Chlorine

Chlorine Tonion SmithCeletia CorralKimberly Fagan

1Electrolysis of brine using the diaphragm cellElectrolysis of brine (sodium chloride) solution can be used to make three useful substances chlorinesodium hydroxide hydrogenThree types of cells are used in the electrolysis of brine:Mercury CellsDiaphragm Cells Membrane Cells

Electrolysis of brine using the diaphragm cellBrine is a concentrated solution of sodium chloride ranging from about 3.5% to 26%Brine consists of high concentrations of Na+ and Cl- ions Water is present in lower concentrationsH+ and OH- ions are produced from the equilibrium of water with its ions:

The Diaphragm Cell

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Electrolysis using the Diaphragm CellAt the Anode:The anode is the positively charged electrodeThe anode is in the diaphragm cell is titaniumNegative ions, Cl- and OH-, are attracted towards the positive anode It is easier to liberate OH- ions (to give oxygen) than Cl- ions (to give chlorine), however, Cl- ions are higher in concentration than OH- ions in brineCl- ions are selectively oxidised at the anode

Major reaction at Anode:Chlorine is produced according to the equation:

Chloride ions lose electrons at the anode (oxidation), and the atoms produced combine to give chlorine gasAt the Anode

Chlorine is contaminated with small amounts of oxygen due to oxidation of OH- ions (from water)

Chlorine is purified by liquefying under pressure to remove oxygen (O2 stays as a gas when compressed)At the Anode

At the CathodeThe cathode is the negatively, charged electrodeThe cathode in the diaphragm cell is steelNa+ and H+ ions (from the water) are attracted to the negative cathodeIt is much easier for a H+ ion to pick up an electron (to form H2) than for a Na+ ion (to form Na metal)At the CathodeMajor reaction at Cathode:Hydrogen gas is produce according to the following equation: 2H+(aq) + 2e H2(g)Hydrogen ions are reducedAs the hydrogen ions are removed and converted into H2 gas, the water equilibrium shifts to the right to replace them (Le Chateliers principle)Net effect:there is a build up of sodium ions and newly-produced hydroxide ions around the cathodeNa+ and OH- ions combine to form dilute solution of sodium hydroxide at the cathode

The Diaphragm Cell (contd)The sodium hydroxide solution leaving the cell:is highly contaminated with unchanged NaCl solutionis concentrated by evaporation during which, NaCl crystallizes out as solid The crystallized salt can be separated, dissolved in water, and passed through the cell againEven after concentration, the sodium hydroxide solution will still contain a small percentage of sodium chlorideFunction of the DiaphragmThe diaphragm: separates the anode and cathode, but allows ions to pass in both directionsis a porous mixture of asbestos and polymersThere is a higher level of liquid on the anode side:to make sure that the flow of liquid is always from left to right (from anode to cathode)to prevent the NaOH solution that is formed from finding its way back to where chlorine is being produced

The need to keep all the products separatedIf chlorine gas comes into contact with hydrogen gas, it will produce a mixture (hydrogen chloride gas) which will explode violently on exposure to sunlight or heat

If chlorine gas comes in contact with sodium hydroxide solution a mixture of sodium chloride and sodium hypochlorite (the main ingredient in bleach) is formedTherefore, if you are trying to manufacture chlorine and sodium hydroxide rather than bleach, you have to keep the chlorine and sodium hydroxide apart as wellThe diaphragm and membrane cells are designed so that all the products are kept separate

The need to keep all the products separatedThe Membrane CellAnode and cathode are separated by a polymer membrane that only allows positively-charged cations to flow through itMembrane can only be permeated by sodium cations (Na+)Membrane restricts the flow of chloride (Cl-) and hydroxide (OH-) anions, as well as water molecules

Electrolysis using membrane cell

Electrolysis using membrane cellAt the Anode:Chlorine anions are reduced at the titanium anode:2Cl-(aq) Cl2(g) + 2e-

Just like in the diaphragm cell, the chlorine gas is contaminated with oxygen gas, so it is liquefied under pressure to separate it from the oxygen (which remains in gaseous state under liquefaction)

Electrolysis using membrane cellAt the cathode:Hydrogen gas is produced just as in the diaphragm cellA sodium hydroxide solution of approximately 33% purity is produced as well:Na+(aq) + e- Na(l)

Na(l) + H2O(l) NaOH(aq)The membrane cell has a major advantage of the polymer membrane not allowing the propagation of OH- ions and Cl- anions from the cathode to the anode thus producing a purer sodium hydroxide solution

The Mercury Cell

Electrolysis using Mercury CellAnode: The anodes may be graphite or titaniumChloride is oxidized as shown:Cl-(aq) Cl2(g) + 2e-

Passing a current through the brine solution causes chloride anions to flow to the anodes

Cl- ions are oxidized into chlorine gas, which escapes the cell and is collected externally

Mercury Cell:Passing a current through the brine solution causes chlorine anions to flow to the anodes. Here, they are oxidized into chlorine gas, which escapes the cell and is collected externally

