(c) doc b(c) doc bSalt: sodium chloride, extraction, uses of halogens

See also Group 7 The HALOGEN elements of the Periodic Table - their physical properties and chemical reactions

Doc Brown's Chemistry KS4 science GCSE/IGCSE/O level Chemistry Revision Notes

Salt deposits and sea water are valuable sources of sodium chloride, from which many useful products are made in the chlor-alkali chemical industry. Sodium chloride solution (brine) is readily electrolysed to produce three products directly - chlorine gas, hydrogen gas and sodium hydroxide solution which can be converted to many other useful industrial and domestic consumer products.

Sources of sodium chloride salt - extracting it from natural resources

In hot countries seawater is evaporated in large open air tanks. As the water evaporates the salt crystals form and be collected up. Seawater is allowed to flow into shallow pools and the water left to evaporate. More seawater can be let in and the water evaporated from the heat of the Sun's rays until a good layer of salt has built up, which can be quite pure.

In the UK there are huge underground deposits from dried up seas from hundreds of thousands or millions of years ago. Rock salt is mined in large quantities from under Cheshire and formed the basis of the Chlor-Alkali industry in the north-west of England. ICI has chemical plants at Runcorn based on the salt extraction industry.

Rock salt solid is conventionally mined by drilling, blasting and digging etc., though there are powerful machines that can just cut away the rock directly with huge rotating blades. On extraction, the solid rock salt is impure sodium chloride, containing silica minerals and other chloride or sulfate salts.

Rock salt can also be extracted by solution mining (diagram on the right). Hot water is pumped under pressure down into the rock salt layers in an out pipe. The hot water dissolves the salt to form a concentrated brine solution, which under pressure, is pumped back to the surface up through the inner pipe. Brine is a name used for aqueous sodium chloride solution. The brine is pumped to storage tanks and onto a refining plant for purification. After impurities (like sand) are removed the brine is boiled to crystallise the salt. A lot of salt, as brine solution, is processed by electrolysis to give several important products (details in a section below).

In areas such as Cheshire and County Durham in the north-east of England there has been (and will be for some time to come) land subsidence due to collapses of old mine workings underground. This subsidence can affect roads and houses and ground collapses can happen without warning! Wherever possible old mines should be filled in to minimise the risk from subsidence. But, by using good mining techniques, such subsidence can almost be prevented. What you do is leave good supporting pillars of rock salt that prevent the rock salt layers and the ground above from collapsing. If larger caverns are used, they should be spaced out and back-filled with waste after use.

All forms of mining use lots of energy, often from fossil fuels, which are finite resources and cause pollution.

 

The Industrial Electrolysis of Sodium Chloride Solution or brine

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

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THE ELECTROLYSIS CELL

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

negative electrode (cathode) product ions from water ions from salt positive electrode (anode) product 

hydrogen gas formed H2(g) <==

hydrogen ion H+(aq) changes to <==

chloride ion Cl(aq) changes to ==>

==> chlorine gas formed Cl2(g)

ion left in solution ==>

OH Na+

<== ion left in solution

When electricity is passed through the concentrated sodium chloride solution (brine) there are three products(1) hydrogen is formed at the negative electrode (–ve cathode), (2) chlorine at the positive electrode (+ve anode) and (3) sodium hydroxide is left in solution (Na+ plus OH). The electrode equations are given and explained below. All three products can be tapped, stored and further processed.
Summary equation for the electrolysis of brine (sodium chloride)
  • sodium chloride + water ==> sodium hydroxide + hydrogen + chlorine
  • 2NaCl (aq) + 2H2O (l) + elec. energy ==> 2NaOH (aq) + H2 (g) + Cl2 (g) 
  • (c) doc bThe 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 splinthydrogen. The (+) anode gas turns damp blue litmus red and then bleaches it white – chlorine.
The electrode equation theory and details
  • The (–) cathode attracts the Na+ and H+ ions. The hydrogen ions are reduced by electron (e) gain to form hydrogen molecules:

    • 2H+(aq) + 2e ==> H2(g)

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

    • 2Cl(aq) ==> Cl2(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

The environmental impact of industrial processes - electrolysis of brine

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.

