Water Science PDF

UNIT I WATER SCIENCE Water, the unique component of nature plays a crucial role in the evolution of life. All the creations of the world emerged on the bands of rivers and have been central to the growth of human society. It is believed that l...

UNIT I WATER SCIENCE Water, the unique component of nature plays a crucial role in the evolution of life. All the creations of the world emerged on the bands of rivers and have been central to the growth of human society. It is believed that life oriented in water and is sustained by it. Now a days, the standard and maturity of the society is indicated by the purity of its rivers. Water is the most essential compound for all living matter on the earth. Water is not only essential for the lives of animals and plants, but also occupies a unique position in industries. It is widely used in drinking, bathing, sanitary, washing, irrigation, fire-fights, air-conditioning and also production of industrial materials. Adequate supply of safe potable water is quite essential and is the basic need of all life on the earth. Because of unplanned industrializations, surface and ground water sources have been contaminated; as a result, preservation of water and managing water quality has become complicated. Conservation of water has become the need of the hour as it has a great impact on the health of human beings. Rivers are known as the cradlers of civilization. But regrettably this has resulted in water pollution because for a long time, streams and rivers have been used for disposal of wastes. In 2002, UN estimates that nearby $100 billion people struggle to access clean drinking water and 2.5 billion people lack of adequate sanitation. It is estimated that by 2024, 2/4 of the population of the world would suffer from extreme water shortage. SOURCES OF WATER The two important sources of water are (1) Surface water and (2) Underground water. 1. Surface water The water available on the earth’s surface is called as surface water. Surface water includes rain water, river water, lake water and sea water. 4  The surface water from various resources evaporates, condense and precipitates as rain, snow or both moves through various organisms and finally enters to sea, surprisingly 90% of water which evaporates returns to the ocean after passing through the hydrological processes. 2. Underground water Underground water is divided into well water and spring water. The ground absorbs rain water and dissolves many soluble minerals present in the soil region. Hence underground water is rich in minerals but contains minimum amount of organic impurities. IMPURITIES IN WATER Water is the most important element through which the life sustains on the earth. But man has exploited the water resources than any other natural resources for his modern livelihood. Pollution is the alteration of physical, chemical and biological characteristics of natural resources mainly water, air and land caused by anthropogenic activities that is harmful to existing or proposed use of the resources. Generally, the pollutants of water are classified as follows 1. Suspended impurities: These impurities impart turbidity, colour and odour to water. It may be organic or inorganic in nature. 2. Colloidal impurities: Products from organic waste, finely divided silica, clay, etc 3. Dissolved impurities: Presence of dissolved salts like carbonates, bicarbonates, chlorides and sulphates of calcium, magnesium, potassium and sodium and dissolved gases like O2, CO2, etc 4. Microorganisms: They include bacteria, fungi and algae. Due to the above mentioned activities water becomes unsuitable for various uses like domestic, industrial and agricultural purposes, mainly the anthropogenic activities such as population explosion, rapid industrialization and over exploitation of natural resources, developmental activity, modern consumerism and various other activities. SOURCES OF IMPURITIES IN WATER Surface water bodies are commonly polluted by soil erosion caused during rainy season that carries both organic and inorganic impurities along with suspended particles. They are also polluted by industrial effluents and chemicals /discharge of sewage, oil and 5 other wastes. 1. Gases are picked up from the atmosphere by rain. 2. Decomposition of plant and animal matter are responsible for organic impurities in water. 3. Agricultural runoff is a major cause by the addition of fertilizers, pesticides and herbicides. 4. Harmful microorganisms such as microbes, pathogens, viruses present in animal and human fecal matters. TYPES OF WATER Water is classified into two types based on its lather forming property with soap solution. (a) Soft Water Soft water is one of that gives good lather readily with soap solution. 2 RCOONa + H2O RCOOH + NaOH Soft water b) Hard Water Water that does not produce lather with soap readily but forms an insoluble precipitate-like white scum is known as hard water. 2RCOONa + H2O (RCOO)2Ca + 2NaCl Hardwater Insoluble soap HARDNESS OF WATER Hardness is the characteristic of water which prevents lathering of soap. This property of water is due to the presence of carbonates, bicarbonates, sulphates and chlorides of calcium, magnesium and heavy metals. A sample of hard water when treated with soap does not produce lather with soap, but it forms white sticky precipitates. This is due to the formation of insoluble soap of calcium and magnesium stearates. 2C17H35COONa + CaCl2/MgCl2 (C17H35COO)2 Ca/Mg + 2NaCl Soap (soluble) salts (soluble) insoluble precipitates Based on hardness, water can be classified into two types 6  1) Hard water: Water which does not produce lather with soap solution readily, but produces white precipitate (scum) is called hard water. This is due to the presence of dissolved calcium and magnesium salts. 2) Soft water: Water, which produces lather, readily with soap solution is called soft water. This is due to the absence of dissolved calcium and magnesium salts. Disadvantages of hardness 1. In Domestic use: a) Washing: When used for washing purposes, hard water does not producing lather freely with soap. This decreases cleaning quality of soap and portion of it is wasted. b) Bathing: Hard water does not lather freely with soap solution, but produces sticky scum on the bath-tub and body. Thus, the cleaning quality of soap is depressed and a lot of it is wasted. c) Cooking: The boiling point of water is increased because of presence of salts. Hence more fuel and time are required for cooking. d) Drinking: Hard water causes bad effects on our digestive system. Moreover, the possibility of forming calcium oxalate crystals in urinary tracks is increased. 