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Chemistry Pool ERT

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chlorine vs other chemical cleaning methods
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    Figure 1: Various  factor relationships TITLE - Swimming in Chemicals? Engaging opening statement To have the perfect swimming pool, two main factors need to be taken into consideration and kept in balance, sanitation and water balance. Sanitation refers to the decomposition of organisms and oxidation refers to the removal of organic debris such as body waste from swimmers and plant matter from the surroundings. There are a number of ways to sanitise swimming pools, the most common of which is a saltwater system using chlorine to eliminate contaminants in the water. Water balance refers to the level of chemicals in the pool such as pH, alkalinity, water hardness and dissolved solids, which have to be managed carefully to fall within certain limits to optimise conditions, preventing harm to the human body when swimming. Chlorine is a popular option which both sanitises and oxidises, and saltwater systems are frequently chosen as the medium to deliver chlorine into the pool, utilising a chlorine generator, an electrolytic cell which after salt is dissolved into the water, transforms the chloride ions produced hypochloric acid through the process below. NaCl(s) → Na + (aq) + Cl - (aq) The salt is added into the pool, which dissolves and disassociates into its ions. The negative chloride (Cl-) ions and hydroxide (OH-) ions are attracted to the positive anode, where the following two reactions can happen: 2 Cl -   →  Cl 2  + 2 e -   E  o ox   = -1.36 V 2 H 2 O (l) →  O 2  + 4 H +  + 4 e -   E  o ox   = -1.23 V From the standard-state potentials, the water seems preferential to be oxidised at the anode. However, because the cell never reaches standard-state conditions and the close potentials, as well as overvoltage, it can be controlled so that the chloride ion is preferentially oxidised to chlorine gas molecules. The negative cathode attracts the Na+ ions in the water. Na +  + e -  -> Na E  o red   = -2.71 V 2 H 2 O (l) + 2 e -  -> H 2  + 2 OH -  (aq) E  o red   = -0.83 V The water is much more easily reduced at the cathode as it has a more positive standard-state potential value than the half reaction of Na+ ions. The water is reduced by electron gain, forming hydrogen molecules at the negative electrode. Cl 2  (g) + H2O →  HOCl + H (+)  + Cl (-)   Hydrogen and chloride ions, along with Hypochlorous acid, the active sanitising species are produced when chlorine gas is filtered into the pool. Hypochlorous acid in water exists in the following pH dependent equilibrium with OCl-, hypochlorite ions. HOCl H +  + OCl -  The concentration of HOCl and OCl -  ions are known as the free available chlorine, being the sanitising species. The hypochlorite ion is relatively ineffective as a disinfecting component, compared to HOCl. Therefore, it is preferable to have a higher amount of hypochlorous acid to hypochlorite ions. Le Châtelier's principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change to reestablish an equilibrium.   As the pH decreases, becoming more acidic, the concentration of H+ ions increase and according to Le Chatelier’s Principle, causes the equilibrium to shift to the left, producing more HOCl. With a more alkaline solution, with pHs of about 8, the opposite occurs with more OCl- ions produced. This relationship can be seen in fig 1. Therefore at higher pH values the concentration of hypochlorous acid will decrease and thus the sanitation capacity is reduced. To reduce and control the fluctuations of pH, a buffer is needed, among other additives that may be used to help control it. Total alkalinity is a measure of the water’s buffering capacity, the ability to neutralise acid and helps to control the pH level. Alkalinity is usually present in bicarbonates and carbonates, such as calcium or sodium bicarbonate, otherwise known as baking soda, which are added to the water to increase resistance against changes in pH. This also causes the build-up of mineral or calcium hardness . At low pH conditions, no calcium carbonate precipitate is formed, and the water becomes acidic, damaging the pool. In high pH conditions, there is too much carbonate formed, and a) b)   causes cloudiness of water and scale, from too much content of calcium in the pool. In ideal conditions of pH, the carbonate ions are in the bicarbonate form to provide buffering. When there are H+ ions present, indicating low pH, the following reaction happens:            The pH is raised, as with the amount of H+ ions reduced. To reduce the pH, HCl or muriatic acid is a widely used solution to increase acidity in the following reaction:      The H +  ions will combine with ClO -  to form   HClO and decrease pH. To provide maximum comfort to the swimmers, the pH should be maintained in between a range of 7.2 to 7.6. Figure 3 below shows the ideal zone of pH, and the conditions caused by various pHs. Chlorine undergoes photolysis and decomposes quickly in the presence of UV rays into hydrochloric acid, and follows the reactions below. 