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Environmental Chemistry- Water

Environmental Chemistry- Water. Importance of Water. Water is the most important and abundant chemical on Earth It makes up 80% of the Earth’s surface 2/3 of our body weight. Uses of Water. Drinking, Washing, flushing toilet Watering gardens etc.

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Environmental Chemistry- Water

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  1. Environmental Chemistry- Water

  2. Importance of Water • Water is the most important and abundant chemical on Earth • It makes up 80% of the Earth’s surface • 2/3 of our body weight

  3. Uses of Water • Drinking, Washing, flushing toilet Watering gardens etc. • Industry uses huge amounts of water for cooling , manufacturing, generating electricity etc.

  4. Hardness of Water • Water circulates around earth in the water cycle

  5. Hardness of water • As water is such a good solvent it is very rare to get pure water, even rainwater contains dissolved gases • In some parts of Ireland water is hard (ie. It will not easily form a lather with soap but will form a grey precipitate referred to as scum) • Hard water contains definite quantities of Ca2+ or Mg2+ ions

  6. Hard water is water that will not easily form a lather with soap. Hardness in water is caused by the presence of Ca2+ or Mg2+ ions

  7. Reactions to learn! • Soap contains sodium stearate (C17H35COONa) • Calcium or magnesium ions react with stearate ions to form a grey insoluble compound called calcium stearate (scum) which floats on water Ca2+ + C17H35COO- (C17H35COO)2Ca

  8. Temporary Hardness • Temporary hardness can be removed by boiling the water • Temporary hardness arises when slightly acidic rainwater (carbonic acid) falls on limestone and reacts with it to form calcium hydrogen carbonate Limestone + Carbonic acid Calcium Hydrogen carbonate CaCO3 + H2CO3 Ca(HCO3)2 Insoluble Soluble

  9. The Ca2+ ions cause the hardness the HCO3- ions have no effect on hardness • If water containing these two ions is heated a chemical reaction occurs which removes the Ca2+ ions from the water thus softening the water

  10. Calcium hydrogencarbonate Calcium Carbonate + Carbon Dioxide + Water Ca(HCO3)2 CaCO3 + CO2 + H2O soluble insoluble • The calcium ions that were dissolved are now precipitated as insoluble calcium carbonate (limestone), this is often referred to as fur or scale and builds up on kettles and water pipes

  11. If temporary hardness is caused by Mg2+ ions • Magnesium ions tend to form dolomite rock which contains a mixture of MgCO3 and CaCO3

  12. Permanent water Hardness • Permanent Hardness is usually caused by the presence of Calcium Sulphate CaSO4 or Magnesium Sulphate MgSO4 • This hardness cannot be removed by boiling

  13. Methods of Removing Temporary and Permanent Hardness • Distillation • Using Washing Soda • Ion Exchange Resin

  14. Distillation • This involves boiling the water and cooling the vapour • All dissolved solids + liquids are removed this way • However it is not practical to use this method on a large scale

  15. Using Washing Soda • Washing Soda softens water as the carbonate ions in the washing soda react with the calcium ions in the water and removes them as insoluble calcium carbonate Ca2+ + CO32- CaCO3

  16. Ion Exchange Resin • These are man made materials which “Swap” ions that cause hardness in water with ions that don’t cause hardness in water • As the ions involved are positive ions they may often be called “Cation exchange resins”

  17. Frequently the Ca2+ ions in the water are replaced with Na+ ions • The calcium ions remain behind in the resin • Eventually the resin loses all of its Na+ ions and it needs to be regenerated by passing a concentrated solution of sodium chloride through it

  18. Deionised Water • For laboratory and medical purposes it may be necessary to remove all ions from water • This is achieved by passing water through a deioniser which contains a mixture of cation and anion exchange resins

  19. The cation exchange removes all the positive ions and the anion exchange resin removes all the negative ions • Positive ions are replaced with H+ ions and negative ions with OH- ions which recombine to form water

  20. Advantages and Disadvantages of Hard Water Advantages Disadvantages Blocks pipes leaves scale on kettles and boilers Wastes soap Produces scum • Provides calcium for bones and teeth • Nicer Taste • Good for brewing and tanning

  21. Determining the Hardness of a Water Sample Total hardness = Magnesium + Calcium ions present • Total water hardness is expressed in terms of mg per litre of CaCO3 (p.p.m. CaCO3) • The concentration of Calcium and Magnesium ions present in water is determined using a chemical called ethylenediaminetetraacetic acid or EDTA for short! • The EDTA forms a complex withCa2+ and Mg2+ ions and reacts in the ration 1:1 with these ions

  22. In the presence of Eriochrome Black T indicator calcium and magnesium ions have a wine red colour • If all the ions have reacted with EDTA the colour changes to blue • By measuring how much EDTA is needed to react with all the ions in a sample of water the hardness of the water may be determined

  23. Hardness in Water

  24. Water Treatment A Water Treatment Plant Criteria for acceptable domestic water Colourless Odourless No active bacteria present Fluoridated to help teeth

  25. Stages in Water Treatment • Screening • Flocculation • Settlement • Filtration • Chlorination • Fluoridation • pH adjustment

  26. Screening Involves passing the water through a wire mesh to remove nay large floating debris eg twigs, plastic bags etc.