2. Industrial Use: a) Textile Industry: Hard water forms insoluble precipitates of calcium and magnesium soaps which adhere to the fabrics and limits its usage. b) Sugar Industry: The water which containing sulphates, nitrates, alkali carbonates are used in sugar refining creates difficulty in the crystallization of sugar. c) Dyeing Industry: The dissolved salts in hard water may reacts with costly dyes forming precipitates. d) Paper Industry: The dissolved calcium, magnesium and ferrous salts in water may affect the quality of paper. e) Pharmaceutical Industry: Hard water may cause formation of some undesirable products. 7 TYPES OF HARDNESS Hardness is classified into two types based on the nature of dissolved salts present in water. 1. Temporary Hardness or Carbonate hardness 2. Permanent Hardness or Non carbonate hardness 1. Temporary Hardness or Carbonate hardness. Temporary Hardness is mainly due to the presence of dissolved bicarbonates of calcium and magnesium (Ca(HCO3)2, Mg(HCO3)2). Temporary Hardness can be removed by two processes: a) By boiling Ca(HCO3)2 Δ CaCO3 + H2O + CO2 Mg(HCO3)2 Δ Mg(OH)2 + 2 CO2 b) Adding lime to water Mg(HCO3)2 + 2 Ca(OH)2 Mg (OH)2↓+ 2CaCO3↓ +2H2O In the above two processes, the bicarbonates are converted into insoluble carbonates and hydroxides and they can be removed by filtration 2. Permanent Hardness or Non carbonate hardness: It is due to the presence of dissolved Chlorides and sulphates of calcium, magnesium, iron and other metals. The salts responsible for permanent hardness are CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3. Permanent Hardness cannot be removed by boiling but it can be removed by following two processes: a) Lime soda process CaCl2 + Na2 CO3 CaCO3 +2Nacl b) Zeolite process CaSO4 + Na2Ze CaZe + Na2SO4 Total hardness Total hardness = Temporary Hardness + Permanent Hardness Expression for hardness – Calcium carbonate hardness The concentration of hardness as well as non hardness producing salts is usually expressed in terms of an equivalent amount of CaCO3. Calcium carbonate is chosen as a standard because: i. Its molecular weight (100) and equivalent weight (50) is a whole number, so the calculations in water analysis can be simplified. ii. It is the most insoluble salt that can be precipitated in water treatment. 8 The conversion of the hardness causing salts into CaCO3 equivalents can be achieved by using the following formula: Calcium Carbonate =Mass of hardness producing salt x Molecular mass of CaCO3 Equivalents Molecular mass of hardness producing salt Calcium Carbonate =Mass of hardness producing salt x Equivalent mass of CaCO3 Equivalents Equivalent mass of hardness producing salt Units of hardness 1. Parts per million (ppm) It is defined as the number of parts of CaCO3 equivalent hardness per 106 parts of water. 2. Milligrams per litre (mg/lit) It is defined as the number of milligrams of CaCO3 equivalent hardness per 1 litre of water. 3. Clarke’s degree (oCl) It is defined as the number of parts of CaCO3 equivalent hardness per 70,000 parts of water. 4. French degree (oFr) It is defined as the number of parts of CaCO3 equivalent hardness per 105 parts of water. ESTIMATION OF HARDNESS – EDTA METHOD This is a complexometric titration method. Ethylene diamine tetraacetic acid (EDTA) is the complexing agent which forms stable complexes with the calcium and magnesium ions present in the hard water. Eriochrome Black T which forms weak complex with calcium and magnesium ions in hard water is used as indicator. During the titration, a pH 8-10 should be maintained by adding NH4Cl – NH4OH buffer. 9  Disodium salt of EDTA The principle of this method is based on the stability of the complexes formed by the EDTA and EBT indicator. The indicator added to the hard water forms a weak complex with Ca2+ and Mg2+ ions in hard water. On titration with EDTA, Ca2+ and Mg2+ ions form stable complex with EDTA. When all the Ca and Mg ions are exhausted in the titrating solution, excess EDTA will pull the Ca2+ and Mg2+ ions from the weak complex. Consequently, EBT is set free and hence the color changes from wine red to blue, the original color of EBT. Ca2+ + EBT [Ca – EBT]2+ Unstable (wine red color) Mg2+ + EBT [Mg – EBT]2+ Unstable (wine red color) [Ca – EBT]2+ + EDTA [Ca – EDTA]2+ + EBT Stable (Colorless) (Steel blue color) [Mg– EBT]2+ + EDTA [Mg – EDTA]2+ + EBT Stable (Colorless) (Steel blue color) Procedure 1) Standardisation of EDTA The EDTA solution is standardized by titrating against standard hard water. 20 ml of the standard hard water is pipette out into a conical flask, 5 ml of buffer solution and 2- 3 drops of EBT indicator and the solution turns wine red color. It is titrated against the EDTA solution taken in the burette. The end point is the colour change from wine red to steel blue colour. The volume consumed is V1 ml. 2) Estimation of total hardness of the water sample. 20 ml of the water sample is pipetted out into a clean conical flask, add 2ml of the 10  buffer solution and 2-3 drops of EBT indicator and the solution turns wine red color. It is titrated against the EDTA solution taken in the burette. The end point is the colour change from wine red to steel blue colour. The volume consumed is V 2 ml. 3) Estimation of permanent hardness of the water sample. Take 100 ml of water sample into a beaker and boil the water till the volume reduces to 50 ml. Cool the solution and filter the water into a beaker. Pipette out 20ml of boiled water sample into a clean conical flask, add 2ml of the buffer solution and 3 drops of indicator. The wine red coloured solution is titrated against EDTA taken in the burette, till a steel blue colour end point is obtained. The volume consumed is V 3 ml. Short Procedure: Titration-III Titration-I Titration-II S.No Content Estimation of Standardization of Estimation of Total Permanent EDTA Hardness Hardness 1 Burette EDTA Standard EDTA Standard EDTA 2 Pipette (20 ml) Standard hard water Sample water Boiled water 3 Additional solution 10ml of NH3 buffer 10ml of NH3 buffer 10ml of NH3 buffer 4 Indicator EBT EBT EBT 5 Endpoint Wine red to steel blue Wine red to steel blue Wine red to steel blue 6 Volume V1 V2 V3 Calculation 1) Standardisation of EDTA 1 ml of standard hard water = 1 mg of CaCO3 equivalent hardness Therefore, 20 ml of standard hard water = 20 mg of CaCO3 equivalent hardness 20 ml of standard hard water consumes V1 ml of EDTA. hence, V1 ml of EDTA solution = 20 mg of CaCO3 equivalent hardness 1 ml of EDTA solution = 20/ V1 mg of CaCO3 equivalent hardness 2) Estimation of total hardness of the water sample. 20 ml of given hard water sample consumes V2 ml of EDTA = V2 X 20/ V1 mg of CaCO3 equivalent hardness Therefore, 1000 ml of hard water sample = V2 X 20/ V1 X 1000/20 mg of CaCO3 11  equivalent hardness = 1000 X V2/ V1 mg of CaCO3 equivalent hardness Thus, total hardness = 1000 X V2/ V1 ppm 3) Estimation of permanent hardness of the water sample. 