Water Chemistry.pdf

Water Chemistry Dr. Bhaskar Devu Mukri Assistant Professor Department of Chemistry Motilal Nehru National Institute of Technology Allahabad, Prayagraj, Uttar Pradesh – 211004, India ...

Water Chemistry Dr. Bhaskar Devu Mukri Assistant Professor Department of Chemistry Motilal Nehru National Institute of Technology Allahabad, Prayagraj, Uttar Pradesh – 211004, India  Water (H2O)  Water covers about 71% of the Earth's surface.  Water resources are natural resources of water that are potentially useful for humans as a source of drinking water supply or irrigation water.  These can be either freshwater from natural sources, or water produced artificially from other sources, such as from reclaimed water (wastewater) or desalinated water (seawater).  97% of the water on Earth is salt water and only three percent is fresh water; two-thirds of this is frozen in glaciers and polar ice caps. Distribution of freshwater resources Glaciers (69%) Groundwater (30%) Other Freshwater (e.g.: Soil Moisture) (0.7%) Directly Accessible Water (0.3%)  States of Water  Water differs from other liquids that it becomes less dense as it freezes. States of water Density Solid Ice 0.9167 g/mL at 0 °C Liquid Water 0.9998 g/mL at 0 °C 0.9999 g/mL at 3.983 °C Gas Water vapor or steam 0.0008 g/mL  The ice is approximately 8.3% less dense than the liquid water.  Greenhouse gases  Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of the Earth.  Without greenhouse gases in the atmosphere, the average temperature of Earth's surface would be about −18 °C, rather than the present average of 15 °C.  The five most abundant greenhouse gases in Earth's atmosphere, which are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3).  Other greenhouse gases are chlorofluorocarbons (CFCs and HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons, SF6, and NF3.  Overall, water vapor is the most important greenhouse gas, which is being responsible for 41– 67% of the greenhouse effect.  Approximately 51% of sunlight (visible wavelengths of radiation) absorbs by the earth’s surface and some of this energy emits back from the earth’s surface in the form of infrared radiation. Since greenhouse gases are infrared active molecules, they absorb and emit back this infrared radiation in all directions, which warms the earth’s surface and lower atmosphere.  Types of Impurities present in water: The natural water is usually contaminated by different types of impurities. They are mainly three types. 1. Physical impurities: Physical impurities in water are visible particles or materials that can affect the appearance, texture, and color of water. Ex: Mud, soil, sewage, rust, organic matters, etc. 2. Chemical impurities:  Inorganic chemicals: Cations (Al3+, Ca2+, Mg2+, Fe2+/3+, Mn2+/4+, Zn2+, Cu2+, Na+, K+) Anions (Cl−, SO42−, NO3−, HCO3−, F−, NO2−)  Organic chemicals: Dyes, paints, petroleum products, pesticides, detergents, drugs textile materials, other organic related materials. 3. Biological Impurities:  Biological impurities are Algae, pathogenic bacteria, fungi, viruses, pathogens, parasite-worms.  Sources of impurities 1. Rain water It is the purest form of natural water. But unfortunately it dissolves the automobiles and industry emission gases like CO2, SO2, NO, NO2, etc and turns into the acid rain. SO2 + ½ O2 + H2O → H2SO4 NO + ½ O2 → NO2 2NO2 + ½ O2 + H2O → 2HNO3 2NO2 + H2O → HNO3 + HNO2 NO2 + ∙OH → HNO3 2. Sea water It is the most impure water containing about 3.5% dissolved salts of which about 2.6 % is sodium chloride. Sulfates, bicarbonates, bromide, chloride of sodium , potassium, magnesium , etc are other salts present in sea water. 3. River water The springs are the origins of the rivers and rain water helps to flow these rivers through the land. While flowing through the land, river water collects the a lot of organic matters (leaf, cow dung, etc) and soluble matters include the salts like Sulfates, bicarbonates, bromide, chloride of sodium , potassium, magnesium , etc 4. Lake water It is purer than river water, dissolved impurities are less but it contains a lot of organic matters and some of microorganisms. 