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THE USES OF HALOGENS and their COMPOUNDS

The Uses of Chlorine, the brine electrolysis products and other halogens and their compounds

 

Uses of Sodium Chloride

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)).

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Uses of CHLORINE from brine electrolysis

Cl2

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  • 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?).
  • 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!

  • 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)
    • (c) doc b
    • 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.
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  • 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.
    • H2(g) + Cl2(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
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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 superceded by digital cameras!

Uses of other halogens

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 fuels 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'.

Uses of HYDROGEN H2 from brine electrolysis
  • 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.

Uses of SODIUM HYDROXIDE

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!

 

APPENDIX - The Use of Salt in the Food Industry and Health Issues

Purified (refined) salt (sodium chloride) is extensively used in the food industry.

It is an ingredient in many processed food, so we take in significant amounts of salt if you eat appreciable amounts of processed food. Salt is used as a preservative to give foodstuffs a longer shelf-life and prevent decay due to mould and bacteria (stop food 'going off'). If meat is treated with salt it dries the meat by absorbing water and this dehydrating effect kills bacteria. 'Cured meat' is preserved in this way.

 

Although we all enjoy the taste enhancing properties of salt there are health issues related to 'over-consumption' of salt. We do need salt in our diet, but not too much.

Consuming too much salt can cause high blood pressure which in turn can cause heart attacks and strokes.

Taking in too much salt has been linked to increased risk of osteoporosis, renal failure (kidney failure) and stomach cancer.

Food products should be labelled with the amount of salt in them, its not always clear. Food labels may give you the amount of salt in the food in grams or the % of the maximum recommended intake of salt.

If you don't eat much processed food, the odd sprinkle on your meals shouldn't cause you any harm.

The Food Standards Agency (FSA) is an independent food safety watchdog that advises the government and the public about food safety and health and issues guidelines on the maximum amount of salt you should take in a day. This is known as the 'Guideline Daily Amount' (GDA).

Two government departments, Department of Health and the Department for Environment, Food and Rural Affairs, are both involved with food safety issues and do risk assessments to check on the safety of chemicals used in food and advise the public on how food, and food additives may affect their health.

We know that too much salt is not good for us, but many food products still have significant amounts of salt in them.

This may due to the cost of producing a replacement 'recipe' that tastes as good, lasts as long too (shelf-life) and sales maintained at previous levels.

This brand of baked beans claims to contain 25% less salt than its 'standard baked beans' product. The product contains 0.4g/100g i.e. 0.4% sodium chloride (as NaCl). A good helping (1/2 can) gives you 15% of the RDA (recommended daily intake).

Sometimes the nutritional information just quotes the mass of sodium ion in the food, but you can do a simple calculation to work out the actual mass of sodium.

Suppose the mass of sodium was quoted as 0.4g per 100g in a food product.

Atomic masses: Na = 23, Cl = 35.5, so formula mass of NaCl = 23 + 35.5 = 58.5

Scaling up gives 0.4 x 58.5 / 23 = 1.0g NaCl/100g of food product, i.e. 1.0% salt in the food.

Note that this calculation assumes all the sodium is associated with sodium chloride, but the sodium may come from other food additives like flavour enhancer sodium glutamate or the preservative sodium nitrate.

However, I can't imagine many consumers doing this sort of 'shopping calculation' in the middle of busy supermarket!

Salt is added to many food products e.g. cereals (above), so it all adds up through the day! and the more processed food you eat, the less control you have over your salt intake.

Note that in cereal 2a they have done the calculation for you, 0.18g Na x 58.5/35.5 = 0.46g of NaCl salt

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See also Group 7 The HALOGEN elements of the Periodic Table

 

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