  27. Flocculation Causing smaller particles in water to come together to form larger particles or “Flocs” This is achieved using flocculating agents such as Aluminium sulphate The larger particles settle out in the next stage

  28. Settelement • Water is passed into settlement tanks • It is allowed to sink then rise very slowly leaving particles at the bottom • 90% of particles are removed at this stage

  29. Filtration • Water from the top of settlement tanks passes through large beds of sand • Sand acts like a sieve or filter paper and removes suspended solids • The sand is cleaned regularly

  30. Chlorination • The water that comes out after filtration is not fit to drink as harmful micro-organisms may still be present • Chlorine is added to kill micro-organisms • Only small amounts of chlorine are added as it gives the water an unpleasant taste and smell if too much is present • Water needs to be monitored for bacteria control

  31. Fuoridation • Fluorine compounds such as Sodium flouridenadhexafluorosilicic acid are added to water to help reduce dental decay • Estimated for every 1Euro spent on fluoridation 30-50 Euro is saved in treating tooth decay!

  32. pH Adjustment • If water is slightly acidic it may damage pipes so Calcium hydroxide may be added to raise the pH to 7 • If the water is very hard it may be softened by adding sodium carbonate • Optimum pH of water for distribution to homes is 7-9

  33. Experiments • Determine the total suspended solids (in p.p.m.) in a sample of water by filtration • Determine the total dissolved solids (in p.p.m.) in a sample of water by evaporation • Find the pH of a sample of water

  34. Water Pollution

  35. Dissolved Oxygen • Dissolved oxygen in rivers, lakes and the sea is vital for fish and other forms of life • O2 is non polar and not very soluble in water • Its solubility depends on the temperature of the water

  36. Organic wastes such as sewage, slurry, effluent from farms, effluents from food processing factories, milk industrial waste etc. Can leak into rivers, lakes etc. • These provide nutrients to bacteria and other organisms naturally present in water and allows them to multiply to large numbers

  37. These large numbers produce more carbon dioxide and use up available oxygen meaning fish life in the water may be reduced or killed off completely • If it gets very bad the dissolved oxygen level may reach 0 and only anaerobic bacteria will survive meaning the water can become foul smelling due to the production of Hydrogen Sulphide gas

  38. Methods used by Chemists to indicate levels of pollution in water • A test to measure the demand water has for dissolved oxygen was introduced in the 1900’s and is called the Biological Oxygen Demand or B.O.D. Of the water

  39. The Biological Oxygen Demand is defined as • The amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20⁰C in the dark for 5 days

  40. Measuring the B.O.D. Of Water • Completely fill two bottles to be tested with water • Measure the dissolved oxygen of one of the samples with a dissolved oxygen meter or by a titration called the Winkler Method • Incubate the second bottle at 20⁰C in darkness for 5 days

  41. The water is kept in darkness to prevent photosynthesis from taking place as this would increase the amount of dissolved oxygen • The temperature must be kept fixed so that a fair comparison is made also the amount of dissolved oxygen is dependent on temperature

  42. After 5 days measure the dissolved oxygen in the second bottle • The B.O.D. Is the difference in the two dissolved oxygen levels as this is the amount of oxygen that has been used up in the test • B.O.D. Is measured in mg/L of oxygen • The higher the B.O.D level the more polluted the water

  43. Sample Results

  44. The more organic waste in water the more bacteria present to feed on it this decreases the available oxygen

  45. Effluents • Effluent means waste water or sewage water • It is possible to measure the B.O.D. of effluents • Oxygen has low solubility in water (9.2mg/L at 20⁰C) so effluents with B.O.D. Higher than 9mg/L must be diluted with Fresh distilled water and well shaken to ensure there will be a measurable amount of oxygen after 5 days • NB Example in book p 275

  46. Precautions for Collecting Water samples for B.O.D. analysis • Fill the bottle under the surface of the water to prevent air getting in and adding extra oxygen • Fill the bottle completely to ensure no air is trapped between the top of the water and the stopper of the bottle • Place the second bottle in the dark immediately to prevent photosynthesis

  47. Eutrophication • Overloading water with plant nutrients ( Nitrate and Phosphate ions) will also reduce the amount of dissolved oxygen • These nutrients cause plants and floating algae to undergo population explosions • When these die and decay they put a high oxygen demand on the water and algal bloom covers much of the water with a green scum

  48. Eutrophication is the enrichment of water with nutrients which leads to the excessive growth of alagae • Natural Eutrophication = gradual increase in nitrogen and phosporous levels in sediments in lakes • Artificial Eutrophication = sudden increase in nutrients caused by artificial fertilisers or domestic sewage and waste getting into water • Concern that nitrate ions in water may cause stomach cancer and death in babies

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