20 ml of given hard water sample (boiled, cooled and filtered) consumes V3 ml of EDTA = V3 X 20/ V1 mg of CaCO3 equivalent hardness Therefore, 1000 ml of hard water sample = V3 X 20/ V1 X 1000/20 mg of CaCO3 equivalent hardness = 1000 X V3/ V1 mg of CaCO3 equivalent hardness Thus, permanent hardness = 1000 X V3/ V1 ppm 4) Temporary hardness Temporary hardness = Total hardness - Permanent hardness = 1000 X V2/ V1 - 1000 X V3/ V1 = 1000 x (V2 - V3) / V1 ppm BOILER FEED WATER Water is used in boilers, steam engines etc., for steam generation. The water fed into the boiler for the production of steam is called boiler feed water. Boiler feed water should be free from dissolved salts, suspended impurities, silica, turbidity, oil, alkali and hardness producing substances. Requirements of boiler feed water 1) Hardness of water should be less than 0.2 ppm 2) Caustic alkalinity should be between 0.15 -0.45 ppm 3) Soda alkalinity should be in the range of 0.415 – 1.00 ppm 4) Excess soda ash should be 0.3 – 0.5 ppm BOILER TROUBLES (OR) DISADVANTAGES OF USING HARD WATER IN BOILERS The hard water obtained from natural sources when fed directly into the boilers, it causes following problems 12  1. Scale and sludge formation 2. Priming and Foaming 3. Caustic embrittlement 4. Boiler corrosion 1. Scale and sludge formation In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reaches saturation point, they are thrown out of water in the form of precipitates which stick to the inner walls of the boiler. If the precipitation takes place in the form of loose or slimy precipitate it is called sludge. On the other hand, if the precipitated matter forms a hard adhering crust/ coating on the inner walls of the boiler, it is a scale. Sludge formation Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge can be easily scrapped off with a wire brush. It is formed at comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow at bends. Sludges are formed by substances which have greater solubilities in hot water than in cold water. Examples are MgCO3, MgCl2, CaCl2, MgSO4 etc. Disadvantages of sludge formation 1. Sludges are poor conductors of heat, so they tend to waste a portion of heat used. 2. When sludges are formed along with scales, they get entrapped in the scales and both get deposited as scales. 13  3. Excessive sludge formation disturbs the working of the boiler. It settles in the regions of poor water circulation such as pipe connection, plug opening, gauge glass connection thereby causing even chocking of the pipes. Prevention of sludge formation 1) By using well softened water. 2) By a frequent blow down operation, i.e., drawing off a portion of the concentrated water and replacing with fresh water. Scale formation Scales are hard deposits, which stick very firmly to the inner surface of the boiler. Scales are very difficult to remove even with the help of hammer and chisel. Scales are the main source of boiler troubles. Formation of scales may be due to 1. Decomposition of calcium bicarbonate Ca(HCO3)2 CaCO3 + H2O + CO2 The scale composed of cacium carbonate is soft and forms the main cause of scale formation in low pressure boilers. In case of high pressure boilers it is soluble. CaCO3 + H2O Ca(OH)2 + CO2 2. Decomposition of calcium sulphate: With increase in temperature of water, the solubility of calcium sulphate in water decreases. In other words, calcium sulphate is soluble in cold water, but almost completely insoluble in superheated water. Consequently calcium sulphate gets precipitated as hard scale on the heated portions of the boiler. This is the main cause in the high pressure boilers. 3. Hydrolysis of magnesium salts: The dissolved magnesium salts present in water undergo hydrolysis at prevailing high temperatures in the boiler forming magnesium hydroxide precipitate, which forms a soft type of scale. MgCl2 + 2H2O Mg(OH)2 + 2HCl 14  4. Presence of silica: Silica present even in small quantities gets deposited as calcium silicate (CaSiO3) and/ or magnesium silicate (MgSiO3). These deposits stick very firmly to the inner walls of the boiler surface and are very difficult for removal. Disadvantages of scale formation 1. Wastage of fuel: Scales are poor conductors of heat, so the heat transfer from boiler to water is decreased. In order to maintain a steady supply of heat to water, over heating is done which in turn increases the fuel consumption. 2. Decrease in efficiency: Scales get deposited in the valves and condenser of the boilers and clog them partially. This results in decrease in efficiency of the boilers 3. Danger of explosion: At high pressure sometimes the scales crack, the water comes in contact with overheated boiler metal. This results in sudden formation of large amount of steam, which may cause explosion of the boiler. 4. Lowering of boiler safety: The boiler metal becomes soft and weak on superheating, which causes the deformation of boiler tube. Prevention of scale formation 1. Soft scales can be removed with the help of wire brush or blow down operation. 2. Brittle scales can be removed by applying thermal shocks i.e heating and sudden cooling with cold water. 3. Hard and adhering scales can be removed by adding chemicals like 5-10% HCl and EDTA. 2. Priming and foaming Priming During the production of steam from a boiler, some of the particles of liquid water are carried out along with steam. This process of wet steam formation is called priming. The wet steam formed carry some dissolved salts and suspended impurities present in water along with it. Hence the phenomenon is also called as carry over. The various causes for priming is as follows 1. Presence of large amounts of dissolved salts (NaCl, Na2CO3), oily matter, alkalies and soaps. 2. Improper boiler design 3. Sudden boiling of water 4. High steam velocities 5. Instant increase in steam production rate. 15  6. Very high water level in boilers Disadvantages of priming 1. The dissolved salts carried by the wet steam get deposited on the turbine blades and reduces the efficiency. 2. The dissolved salts may enter the machinery parts, thereby decreasing the life of machinery. 3. Actual height of the water column cannot be assessed. Prevention of priming 1. Fitting mechanical steam purifiers 2. Efficient water softening 3. Maintaining optimum water level in boilers 4. Good boiler design. 