 'escapes' via an accident into the environment and enters river systems fish are poisoned and people who eat the fish poisoned too.

Electrolysis plants have used asbestos insulation (hopefully not now?), asbestos is toxic mineral that causes lung damage leading to lung cancer.

NaCl

'salt' & 'rock salt'

• sodium chloride is an important raw material found as 'rock salt' or in seawater.
• Purified sodium chloride, 'salt' is used directly for food flavouring and food preservation.
• Table salt is produced by evaporating purified brine solution.
• Rock salt is used for de–icing roads. Raw rock salt can be used directly on roads to melt ice and preferably, stop ice forming in the first place. Dissolved salts lower the freezing point of water. The rock salt is mixed with grit to give better tyre grip on icy roads.
• BUT sodium chloride is mainly converted into other products by electrolysis (see above for the production of chlorine, hydrogen and sodium hydroxide by electrolysis of brine (aqueous sodium chloride solution).

Uses of CHLORINE from brine electrolysis

Cl₂

• All the Halogens are potentially harmful substances and chlorine in particular is highly toxic. It is dangerous to ingest halogens or breathe in any halogen gas or vapour.
• Chlorine, a disinfectant, is used to kill bacteria and so sterilise water for domestic supply or in in swimming pools. Chlorine kills most harmful microorganisms (all of them?).
  • For further discussion see Water Treatment page.

• The sodium hydroxide and chlorine can be chemically combined at room temperature to make the household bleach, sodium chlorate(I) NaClO. This is used in some domestic cleaning agents, it chemically 'scours' and chemically 'kills' germs!

  • sodium hydroxide + chlorine ==> sodium chloride + sodium chlorate(I) + water

  • 2NaOH_(aq) + Cl_2(aq) ==> NaCl_(aq) + NaClO_(aq) + H₂O_(l)

  • More details for Advanced A Level Chemistry Students

• Organic phenolic chlorine compounds are used as antiseptics and  disinfectants like 'Dettol' or 'TCP'
• Organic chlorine compounds are used as pesticides, including the now mainly banned DDT.
• Chlorine is used in making CFC refrigerant gases/liquids but their production and use are being reduced. They break down in the upper atmosphere and the chlorine atoms catalyse the destruction of ozone O₃ which absorbs harmful uv radiation.
• PVC plastic: Chlorine (from electrolysis of NaCl) and ethene (from cracking oil fractions) are used to make a chemical called chloro(ethene), which used to be called vinyl chloride, this is then converted into the plastic–polymer poly(chloroethene) or PVC, because it is shorthand for the old name polyvinylchloride! (equation below)
  • 
  • Poly(chloroethene), old names PVC, from chloroethene (vinyl chloride) is much tougher than poly(ethene) and very hard wearing with good heat stability. so it is used for covering electrical wiring and plugs. It is also replacing metals for use as gas and water drain pipes and has found a use as artificial leather and readily dyed to bright colours!
  • PVC is very tough hard wearing useful plastic and a good electrical insulator and is used for water piping, window frames, part of electrical fittings e.g. plug covers etc.
    • (old names : polyvinyl chloride, shortened to PVC)
• Liquid organic chlorine compounds are used as dry cleaning or de–greasing solvents.
• Chlorine is used in the manufacture of potassium chlorate weed killer, KClO₃.
• Chlorinated organic compounds are used as insecticides.

• Hydrogen chloride & hydrochloric acid - a joint use of chlorine plus hydrogen
• HCl_{(g => aq)} As described above, some of the hydrogen and chlorine from the electrolysis of sodium chloride solution are combined to form hydrogen chloride gas.
  • H_2(g) + Cl_2(g) ==> 2HCl_(g) 
• This gas is dissolved in water to manufacture hydrochloric acid.
  • HCl(g) + aq ==> HCl_(aq)   or   ==> H⁺_(aq) + Cl^–_(aq) 
• This is a very important acid used in the chemical industry to make chloride salts.
• If there is an excess of hydrogen chloride (for some reason?) it can be oxidised back to chlorine.
  • Hydrogen chloride and oxygen are passed over a catalyst at high temperature
  • hydrogen chloride + oxygen  ==>  chlorine + water
  • 4HCl  +  O₂  ===>  Cl₂ +  2H₂O

Uses of silver salts Ag⁺X^–

• Silver chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI) are all sensitive to light ('photosensitive'), and all three are used in the production of various types of photographic film used to detect visible light and beta and gamma radiation from radioactive materials.

• Each silver halide salt has a different sensitivity to light. When radiation hits the film the silver ions in the salt are reduced by electron gain to silver.

  • Ag⁺ + e^– ==> Ag, the halide ion is oxidised to the halogen molecule

    • 2X^– ==> X₂ + 2e^–

• AgI is the least sensitive and used in X–ray radiography, AgCl is the most sensitive and used in 'fast' film for cameras, and AgBr is used in most standard photographic films – but much of their use is being superseded by digital cameras!

FLUORINE F₂

BROMINE Br₂

IODINE I₂

• Fluorine is used as fluoride salts in toothpaste or added to domestic water supplies to strengthen teeth enamel helping to minimise tooth decay. (e.g. potassium fluoride).

• Fluorine is used in the manufacture of the tough non–stick plastic PTFE coating of cooking pans.

• Fluorine is used in manufacture of aerosol propellants and refrigerant gases.

• Apart from its silver salt use in photography, bromine is used to manufacture organic pesticides and fungicides because of their poisonous nature

• Organic bromine compounds are used as flame inhibitor chemicals (flame retardants) for plastic products to reduce their flammability and as petrol additives to reduce the build–up of lead in car engines (a use decreasing as 'green' unleaded are becoming more popular).

• Bromine and iodine are both used in 'halogen' car headlamps.

• Iodine is used in hospitals in the mild antiseptic solution 'tincture of iodine'.

• Hydrogen is used in the manufacture of ammonia for fertilisers, nitric acid, domestic cleaning products.

• The hydrogen is used to partially hydrogenate unsaturated vegetable oils/fats to make margarine by adding hydrogen to the C=C double bonds in these natural molecules.

• The hydrogen can be combined with chlorine to make hydrogen chloride, which dissolved in water becomes the important industrial chemical hydrochloric acid.

• Hydrogen is used in hydrogen-oxygen fuel cells.

• Hydrogen is used as fuel in welding and metal cutting equipment.

• Hydrogen isn't a halogen, but it is made from the electrolysis of salt solution.

NaOH

from brine electrolysis

• Sodium hydroxide, a very strong alkali, is used in the manufacture of soaps, detergents, paper pulp, oven cleaner, ceramics and to make soluble salts of organic acids with low solubility in water (e.g. soluble Aspirin).
• It isn't a halogen compound, but it is made from the electrolysis of salt solution.

• The sodium hydroxide and chlorine can be chemically combined at room temperature to make the household bleach, sodium chlorate(I) NaClO. This is used in some domestic cleaning agents, it chemically 'scours' and chemically 'kills' germs!

  • sodium hydroxide + chlorine ==> sodium chloride + sodium chlorate(I) + water

  • 2NaOH_(aq) + Cl_2(aq) ==> NaCl_(aq) + NaClO_(aq) + H₂O_(l)

  • More details for Advanced Level Chemistry Students