20Electrolysis using Mercury CellCathode: A thin layer of flowing mercury used as the cathodeSodium cations are reduced to sodium metal:Na+(aq) + e- Na(l) The sodium metal that is formed dissolves in the mercury to form an amalgam:Na(l) + Hg(l) Na-Hg (Na amalgam)

An amalgam is any substance formed by the reaction of mercury with another metal21Electrolysis using Mercury CellSee diagram on slide 19:Brine floats on top of the amalgam, allowing the metals to be tapped off of the bottom of the vesselFrom there, they flow to a second vessel that has been partly filled with waterThe amalgam reacts with the water as shown:2Na(l) (+Hg(l)) + 2H2O 2NaOH(aq) + H2(g) (+Hg(l))H2 gas is releasedNaOH solution is left in a pure, aqueous formAfter Na metal is removed, the remaining mercury is recycled back into the electrolytic cell

Summary of three methodsDiaphragm CellMercury CellMembraneAnode (+ve)TitaniumGraphite or TitaniumTitaniumCathode (-ve)SteelMercuryNickelProduct at AnodeCl2Cl2Cl2Product at CathodeH2, NaOHNa Amalgam, NaOH, H2H2, NaOHComparison of three methodsDiaphragm Cell:cell is relatively cheap to producekeeps the chlorine gas and sodium hydroxide products separated at the end of the electrolysis processdoes not produce a pure NaOH solution, even after concentration it will still be slightly tainted with a small percentage of NaClComparison of three methodsMembrane Cell:more expensive than a diaphragm cell but cheaper than a mercury cellpreferred over the diaphragm cell or mercury cell method because it uses the least amount of electric energy has a major advantage since the cation-exchange membrane does not allow the passage of OH- and Cl- anions from the cathode to the anode (thus producing a purer sodium hydroxide solution)

Comparison of three methodsMercury Cell:are the most expensive cells to buildreaction needs a higher voltage than the diaphragm cell Sodium formed at the cathode forms an amalgam with mercury from the cathodeMercury can be recycled potential damage to the environment due to mercury deposits (toxic effects of Hg)

Industrial importance of chlor-alkali productsChlorine:Used to make solvents for dry cleaning, metal work and the electronics industryUsed in the manufacture of products such as :DyesAntisepticInsecticidesPaints Chlorine (contd):Used as a disinfectant in water treatmentkills bacteria and other microbes in drinking water and swimming poolsIt is used to make household bleachLargest use of chlorine is the production of organic compounds such as poly vinyl chloride (PVC) and chlorofluorocarbons (CFCs) Used along with other compounds to bleach wood pulp in paper production

Industrial importance of chlor-alkali productsSodium Hydroxide:Sodium hydroxide is strongly alkaline and is used in the production of:SoapsCeramicsOven cleanerIndustrial importance of chlor-alkali productsIndustrial importance of the chlorine and its compounds Poly vinyl chloride (PVC)PVC is predominant in the construction industry due to its low production cost, ease to mold and lightweightIt is used as a replacement for metal in many applications where corrosion can compromise functionality and escalate maintenance costsUsed in industrial and municipal applications to make pipe fitting and pipe conduitsUsed in the health care industry it is used to make feeding tubes, blood bags, intravenous (IV) bags, parts of dialysis devices etc.Common consumer products such as raincoats, plastic bags, toys, credit cards, hoses, doors and window frames etc.

Industrial importance of the the chlorine and its compoundsChlorofluorocarbons (CFCs)CFCs are generally unreactive and stable, non-toxic and non-flammableUsed in refrigeration systems and air conditionersUsed as solvents to clean electronic componentsUsed as blowing agents in the production of plastic foamsUsed as propellants in air conditionersImpact of the chlor-alkaline Industry on the environmentMercury The pollutant of most concern from the chlor-alkali industry is mercury, which is specific to the mercury cell. Between 1930 and 1960, several tonnes of mercury waste were dumped in Minamata Bay in Japan. Thousands of people living around the bay developed methyl mercury poisoning through the consumption fish. The victims suffered from severe neurological damage, which later became known as Minamata DiseaseSince then, there was a significant move away from mercury-cell to diaphragm and membrane cellImpact of the chlor-alkaline Industry on the environment (contd)Mercury (contd)Japan was the first country to install the membrane cells on a massive scale in the mid-1980s. In other countries, the replacement of the existing mercury and diaphragm cell with membrane cells is taking place at a slower rate because of the high capital costs of replacement and the fact that most existing chlorine plants were installed with a plant life of 40-60 yearsAsbestosThe main problem with the diaphragm cell is asbestos. Dry asbestos fibres can cause serious health problems if the not properly used and handled in the diaphragm process. Asbestos fibres are toxic, thin , sharp crystals, which are considered to be carcinogenic. They can enter the human body by either inhalation or ingestion causing lung cancer, asbestosis, and or mesothelioma.Impact of the chlor-alkaline Industry on the environmentChlorine and chlorinated productsOther concerns of the chlor-alkali industry are associated with the chemical processes that use chlorineChlorine bleaching of wood pulp releases dioxins in the environment. The production of chlorofluorocarbons (CFCs) contributes to the depletion of the ozone layer.PVC is not biodegradableImpact of the chlor-alkaline Industry on the environment