5. Controlling the steam velocity Foaming Foaming is the production of persistent foam or bubbles in boilers which do not break easily. This is due to the presence of oil, grease and finely divided particles in water. Oil substances and alkali substances react to form soaps, which reduce the surface tension and thus increasing the foaming tendency of the soap. Priming and foaming usually occur together. Prevention of Foaming 1. Adding coagulants like Ferrous sulphate, sodium aluminate, aluminium hydroxide, etc 2. Adding anti foaming chemicals like synthetic polyamides, castor oil etc 3. Caustic embrittlement This is the special type of boiler corrosion due to usage of highly alkaline water in boiler. In this phenomenon, boiler metal becomes brittle with the accumulation of caustic substances. During water softening process by lime soda process, usually small fraction of free Na2CO3 is present. In high pressure boilers, Na2CO3 decomposes to give NaOH making the boiler water caustic. Na2CO3 + H2O 2NaOH + CO2 The sodium salts in natural water causing alkalinity also forms sodium hydroxide in water. When the concentration of NaOH increases on evaporation of water, it attacks the boiler metal forming sodium ferroate Na2FeO2 which decomposes and forms rust. Fe + 2NaOH Na2FeO2 + H2 Na2FeO2 + 4 H2O 6 NaOH + Fe3O4 + H2 16  Rust This causes embrittlement of boiler parts, particularly stressed part like bends, joints, rivets, etc., causing even failure of the boiler. Prevention of caustic embrittlement 1. By adding sodium phosphate as softening agent instead of sodium carbonate. 2. By adding sodium sulphate which blocks the minute cracks thereby prevents the entry of sodium hydroxide. 3. By adding tannin, lignin to boiler water, which blocks the hair cracks. Boiler corrosion Boiler corrosion is the destruction or decay of boiler material either due to chemical or electrochemical reaction with its environment. Boiler corrosion is due to the presence of 1. Dissolved oxygen 2. Dissolved carbon dioxide 3. Dissolved salts such as MgCl2 1. Dissolved oxygen Water usually contains 8mg of dissolved oxygen per litre at room temperature. At high temperatures, dissolved oxygen in water attacks the boiler material. 2Fe + 2 H2O + O2 2 Fe(OH)2 4 Fe(OH)2 + O2 2 [Fe2O3. 2 H2O] (rust) Removal of dissolved oxygen It can be removed by following two methods a) Chemical method Dissolved oxygen can be removed by adding calculated quantities of sodium sulphite or hydrazine or sodium sulphide. 2Na2SO3 + O2 2Na2SO4 N2H4 + O2 N2 + 2 H2O Na2S + 2O2 Na2SO4 Hydrazine is found to be an ideal internal compound for removing dissolved oxygen as the products formed were water and inert N2 gas. 17  b) Mechanical deaeration Dissolved oxygen can also be removed from water by mechanical de-aeration. It is one of the method, used to remove dissolved gases such as O 2and CO2. In this process, water is allowed to fall slowly on the perforated plates fitted inside the tower. To reduce pressure inside the chamber, the de-aerator is connected to a vacuum pump. The sides of the tower are heated. The water flowing down through perforated plates under goes deaeration at high temperature and low pressure. High temperature, low pressure, and large exposed surface (provided by perforated plates) reduces the dissolved oxygen in water. This is because the solubility of a gas in water is directly proportional to pressure and inversely proportional to temperature. Fig. 1 Mechanical deaeration 3) Dissolved carbon dioxide Dissolved carbon dioxide in water forms carbonic acid which has slow corrosive effect on the boiler material. Water containing bicarbonate salts also releases carbon dioxide inside the boiler. CO2 + H2O H2CO3 Ca(HCO3)2 CaCO3 + H2O + CO2 Removal of Dissolved carbon dioxide a) Adding required amount of NH4OH into water. 2 NH4OH + CO2 (NH4)2CO3 + H2O b) By mechanical deaeration method along with dissolved oxygen c) 18  4) Dissolved salts The dissolved magnesium salts present in water liberate acid on hydrolysis at high temperature, e.g MgCl2 + 2 H2O Mg(OH)2 + 2HCl The liberated HCl reacts with boiler metal (Fe) to form hydroxide which is converted into rust in a chain reaction producing HCl again and again. Fe + 2HCl FeCl2 + H2 FeCl2 + 2 H2O Fe(OH)2 + 2HCl Fe(OH)2 + O2 2[Fe2O3. 2 H2O] The acid formed can be removed by the addition of alkali to the boiler water HCl + NaOH NaCl + H2O WATER SOFTENING OR CONDITIONING METHODS We know that dissolved impurities present in water are responsible for the hardness. The water used in heating units involving boilers and steam generators should be free from the hardness producing salts. The process of removing hardness producing salts from water is known as water softening or conditioning. Water softening can be done by the following two methods 1) External conditioning or treatment 2) Internal conditioning or treatment External conditioning In external conditioning, the hardness producing salts are removed before feeding the water into the boiler. There are two methods: 1) Zeolite or Permutit process 2) Demineralisation process or Ion exchange process Demineralisation process or Ion exchange process In this process cations and anions present in water and which are responsible for hardness are removed by ion – exchange process. Ion exchange resins are insoluble, cross linked, long chain organic polymers with a microporous structure. The functional groups attached to the chains are responsible for the ion exchanging properties. Ion exchange resins are mainly 1) Cation exchange resins 2) Anion exchange resins 19 Cation exchange resins These resins have acidic functional groups such as – COOH, -SO3H etc., which are capable of exchanging their H+ ions with other cations present in hard water. Hence they are also called as cation exchangers. It is represented as RH2, where R is the insoluble polymeric heavy part. Example: Sulphonated coals, sulphonated polystyrene Anion exchange resins These resins have basic functional groups such as –NH2, -OH etc., which are capable of exchanging their anions with other anions present in hard water. Hence they are also called as anion exchangers. It is represented as R’(OH) 2. Example: Cross linked quaternary ammonium salts, Urea formaldehyde resin Process The hard water is first passed through cation exchanger, which removes all the cations like Ca2+, Mg2+, Na+ etc., present in hard water and release equivalent amount of H+ into water. RH2 + CaCl2 RCa + 2HCl RH2 + MgSO4 RMg + 2H2SO4 The cation free water is then passed through anion exchange column, which removes all the anions like Cl-, SO42-, HCO3- etc., present in water and equivalent amount of OH- ions are released from the exchanger into water. R’(OH)2 + 2HCl R’Cl2 + 2 H2O R’(OH)2 + 2H2SO4 R’ SO4 + 2 H2O – The H+ ions and OH ions (released from cation exchanger and anion exchanger respectively) combined to give water molecule. Thus water coming out of the exchanger is free 20 from both cations and anions, hence called as deionized or demineralised water. Regeneration When the exchange capacities of the cation exchange resin are exhausted, it can be regenerated by passing a solution of dil. HCl or dil. H 2SO4. RCa + 2HCl RH2 + CaCl2 RMg + 2HCl RH2 + MgCl2 Similarly, when the exchange capacities of the anion exchange resin are exhausted, it can be regenerated by passing a solution of dil. NaOH. R’Cl2 + NaOH R’(OH)2+ 2NaCl Advantages 1. Highly acidic or alkaline water can be treated by this process 2. Water obtained from this process contains very low hardness (< 2 ppm) 3. Water treated by this process can be used in high pressure boilers. Disadvantages 1. The equipment is costly and more expensive chemicals are needed. 