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Chlorine can be produced by use of an electrolytic cell, the "Diaphragm Cell". An electrolytic cell is a device that converts electrical into chemical energy. An electrolytic cell contains both an electrolyte, as well as two electrodes, known as the Anode and the Cathode. The electrolyte is typically a solution of water as well as other solvents, or molten salts. When an electrolytic cell is connected to an electric current, this electrolyte gives provides ions to the cathode and the anode. Redox reactions will therefore take place.
The Diaphragm Cell is used primarily in the chlor-alkali industry, in order to produce chlorine. This cell is made up of a steel-mesh container, lined with a porous asbestos diaphragm, which separates the anode and the cathode, and prevents chlorine from forming at the anode and re-mixing with the sodium hydroxide and the hydrogen, to be formed at the cathode. The anode is typically made of titanium, although carbon too is used and the cathode is steel. For this process, a solution of saturated sodium chloride (commonly known as brine) is poured into the anode compartment. The level of the brine is kept as greater than that of the level in the cathode section, so that this brine pours through the asbestos. This too reduces the flow of hydroxide ions and prevents ClO- ions being produced. Hydrogen, being produced on the outside of the asbestos diaphragm, escapes the cell, and is collected and used to make other products. The steam inside the cell produces heat, and therefore condensation (on the steel mesh) removes the sodium hydroxide. This sodium hydroxide collects at the bottom of the cell, and this makes up around 10% of the cell liquor (the solution leaving the cell), 15% of this being made up for sodium chloride. After this liquor has left the cell, the remaining solution contains around 1% sodium chloride. The sodium chloride crystallises, and so can easily be recycled. Chlorine gas is produced at the anode, which escapes the cell and is collected for additional treatment.
In this process, half reactions take place at the anode and cathode, in order for the products to be produced.
At the anode, this half reaction takes place:
2 Clâˆ’ (aq) â†’ Cl2 (g) + 2 eâˆ’
At the cathode, the half reaction takes place:
2 H+ (aq) + 2 eâˆ’ â†’ H2 (g)
These two half reactions produce the overall cell reaction, representing the products produced by the diaphragm cell:
2 NaCl (or KCl) + 2 H2O â†’ Cl2 + H2 + 2 NaOH (or KOH)
This diaphragm cell produces dilute alkali, with lower purity achieved by the Mercury and diaphragm cell. It too, is less harmful on the environment than the Mercury cell, as there is no poisonous mercury releases into the environment. They too, operate a low voltage, saving electricity. This cell is, however being phased out of production and is hardly, if ever used today. This is due to its use of an asbestos membrane. In this production, the asbestos membrane could very easily be damaged. When this material is damaged, its tiny fibres become airborne and are easily inhaled. When these fibres penetrate lung tissue, they cause the lungs to become dangerously inflamed. As a result, the body sends white blood cells to destroy these fibres. These fibres, however resist and destroy these cells, causing further inflammation and often deadly scarring of the lungs. This condition is known as asbestosis.
A diagram illustrating the diaphragm cell:
A use of the chlor-alkali process:
Chlorine, produced by the chlor-alkali process is extremely important for a large number of products, one of the most essential to humans being the process used for the purification of water. This chemical was first used in the 19th century to prevent the spread of water-bound diseases such as cholera, typhoid and gastro-enteritis; all caused by a lack of purified, clean water to the people. These diseases, have statistically, killed more people than a combination of every war ever experienced by humankind. The World Health Organisation estimates that over three million people are killed each year due to a lack of clean drinking water. The addition of chlorine to water, or chlorination, is both a safe, and economically viable process in order to combat water-dwelling diseases. It, unlike other methods of water purification ensures clean water all the way to the tap, whereas other methods of purification are only temporary. It too prevents the growth of algae and slime in pipes and irrigation systems.
When chlorine is added to water, this causes a reaction, creating a pH dependant equilibrium mixture of the chlorine, hydrochloric acid and hypochlorous acid. The following reaction takes place:
Cl2 + H2O â†’ HOCl + HCl
Hypochlorous acid, depending on the pH of the solution, partly dissociates into hydrogen and hypochlorite ions, as shown below:
HClO â†’ H+ + ClO-
Depending on the pH of the solution, different species are present: in an acidic solution, the largest species, Cl2 and HOCl are present, whereas in a basic solution, only ClO-. There too, are tiny samples of ClO2-, ClO3-, ClO4- in solution.
This process can, therefore purify both acidic and basic solutions of water, making them safely drinkable to all. Chlorine, therefore acts as a powerful disinfectant and when added to water in tiny quantities, it effectively kills bacteria, germs and other microbes extremely quickly. This is too, the primary reason why chlorine is used to clean swimming pools. It is placed in small quantities, into pools, cleaning them by removing potentially hazardous microbes that thrive in stagnant water, allowing us as humans to swim at our leasure.
Despite the large success of chlorine as a means of water purification, there are certain drawbacks that very seldom can result in water becoming poisonous. Chlorine has been known to react with certain organic compounds in water, producing Disinfection Byproducts, or DBPs. The most common of these are haloacetic acids, and trihalomethans, which have been proven to be potentially carcinogenic. One Dr. Joseph M. Price, MD, labeled chlorine as the "greatest crippler and killer of modern times," he too named it an "insidious poison." In a study, conducted in 1992, researchers at the Medical College of Wisconsin in Milwaukee found that those drinking chlorinated water with large amounts of DBPs, have a considerably greater risk of developing bladder and rectal cancers than those drinking unchlorinated water. This study estimated that a shocking 9% of all bladder cancer and 18% of all rectal cancer cases were directly resultant of drinking chlorinated water. This has added an additional approximately 20 000 people to the cancer death toll each year. Despite this carcinogenetic nature of chlorination, it is important to understand that there are thousands if not millions of everyday household items, foodstuffs or electronics, which too have been linked to cancer. If we are going to worry about our own tap water supply causing cancer, we should to worry about the links between wireless networks, televisions, mobile phones, computers and so many other everyday items to cancer. Cancer is a reality that has to be faced, but it is however, a great overreaction to condemn chlorine as a killer, when it has save millions of lives from water bound diseases. The risks of cancer, associated with chlorination are therefore relatively miniscule compared with that of the risks of drinking unpurified water, or using the alternative methods of water purification which are often unsuccessful, far more expensive and power consuming, and often to do fully purify water. These other methods have a greater impact on the environment, using more electricity, which therefore destroys more of the ozone layer, creates more dangerous gases in our environment and generally has far more risks than the use of chlorination.
Chlorination, as well as other products or processes chlorine is used to make or perform, often have dangerous effects on the environment. Because of its reactivity, chlorine is unlikely to move through the ground and enter natural water supplies, although in some cases this has directly and indirectly happened. Chlorine can be hazardous to organisms living in fresh water. Many companies often filter the Disinfection Byproducts produced by the process of chlorination, in order to prevent these from reaching taps. These byproducts are then often dumped into rivers and dams where fish and other living organisms reside. As mentioned above these substances are known to be carcinogenic in very dilute form, and when in such a concentrated, filtered form, their effects can be hazardous when dumped into the environment. To make it worse, when these products are in the "waste stream", filled with other garbage products dumped by factories, they are exposed to other organic substances, with which they combine to form toxic organic chemicals. The water will then in turn become contaminated, which will result in fish being contaminated, which will contaminate the entire food chain, with larger animals eating fish and humans eating both of these. Many environmental groups however are protesting this problem and a ban on dumping byproducts of chlorination in water is being called for.