Iodine is present at 0.05 p.p.m of seawater. However certain varieties of seaweed concentrate iodides greatly. The main source of iodine is sodium iodate (V) NaI03 found only in Chile.
The chlor-akali industry is the production of chlorine by electrolysis of brine (concentrated rock salt in water) and the production of sodium hydroxide. Flowing mercury is cathode used and the sodium produced dissolved into the mercury to form an amalgam. The sodium is then extracted and the mercury is recycled. The use of recycling the mercury means that all the products are useful. Also no energy has to be inputted to produce a liquid metal in this reaction, as mercury is a liquid at room temperature.
During the reaction, the chlorine is formed at the graphite anodes and liberated; sodium is produced and dissolved at the mercury cathode.
Anode: 2Cl-(aq) Cl2(g) + 2e-
Cathode: 2Na+(aq) + 2e- 2Na(Hg)(l)
The chlorine is collected and pressurised for storage whilst the sodium – mercury amalgam passes onto a second soda cell to form sodium hydroxide. The sodium reacts with the water to form sodium hydroxide solution (caustic soda) and hydrogen.
2Na(Hg) + 2H2O(l) 2Na+(aq) + 2OH-(aq) + H2(g) + 2Hg(l)
Mercury cells are not used as in the 1950s a disease occurred in Mina Mata Japan. Where many citizens and domesticated animals that fed on fish developed symptoms including loss of balance, muscle wasting, paralysis, and eventually death. This was discovered to be mercury poisoning. A major source of mercury was from a plant producing chlorine from brine using flowing mercury cells.
The anode was made from titanium coated with ruthenium (IV) oxide and titanium (IV) oxide. This is highly resistant to corrosion by chlorine and also aids production of the gas. Sodium-mercury amalgam contains about 50% sodium hydroxide. Again the mercury is recycled and so is fed back into the cells.
The move from mercury cells was made to membrane cells, which does not contain a risk of harming the surrounding population.
In the membrane cells there is no net flow of liquid. Na+ ions are able to pass through the membrane whilst Cl - and OH – cannot. In this way a current can pass through the cell, this also helps to prevent contamination of the sodium hydroxide and the sodium chloride.
The materials used for the membrane are synthetic polymers selected for their ability to transport cations and for their resistance to corrosive solutions and high temperatures.
In 1924 an Austrian chemist named Friedrich Paneth wrote: -
“The unreactivity of the noble gas elements belong to the surest of experimental results”
A chemist by the name of Neil Bartlett in 1962 discovered the first compound containing a noble gas and was stable at room temperature. It was a yellow-orange solid with the formula XePtF6.
Bartlett had been working for some time on a compound containing platinum and fluorine PtF6. He discovered this to be an extremely strong oxidising agent, which proved possible to make a compound, which the O2+ cation appeared. [O2]+ [PtF6]-. His success in making a compound using a noble gas was due to the fact that he realised that the first ionisation energy of Xenon was less than that of the Oxygen molecule. Subsequently reasoned that Xenon reacted the same was as oxygen producing XeF2, XeF4, XeOF4, and XeO4.
Disproportion is the reaction of chlorine, bromine and iodine with an alkali. It is a redox reaction in which a molecule, atom, or ion is simultaneously oxidized and reduced. When chlorine is reacted with an alkali, Cl – ions and ClO – ions are produced. This entails a change in oxidation states from being 0 in Cl2 to –1 in Cl- and +1 in ClO-.
Cl2 + 2NaOH(aq) NaCl(aq) + NaClO(aq) + H2O(l)
In the same way the disproportion of ClO- also results in both an increase and decrease in the oxidation states of chlorine.
3ClO-(aq) 2Cl-(aq) + ClO3- (aq)
Halogens have many uses such as pesticides e.g. Dichlorodiphenyltrichloroethane (DDT), which was aimed at removing the threat of tropical diseases such as the anopheles mosquito that carries malaria. DDT removed the treat of malaria to thousand of people and saved millions of lives throughout the world, and also helped to stop the spread of typhus and yellow fever.
Another example of the uses of halides is water purification. The oxidising power of chlorine is used in the treatment of drinking water, which is piped to our homes. After the removal of waste solid matter like fine particles of dirt: the water in the reservoir is treated with chlorine to kill any bacteria. Small amounts of chlorine remain in the water to destroy any bacteria, which may prevent recontamination the water.
However halide compounds are not all good things e.g. CFCs otherwise know as Chlorofluorocarbons. This compound contains carbon, chlorine, and fluorine. These compounds cause damage to the ozone layer, which increases global warming.
DDT is another example as a harmful halide, despite treating malaria, DDT remains in the environment, as they do not break down naturally. The level of these chemicals was not high – it was at a level enough to kill pests but not larger organisms. However because they are stable they have gathered in the tissues of animals within food chains. Its main effect was on their reproduction within the species, they became less fertile and offspring were less likely to survive.
These particles have also made themselves into the human food chain. However some of the organisms they were originally used on some have developed a resistance to these pesticides and so are carefully regulated.