2. Water containing Fe and Mn salts cannot be treated as it forms stable complex with the resin 3. If water contains turbidity, it reduces the output of the process. Desalination of water The process of removing common salt (Sodium Chloride) from the water is known as desalination. The water containing dissolved salts with a salty or brackish taste is called brackish water. Oil rich West Asian countries, which are surrounded by sea, have no potable water but only sea water. Brackish water is totally unfit for drinking purpose. But, it can be made into drinking water by desalination process. Commonly used methods for the desalination of brackish water are reverse osmosis and electrodialysis. Depending upon the quantity of dissolved solids, water is graded as: i. Fresh Water: Contains less than 1000 ppm of dissolved solids. ii. Brackish Water: Contains more than 1000 ppm to less than 35000 ppm of dissolved 21  solids. iii. Sea Water: Contains more than 35000 ppm of dissolved solids. Sea water and brackish water can be made available as drinking water through desalination process. Desalination is carried out either by reverse osmosis or electro dialysis. Reverse Osmosis When two solutions of different concentrations are separated by a semi permeable membrane, flow of solvent takes place from dilute to concentrated side. This process is called osmosis and the driving force for this phenomenon is known as osmotic pressure. If, however a hydrostatic pressure in excess to osmotic pressure is applied on the concentrated side, the solvent flow is reversed, i.e, solvent is forced to move from concentrated side to dilute side across the membrane. This is the principle of reverse osmosis. Thus in reverse osmosis method, pure solvent is separated from its contaminants, rather than removing contaminants from the water. The membrane filtration is sometimes also called super-filtration or hyper filtration. Process In this process, pressure (15 – 40 kg cm-2) is applied to the sea water or impure water to force the pure water content of it out the semi-permeable membrane, leaving behind the dissolve solids. The principle of reverse osmosis as applied for treating saline/sea water. The membrane consists of very thin film of cellulose acetate, affixed to either side of a perforated tube. However, more recently superior membranes made of polymethacrylate and polyamide polymers have come into use. 22 Advantages 1. Reverse osmosis possesses distinct advantages of removing ionic as well as non-ionic, colloidal and high molecular weight organic matter. 2. It removes colloidal silica, which is not removed by demineralization. 3. The maintenance cost is less. 4. The life time of membrane is quite high, about 2 years, 23  UNIT II BATTERIES AND SENSORS Introduction Batteries Batteries are the storehouses of electrical energy. A cell is a single unit consisting of minimum pair of electrodes and an electrolyte. A battery is one, which consists of series of cells. A cell or battery is a device which converts chemical energy into electrical energy. In an electrochemical system, oxidation and reduction takes place at the anode and cathode simultaneously and to the same extent. If external current is passed and the electrochemical reactions are made to occur, then the system is called a voltaic cell. When two electrodes in contact with suitable electrolytes are combined, electrochemical reactions occur spontaneously in the cell and electricity is generated. This is called a galvanic cell. A look at the emf series indicates that the electrode with maximum oxidation potentials, if combined with the electrode with maximum reduction potential will form a cell with high voltage of about 4 to 5 volts. Cells of this type are known. But the goal of battery research is to develop a cell of as high potential as possible giving a high sustained content. For the application of battery, two electrical parameters are most essential. They are potential or emf of the battery and the current the battery can supply. Students are required to have a clear understanding of these two parameters. 1. Potential or emf of a cell The emf of a battery is theoretically the sum of oxidation and reduction potentials. If the reversible, equilibrium potentials are considered, the emf will be the maximum. For example, in Daniel cell the oxidation potential of Zn in contact with 1M Zn 2+, cannot exceed +0.76V. The reduction potential at copper electrode in 1M Cu2+ solution cannot exceed +0.34 V. The sum of the two potentials is 1.10 V, which is the emf of the cell. Any battery is similar to Daniel cell and its emf depends on 1. the nature of the electrode – electrolyte system 2. concentration of the electrolyte (Nernst equation) 3. temperature Hence, for a particular system with a particular concentration and at a particular temperature, the potential depends on the nature of the electrode – electrolyte system. That is the reason the potential or emf of a battery or cell is constant. 2. Current The more important parameter in battery from a practical point of view is current. One should remember that this is not an inherent property of the electrochemical system. it denotes the extent or strength of the electrons that flow in the circuit. It is decided by the voltage and the resistance 24  offered to the flow of electrons. It is governed by ohm’s law which is stated as, E = IR E is the voltage in volts and R is the resistance in ohms, and I is the current in amperes. The potential of the battery is the energy which is responsible for the continuous occurrence of the reactions. The energy drives the electrons through the circuit from anode to cathode overcoming the resistance. At anode, oxidation occurs and electrons are released. M Mn+ + ne- these electrons reach the cathode through the external wire and into the electrolyte system and reduces the cations present in the cathode. n+ M + ne- M 1 1 The extent, to which these two reactions, namely the oxidation and reduction occur, is measured as current. Hence the electrical parameter namely the current is a measure of quantity of chemical species that undergo oxidation and reduction in the electrochemical system. Since the reactions occur at the surface of the electrodes, the rate of these reactions relies upon the surface area