5. Ground water The rain water and other surface water percolate down through the soil and rocks, which get filtered and finally collected on the rock’s surface or again come out as springs. It contains less suspended matters, but high dissolved minerals. Effect of water on rocks while percolation: Dissolution: NaCl, gypsum (CaSO4∙2H2O), Mg(HCO3)2, etc. Hydration: CaSO4 + 2H2O CaSO4∙2H2O Mg2SiO4 + xH2O Mg2SiO4∙xH2O Oxidation Hydration Oxidation: 4/3 Fe3O4 2Fe2O3 2Fe2O3∙2H2O Reaction of carbon dioxide: CaCO3 + H2O + CO2 Ca(HCO3)2 Insoluble in water Soluble in water  Hardness of water Hardness of water is the characteristic of preventing the lather formation of water with soap, which is due the presence of bicarbonates, chlorides, and sulfates of Ca2+ and Mg2+ salts. 2 C17H35COONa + CaCl2 (C17H35COO)2Ca + 2 NaCl Soap or MgCl2 (C17H35COO)2Mg + 2 NaCl Hard water Insoluble (soluble salts) Hardness of water is mainly two types: 1. Temporary hardness: It is due the presence of dissolved bicarbonates of Ca2+ and Mg2+ salts. Since carbonates readily get precipitated on boiling the hard water, temporary hardness of water can be easily removed. Heat Ca(HCO3)2 CaCO3  + H2O + CO2 Insoluble Heat Mg(HCO3)2 Mg(OH)2  + 2CO2 Insoluble 2. Permanent hardness: It is due the presence of dissolved chlorides and sulfates of Ca2+ and Mg2+ salts. These salts cannot be easily removed by boiling the hard water.  Units of hardness Hardness produced salts usually expressed in terms of an equivalent amount of CaCO3. The choice of CaCO3 is due to the facts that (i) its molecular weight (100) and equivalent weight (50) are whole numbers, so that calculations will be simplified. (ii) It is the most insoluble salt in water. Mass of the hardness produced salt Degree of hardness = × Molecular weight of CaCO3 Molecular weight of hardness produced salt or Mass of the hardness produced salt Degree of hardness = × Equivalent weight of CaCO3 Equivalent weight of hardness produced salt Mainly, unit of hardness is expressed in terms of mg/L or ppm. 1 part of hardness 1 ppm = 106 parts of water In French degree of hardness (°Fr ), 1 part of hardness 1 °Fr = 106 parts of water In English degree of hardness or Clark (°Cl), 1 part of hardness 1 °Cl = 7 × 104 parts of water Relationship between various units of hardness: 1 ppm = 1 mg/L = 0.1 °Fr = 0.07 °Cl Disadvantages of hard water: (i) In domestic uses, Washing and bathing: Hard water prevents the formation of lather with soap and it also creates sticky precipitates that deposit on the bath tub, body, clothes, etc. Cooking and drinking: Hard water produces scum on the bottom of the vessel that affects to consume more fuel for cooking. Drinking of hard water affects the digestive system and urinary tract. (ii) In industrial uses, Dyeing in textile industry: Hard water causes the deposition of insoluble salts that interfere with the proper dyeing of the fabrics. In Sugar industry, hard water creates the problems in refining and crystallization of sugar. In paper industry, hard water spoils the smooth and glossy finishing of the papers. In boilers, hard water creates many problems like scale formation, corrosion, etc.  Determination of hardness in water  Complexometric titration with EDTA (Ethylenediaminetetraacetic acid) is one of the methods to determine the hardness of water.  Since [M(EDTA)]2− complex is a colorless complex, EBT (Eriochrome Black T) or Murexide indicator is used to determine the endpoint of the titration. Eriochrome Black T Murexide Determination of Total hardness, Ca2+ hardness, and Mg2+ hardness: Buffer solution EDTA titration with EBT indicator Total hardness of water sample Water sample pH 10 NaOH Mg(OH)2  + Ca2+ ion EDTA titration with Murexide indicator Ca2+hardness of water sample pH >11 Mg2+hardness of water sample = Total hardness − Ca2+hardness Determination of Total hardness, Permanent hardness, and Temporary hardness: Buffer solution EDTA titration with EBT indicator Total hardness of water sample Water sample pH 10 Boil Filter Buffer solution EDTA titration with EBT indicator Mg(OH)2  + CaCO3  Filtrate pH 10 Permanent hardness of water sample Temporary hardness of water sample = Total hardness − Permanent hardness  Alkalinity  It is a measure of water’s ability to neutralize the acid.  The alkalinity of surface water is primarily due to the carbonate (CO32−), bicarbonate (HCO3−), and hydroxide (OH−) content.  It is measured by titrating the solution with an acid such as HCl until its pH reaches to less than 4.5.  It is expressed in terms of an equivalent amount of CaCO3 and its unit is mg/L or ppm.  Phenolphthalein and methyl orange are commonly using indicators to determine the alkalinity of water. Phenolphthalein: pH 8.2 − 10 Colorless (Acidic) Pink (Basic) Methyl orange: pH 3.1 − 4.4 Red (Acidic) Yellow (Basic)