2 OCl -  + UV → 2 Cl -  + O 2  (g) 2 HOCl -  + UV → 2 HCl + O 2   The prevention of free chlorine decay requires a stabiliser such as cyanuric acid, which has the characteristics and effects of both stabiliser and a buffer. As a stabiliser, it greatly reduces the rate of decomposition of chlorine by forming a weak bond with available HOCl and OCl-, forming compounds which are not photosensitive, stabilising it from the effect of UV light in a manner which does not use up the chlorine. According to Le Chatelier’s Principle, as  the concentration of OCl- decreases via photolysis, the equilibrium balances by shifting from right to, resulting in the decrease in the amount of dichlorocyanuric acid and OH- and the production of cyanuric acid and OCl-. Thus the net depletion rate of OCl- is also slowed because as OCl- and HOCl decomposes, it is replaced equally by a shift in the equilibrium above. The free chlorine then is maintained at a constant level, as long as there is sufficient cyanuric acid and constant pH. Testing has shown that only approximately 15% of free chlorine residual is lost over the day with the use of cyanuric acid, whereas 90% is lost without. However, the addition of cyanuric acid also reduces the disinfectant power of hypochlorous acid slightly because of the bonds formed between them. This can be compensated for by increasing the free chlorine concentration from 1mg/L to 3mg/L. The stabilising effect of cyanuric acid becomes “locked in” at levels over 100 ppm and may not be as effective in killing bacteria and algae. During the oxidisation of organic compound impurities containing nitrogen such as perspiration, saliva, body oils and urine by hypochlorous acid, if there is an insufficient supply of HOCl to completely oxidise the impurities, chloramines are formed, as shown in the equations below.                  Ammonia, as waste from swimmers react with HOCl to form monochloramine, which then reacts with more HOCl to form dichloramine, and repeats the process to form trichloramine, which happens when there is an abundance of hypochlorous acid. Chloramines are still sanitisers; however they are 40 to sixty times less effective than the hypochlorous acid. As well as the reduction in effectiveness, chloramines also cause eye irritation and a distinctive odour, whereas free chlorine has no apparent smell.         A low level of chloramines is tolerable as they react with each other in the following reaction:         The nitrogen gas is then lost and the HCl can disassociate into H+ and Cl- ions. However, as it is in equilibrium and a natural byproduct of the disinfectant process, the overall content of chloramines still accumulate. The amount of chloramines can be reduced with superchlorination, which involves an extra-large dose of chlorine past the breakpoint to completely oxidise contaminants. The process of superchlorination and its effect on chloramine concentration can be seen in fig 1b. If there is an overabundance of HOCl in the pool, nitrogen trichloride, which especially irritates the eyes and Figure 3   mucous membranes of swimmers can be produced. Fig 1b shows the positive slope shows the increasing amount of chloramines, as the amount of chlorine residual increases. The continual addition of chlorine into the system causes a corresponding rise in chlorine residual, and at a certain point, named the ‘breakpoint’, the content of residual chlorine has a sudden drop. Chlorine added after the breakpoint remains as residual. The periodic use of superchlorination maintains a lower level of accumulated chloramines and harmful compounds in the pool. Part B  –  Comparison of Sanitizers   ‘ Chlorine ’ is the basis of the most common form of steriliser or sanitiser used in swimming pools. Some companies have developed alternatives to ‘chlorine’ as the sanitiser in swimming pools. Identify and describe   two alternative ‘non - chlorine’  methods of sanitising pool water? Explain  simply the chemistry involved in these alternative methods and evaluate  their effectiveness as pool water sanitisers. Present a recommendation  as to which of the three methods might be the best