of the electrodes. Hence, current rely on the surface area of the electrodes. For a large surface area, more number of electrons or the ions can get discharged in unit time. Hence, the term current density, which is current per unit area, is more relevant. It is expressed in Ampere per decimeter 2. Therefore, for a successful battery system, the voltage must be sufficient to effect the flow of electrons overcoming the resistance and sufficient area of the electrode must be available to get the needed current. TYPES OR CLASSIFICATION OF BATTERIES Batteries can be categorized into the following three types 1. Primary cells or primary batteries or reversible batteries 2. Secondary cells or secondary batteries or irreversible batteries 3. Fuel cell or Fuel battery or Flow cells Primary cells or primary batteries or reversible batteries A primary battery is one in which the reaction products cannot be brought back to the original state by any means. Once the battery is discharged it cannot be recharged and has to be disposed of. The first primary battery was designed by Leclanche and the present day eveready, novino and other similar branded cells are all known as leclanche type batteries. They were earlier called dry cells since the electrolyte used was a paste. Example : Dry cell, Zn – HgO cell, alkaline battery etc. Alkaline Battery It is an improved form of dry cell. It uses zinc as anode and MnO2 as cathode. KOH is used as the 25 electrolyte in this cell and hence the name as alkaline battery. This cell gives its power from the reduction of the MnO2 (cathode) and oxidation of the zinc (anode). The emf of the cell is 1.5 V. Fig. Alkaline battery Cell reactions Anode: Zn(s) + 2 OH-(aq) Zn (OH)2 (s) + 2 e- Cathode: 2MnO2 + H2O (l) + 2e- 2OH- +Mn2O3 Net reaction: Zn(s) + 2MnO2(s) + H2O (l) Zn (OH) 2(s) + Mn2O3(s) Alkaline battery is otherwise called as heavy duty battery, because it sustains heavy use and has a longer shelf life than Zinc – carbon battery. It performs better in cold weather than other types of batteries. Advantages 1. The life of alkaline battery is longer than the dry battery, since no corrosion takes place on zinc. 2. Zinc does not dissolve readily in a basic medium 3. Alkaline battery maintains its voltage, as the current is drawn from it. Uses It is used in toys, cameras, calculators, watches, transistors, etc. Secondary cells or secondary batteries or irreversible batteries The secondary battery is the one in which the products can be brought back to the original state by passing electricity from an external source. They are rechargeable. The discharge reactions can be exactly 26 reversed. Primary batteries are not capable of delivering high current. Hence secondary batteries were 27 developed. Example: Lead acid storage battery, Ni – Cd battery Lead acid storage battery or Lead Accumulator A lead acid storage cell or battery is a secondary battery which can act both as a voltaic cell and as an electrolytic cell. When acting as a voltaic cell, it gives electrical energy and becomes “run down”. It must then be recharged. When it is recharged, the cell starts to function as an electrolytic cell. Construction and description of lead acid battery A lead acid battery consists of 3 to 6 identical cells connected in series. In each cell, the anode is made up of lead and the cathode is made up of lead dioxide PbO2 on Pb metal plate. A number of lead plates (anode) and number of PbO2 (cathodes) are connected in parallel. These plates are separated from the adjacent ones by insulators like rubber or glass fibre. The electrodes are immersed in 20% H 2SO4 having a density of 1.2 g/mL. Fig. Lead acid battery Working of lead acid battery During discharging of the cell, the following reactions occur At anode Lead is oxidized to Pb2+ ions, which further combines with SO 2- and forms insoluble PbSO. 4 4 discharging Pb (s) + SO42- PbSO4 (s) + 2e- 28  charging At cathode PbO2 is reduced to Pb2+ ions, which further combines with SO 42- and forms insoluble PbSO4. discharging PbO2(s) + SO42- + 4H+ + 2e- PbSO4 + 2 H2O Charging Over all reaction during use (Discharging) discharging Pb (s) + PbO2(s) + 2 H2SO4 2 PbSO4 + 2 H2O + Energy charging The voltage of the lead acid storage battery is 2.0 V. In the above cell reactions, PbSO4 is precipitated at both anode and cathode. The concentration of H2SO4 decreases, and hence, the density of H2SO4 is reduced to 1.0 g/ml. So, the battery needs charging. Recharging The lead acid storage battery is recharged by passing electric current in the opposite direction. The result is lead is deposited at the anode and PbO2 at the cathode. The electrode reaction gets reversed and the density of H2SO4 is increased. At anode: charging PbSO4 (s) + 2e- Pb(s) + SO 42- discharging At cathode charging PbSO4 + 2 H2O PbO2(s) + SO42- + 4H+ + 2e- discharging Net reaction charging 2 PbSO4 + 2 H2O + Energy Pb (s) + PbO2(s) + 2 H2SO4 discharging 29 Uses 1. It is used to supply current mainly in automobiles. 2. It is also used in telephone, mines, laboratories, gas engine ignition, hospitals, power plants etc. Lithium Ion Battery Lithium battery is a solid state battery because solid electrolyte is used instead of liquid or a paste of electrolyte. The lithium battery consists of lithium which acts as anode and TiS 2 acts as cathode. Electrolyte is generally a polymer (polypropylene), is packed in between the two electrodes. The solid electrolyte allows the passage of ions through two electrodes but not that of electrons. During discharging When the anode and cathode are connected through solid polymer, Li+ ions move from anode to cathode and electrons are generated at the anode. The cathode receives lithium ions and electrons through the external circuit connecting anode and cathode. At anode: Li(s) Li+ + e- At cathode TiS2 (s) + e- TiS2 - Net reaction Li(s) + TiS2 (s) LiTiS2 + energy The voltage of lithium battery is 3.0V During recharging 30  It can be recharged by supplying an external current, which drives the lithium ions back to the lithium anode. The net reaction is LiTiS2 + energy Li(s) + TiS2 (s) Advantages 1. Lithium is a light weight metal, only 7g (1 mole) material is required to produce 1 mole of electrons. 2. Electrode potential is more negative which generates a high voltage than the other types of cells. 3. Battery constituents are in solid state so no chance of leakage from the battery. 4. Li batteries are available in different sizes and shapes. Uses It is used in calculators, transistors, head phones and cordless applications. Fuel cells A fuel cell is a device which converts the thermal energy is directly converted into electrical energy. In a conventional fuel system, thermal energy is being converted into mechanical energy and mechanical energy to electrical energy. Thereby the efficiency of the whole process is very less. In a fuel cell system since the conversion is direct, the efficiency is high. In principle, the combustion reaction, that is, oxidation of the fuel is viewed as an electrochemical reaction in the fuel cell system. The fuel gets oxidized and oxygen gets reduced, which is for an electrochemical system. As in other electrochemical systems, the fuel cell also has two electrodes and an electrolyte. Fuel + oxygen Oxidation products + Electricity Example: Hydrogen – oxygen fuel cell, Methanol – oxygen fuel cell, Propane – oxygen fuel cell Hydrogen – Oxygen fuel cell Hydrogen – Oxygen fuel cell is one of the earliest and most successful fuel cell. These fuel cells make use of the fuel hydrogen and oxidizer oxygen continuously passed through the cell in the presence of liquid electrolyte. Description of Hydrogen – Oxygen fuel cell It consists of two porous electrodes (anode and cathode), which are made up of compressed carbon containing a small amount of catalyst (Pt, Pd, Ag and Ni). In between the two electrodes an electrolytic solution such as 25% KOH or NaOH is filled. The two electrodes are connected through the voltmeter. This cell develops an emf of 1.23 V. 31 Working In H2 – O2 fuel cell, pure hydrogen gas is bubbled through the anode compartment, where it is oxidized. The oxygen gas (oxidizer) is bubbled through the cathode compartment, where it is reduced. At anode Hydrogen molecules get oxidized at the anode which liberates electrons which then combine with hydroxide ions to form water. 2H2 + 4OH- 4H2O + 4e- Fig. Hydrogen – Oxygen fuel cell At Cathode The electrons produced at the anode pass through the external wire to the cathode, where it is absorbed by oxygen and produce hydroxide ions. O2 + 2H2O + 4e- 4OH- Cell reactions At anode: 2H2 + 4OH- 4H2O + 4e- At cathode: O2 + 2H2O + 4e- 4OH- Net cell reaction: 2H2 + O2 2H2O The emf of the cell is 0.8 to 1.0V. Advantages 1. H2 – O2 fuel cells does not produce noise or thermal pollution (eco friendly). 2. It produces 100% pure water 3. It is highly reliable. Applications 1. H2 – O2 fuel cells are used as ancillary energy source in space vehicles, submarines or other military vehicles. 2. In hydrogen - oxygen fuel cells, the product water acts as a valuable source of fresh water to the astronauts. 32 Solar cells A solar cell (also known as a photovoltaic cell or PV cell) is defined as an electrical device that converts light energy into electrical energy through the photovoltaic effect. A solar cell is basically a p-n junction diode. Solar cells are a form of photoelectric cell, defined as a device whose electrical characteristics – such as current, voltage, or resistance – vary when exposed to light. Individual solar cells can be combined to form modules commonly known as solar panels. The common single junction silicon solar cell can produce a maximum open-circuit voltage of approximately 0.5 to 0.6 volts. These solar cells are tiny, when combined into a large solar panel, considerable amounts of renewable energy can be generated. Construction of Solar cell A solar cell functions similarly to a junction diode, but its construction differs slightly from typical p- n junction diodes. A very thin layer of p-type semiconductor is grown on a relatively thicker n-type semiconductor. We then apply a few finer electrodes on the top of the p-type semiconductor layer. These electrodes do not obstruct light to reach the thin p-type layer. Just below the p-type layer there is a p-n junction. A current collecting electrode is provided at the bottom of the n-type layer. The entire assembly is encapsulated by thin glass to protect the solar cell from any mechanical shock. Working Principle of Solar Cell When light photons reach the p-n junction through the thin p-type layer, they supply enough energy to create multiple electron-hole pairs, initiating the conversion process. The incident light breaks the thermal equilibrium condition of the junction. The free electrons in the depletion region can quickly come to the n- type side of the junction. Similarly, the holes in the depletion can quickly come to the p-type side of the junction. Once, the newly created free electrons come to the n-type side, cannot further cross the junction because of barrier 33 potential of the junction. Once the newly created holes reach the p-type side, they cannot cross back over the junction due to the barrier potential. This separation of electrons and holes across the p-n junction allows it to function like a small battery cell. A voltage is set up which is known as photo voltage. If we connect a small load across the junction, there will be a tiny current flowing through it. Advantages of Solar Cell 1. No pollution associated with it. 2. It must last for a long time. 3. No maintenance cost. Disadvantages of Solar Cell 1. It has high cost of installation. 2. It has low efficiency. 3. During cloudy day, the energy cannot be produced and also at night we will not get solar energy. Uses of Solar Generation Systems 1. It may be used to charge batteries. 2. Used in light meters. 3. It is used to power calculators and wrist watches. 4. It can be used in spacecraft to provide electrical energy. Biosensors The term “biosensor” is short for “biological sensor” and is a device made up of a transducer and a biological element that may be an enzyme, an antibody, or a nucleic acid. The biological element or bio element interacts with the analyte being tested and the biological response is converted into an electrical signal by the transducer. Every biosensor has a biological component that acts as the sensor and an electronic component that detects and transmits the signal. Some examples of the fields that use biosensor technology include: 1. General healthcare monitoring 2. Screening for disease 3. Clinical analysis and diagnosis of disease 4. Veterinary and agricultural applications 5. Industrial processing and monitoring 6. Environmental pollution control 34  Biosensors can provide cost-effective, easy-to-use, sensitive and highly accurate detection devices in a variety of research and commercial applications. Principle of biosensors Biosensors can be categorized according to the basic principles of signal transduction and bio recognition elements. In the general scheme of a biosensor the bio recognition element responds to the target compound and the transducer converts the biological response to a detectable signal, which can be measured electrochemically, optically, acoustically, mechanically, calorimetrically, or electronically, and then correlated with the analyte concentration. Biological elements include enzymes, antibodies, microorganisms, biological tissue, and organelles. When the binding of the sensing element and the analyte is the detected event, the instrument is described as an affinity sensor. When the interaction between the biological element and the analyte is accompanied or followed by a chemical change in which the concentration of one of the substrates or products is measured the instrument is described as a metabolism sensor. Finally, when the signal is produced after binding the analyte without chemically changing it but by converting an auxiliary substrate, the biosensor is called a catalytic sensor. The method of transduction depends on the type of physicochemical change resulting from the sensing event. Often, an important ancillary part of a biosensor is a membrane that covers the biological sensing element and has the main functions of selective permeation and diffusion control of analyte, protection against mechanical stresses, and support for the biological element. 