to use in a backyard pool and  justify   your decision. PART B. Apart from chlorine, which contraversially has many negative side effects, such as _____, there are many other ‘non - chlorine’  alternate sanitation methods to be considered, two of them ozone and UV pools. Ozone has long been recognised as an effective ways to purify and sanitise water. It is a form of oxygen, O3, which naturally occurs in the atmosphere of the earth, created by the ultraviolet rays or corona discharge with   the combination of oxygen present. Oxygen molecules are split by the addition of energy, resulting in two oxygen atoms, which bonds with O2 molecules to produce O3, with the oxygen atom held by a weak single bond. Thus ozone is a very powerful oxidiser. When the ozone molecule collides with an oxidisable substance, the weak bond splits and an oxidation reaction occurs between the oxygen atom and organic matter, breaking down chloramines and other chemical by-product efficiently. Ozone is produced for swimming pools using s   There are several methods of ozone generation that include UV light, radiation, corona discharge, and electrolysis. All methods involve applying energy to break the bonds of oxygen molecules allowing them to dissociate and reform as ozone. Since this process is random, ozone generation is very inefficient converting only a fraction of the source oxygen to ozone. Furthermore, due to the instability of the molecule, Ozone is not stored but generated on-site at the point of application. At present, only two generation methods are commercially viable, corona discharge and UV light generation. When exposed to UV light, an oxygen molecule in the ground state will absorb the energy and dissociate to a certain degree depending on the intensity and wavelength of the absorbed UV light. The released oxygen atoms then react with other unreacted oxygen molecules to form ozone. The breakdown of oxygen molecules is most favorable at wavelengths below 200nm. The typical UV lamp for water treatment produces monochromatic light at 254nm. As referenced earlier, ozone also absorbs UV light causing its destruction that also happens to be in the peak range near 254 nm. For this reason, UV lamps that produce a wavelength near 185nm are used instead. The best results for ozone generation with UV light occur with a 185nm lamp and pure oxygen source. Even at these conditions, UV light produces only minimal amounts of ozone (~0.100%). Due to the insoluble nature of ozone, this situation is worsened and makes ozone generation less effective. Most disinfection by UV light generated ozone occurs indirectly by the chemical disinfectant, chlorine, and free radicals. http://www.aquaticdesigngroup.com/images/press/PRG1009_Mendioroz_Pool.pdf  What about ozone and/or UV? Can't that reduce or eliminate the need for chlorine? In a word, No, and for a very simple reason. Ozone and UV have NO residual effect so again, a residual sanitizer is still needed. The only place these will kill pathogens is in the contact chamber (where the water is exposed to the ozone or UV), not in the pool. Ozone will destroy chlorine but will oxidize organics so it's a two edge sword. You will generally have higher chlorine consumption with ozone than without Recent research has also discovered several microbes that are particularly resistant to chlorine disinfection. Two of these protozoan pathogens are Cryptosporidium and Giardia. Even with good maintenance of chlorine residual, cryptosporidium may remain active in pool water for more than a week. If an outbreak is expected, pools need to be closed for a day or two with excess levels of chlorine to kill off the germs. Prevention is often the best cure by scheduling periodic superchlorination of the pool. Obviously, this adds considerable cost for chemicals, maintenance, and pool down-time. http://www.flasolar.com/pdf/water_chemistry_for_swimming_pools.htm  Ozone vs UV The two key technologies that form the mainstay of the discussion around secondary disinfection are UV and ozone. While UV has been relatively well understood by aquatics designers and operators, ozone, a rapidly-advancing technology, has been given less attention despite the many compelling reasons to choose ozone over UV. What is Ozone? Ozone is a gas that is dissolved in water to kill microorganisms, destroy organics, and break down chloramines by oxidation. This occurs immediately at the ozone gas injection point, and continues as the side-stream remixes with the main return. A small residual (~0.1 PPM) of dissolved ozone will enter the pool, providing further oxidation of contaminants.
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