35 Types of biosensors Biosensors can be grouped according to the type of biological element and transducer they contain. They may also be named according to how the biosensing takes place. The types of biological elements include: 1. Enzymes 2. Antibodies (also called immunosensors) 3. Micro-organisms 4. Biological tissue 5. Organelles Types of biosensing The different ways that biosensing may occur are described below: 1. If the bio element binds to the analyte, the sensor is referred to as an affinity sensor. 2. If the bio element and the analyte give rise to a chemical change that can be used to measure the concentration of a substrate, the sensor is called a metabolic sensor. 3. If the biological element combines with the analyte and does not change it chemically but converts it to an auxiliary substrate, the biosensor is called a catalytic sensor. Types of sensing elements 1. Enzymes An enzyme is a protein that has a high selectivity for a particular substrate, which it binds to, bringing about a catalytic change. Enzymes are commercially available in highly purified states and are therefore useful in the mass production of enzyme sensors. Enzymes can be fixed onto the surface of a transducer through adsorption, covalent attachment, and entrapment in a gel or an electrochemically generated polymer. 36  2. Antibodies or immunosensors Antibodies are produced by B-lymphocytes in response to antigenic stimuli such as foreign invaders or microbes. When used as biosensors in immunoassays, antibodies are immobilized on the surface of a transducer through covalent attachment by conjugation of amino, carboxyl, aldehyde or sulfhydryl groups. Antibodies are sensitive to changes in pH, ionic strength, chemical inhibitors and temperature. Immune sensors usually employ optical, fluorescence or acoustic transducers. 3. Microorganisms Microbes may be used to detect the consumption of oxygen or carbon dioxide in an environment using electrochemical techniques. Microbe biosensors have the advantage of being cheaper than enzymes or antibodies and are more stable. However they may be less selective than enzymes or antibodies. 4. Other bioelements Organelles, nucleic acids and biological tissues have been researched as biosensors. Types of transducer 1. Electrochemical transducers These are useful in electrochemical, amperometric and potentiometric signals. These electrodes are commonly made of platinum, gold, silver, stainless steel, or carbon-based inert materials. Amperometric transducers, detect changes in current that occur due to oxidation or reduction. The current reflects the reaction that takes place between the analyte and the bioelement. Potentiometric transducers can measure the charge accumulation (potential) of an electrochemical cell. The transducer is usually made up of an ion-selective electrode and a reference electrode. 2. Optical transducers Fluorescence is commonly used in signal transduction, especially when using enzymes and antibodies. Fibre optic probes consist of at least two fibres. One is connected to a light source of a given wavelength range and produces the excitation wave. The other is linked to the photodiode that detects the change in optical density at a selected wavelength. Plasmon resonance transducers measure alterations in the refractive index at and close to the sensing element’s surface. 3. Acoustic transducers These are devices in which mechanical acoustic waves act as the transduction system. The membrane contains chemically interactive materials in contact with a piezoelectric material. The devices vary according to the wave guiding process used. Usually, bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices are used. 37  4. Calorimetric transduction These measure the heat from the biochemical reaction between the sensing element and the analyte. Application of Biosensors 1. Clinical and Diagnostic Applications One well known example of a clinically applied biosensor is the glucose monitor, which is used on a routine basis by diabetic individuals to check their blood sugar level. These devices detect the amount of blood glucose in undiluted blood samples allowing for the easy self-testing and monitoring that has revolutionized diabetes management. 2. Applications in industry Biosensors are used in the food industry to measure carbohydrates, alcohols and acids, for example, during quality control processes. The devices may also be used to check fermentation during the production of beer, yoghurt and soft drinks. Another important application is their use in detecting pathogens in fresh meat, poultry or fish. 3. Environmental applications Biosensors are used to check the quality of air and water. The devices can be used to pick up traces of organophosphates from pesticides or to check the toxicity levels of wastewater. 38  UNIT - III ORGANIC ELECTRONIC MATERIALS Organic electronic materials are a fascinating field within materials science and electronics, focusing on substances made from organic (carbon-based) compounds that exhibit electronic properties. These materials are notable for their potential applications in flexible, lightweight, and low-cost electronic devices. Conducting Polymers Most polymeric materials are poor conductor of electricity, because of the non-availability of large number of free electrons in the conduction process. Within the past several years, polymeric materials has been synthesized which possess electrical conductivities on par with metallic conductors. Such polymers are called conducting polymers. Conductivities as high as 1.5x107 ohm-1 m-1 have been attained in these polymeric materials. On a volume basis, this value is equal to one-fourth of the conductivity of copper, or is twice its conductivity on the basis of weight. Electrical conductivity of some polymers Polymers Electrical Conductivity (ohm-1 m-1) Phenol Formaldehyde 10-9 – 10-10 Poly methyl methacrylate