WATER

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WATER

  1. 1. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry Introduction:UNIT –I Water is the most useful, wonderful and abundant compound on earth. Its a vital component of the life forms. Its a proven theory that one can live without food for many number of days but one cannot live without water. Water is the important constituent of all the body fluids, without which all the cells or organisms are in crystalline or dead state. (For example, the human body contains 70%, land plants 50 – 75% and aquatic plants 95 – WATER 99%).Water is not only essential for the lives of animals and plants, but also occupies a unique position in industry.  Water is used in power generation. As steam in steam turbines and as coolants.  Blasting and water jet cutters. Very high pressure water guns are used for precise cutting. It is also used in the cooling of machinery to prevent over-heating, or prevent saw blades from over-heating.  Industry requires pure water for many applications and utilizes a variety of P. V. NARAYANA REDDYM.Sc., Ph.D . purification techniques both in water supply and discharge.  Water plays many critical roles within the field of food science.  Water is widely used in the production of steel, rayon, paper, atomic energy, textiles, chemicals, ice and for air conditioning, drinking, bathing, sanitary, washing, irrigation and fire-fighting etc. Sources of water:  Surface water · Flowing waters, such as rain water, rivers, streams etc. · Still waters, such as lakes, ponds etc. · Sea water. Its use is very limited as its uses entails very great problems of chemical engineering.  Ground water. ◦ Water from springs.UNIT I: ◦ Water from shallow wells. In the case of shallow wells, the boring is done only through one geological stratum.Water: Sources of Water,Types of impurities in Water, Hardness of Water –Temporary and ◦ Water from deep wells. Here the boring is done through many geological strata.Permanent hardness. Units. Estimation of hardness by EDTA Method. Analysis of Water - Source and nature of impurities of water:Dissolved Oxygen. Disadvantages of Hard Water. Problems on hardness of water. Methods Water in the form of vapour in clouds is said to be pure. Yet when it condenses as rain and flows on the ground it takes many impurities from atmosphere and ground. Theof Treatment of Water for Domestic Purpose – Sterilisation: Chlorination, Ozonisation. major situations in which water gets impure are as followed:Water for Industrial purpose - Water for Steam Making, Boiler Troubles – Carry Over 1. Dissolved Substances :(Priming and Foaming), Boiler Corrosion, Scales and Sludges, Caustic Embrittlement. Water Inorganic salts like carbonates, Chlorides, Sulphates of calcium and magnesium in theTreatment: - Internal Treatment – Colloidal, Phosphate, Calgon, Carbonate, Sodium dissolved form causes hardness to water. The dissolves impurities like the salts of sodium ,aluminates Conditioning of Water. External Treatment - Ion- Exchange Process; potassium causes alkalinity to water.Demineralization of Brakish Water – Reverse Osmosis. Dissolved gases like CO2, N2,O2, H2S etc., are responsible for the odour and acidity in water . The Presence of O2 causes corrosion in boilers.1 2 Narayana Reddy
  2. 2. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. ChemistryThe total dissolved salts may range from a few parts per million in snow water to several Mg(HCO3)2 → Mg(OH)2↓ + CO2↑thousand parts per million in water obtained from mineral springs. soluble insoluble2.Suspended Matter: It contain two types of impurities .  Permanent or Non – Carbonate Hardness:i)Inorganic suspended matter , like clay and sand Permanent hardness is due to the presence of chlorides, sulphates of calcium, magnesium,ii)Organic suspended matter like animal and vegetable matter. iron and other heavy metals. It cannot be removed by boiling, it is removed by usingRiver water mainly contains suspended matter , which is responsible for turbidity. softeners or chemical treatment.3.Biological Impurities : Micro organisms like Bacterias , algae and fungi. Disadvantages of Hard water:Hardness of water: 1.Domestic Purpose:Hardness in water is the character, which “prevents the lathering of soap”. This is due to  Washing & Bathing: Hard water does not form lather easily with soap. As a result, athe presence of bicarbonates, chlorides and sulphates of calcium, magnesium and other large amount of soap is wasted.heavy metals like strontium and iron.  Drinking: Hard water causes bad effects on our digestive system. Sometimes, stoneFor example: formation takes place in kidneys.Insoluble carbonates of Ca, Mg and Fe readily transform into soluble bicarbonates in the  Cooking: The boiling point of water is increased due to the presence of salts. Hence,presence of carbon dioxide. more fuel and time are required for cooking. CaCO3 + CO2 + H2O → Ca(HCO3)2 MgCO3 + CO2 + H2 O → Mg(HCO3)2 2.Industrial Purpose: Insoluble soluble Insoluble soluble  Textile industry: Hard water causes wastage of soap. Precipitates of calcium andMany of the mineral constituents of rocks like NaCl, CaSO4.H2O readily dissolve in water magnesium soaps adhere to the fabrics and cause problem.and collects in it.  Paper industry: Calcium and magnesium salts in water may affect the quality of CaSO4 + 2H2 O → CaSO4.2H2O paper. Anhydrite Gypsum  Sugar industry: Water containing sulphates, carbonates, nitrates affects theIf a sample of water is treated with soap (Na/K salt of higher fatty acids like oliec, crystallization of sugar.palmitic/steric)like sodium sterate and if the water does not produces lather, then the  Dyeing industry: The salts of calcium and magnesium in hard water react with dyeswater is said to be hard water, else soft water. and spoil the desired shade.In the hard water the soap produces precipitate or white scum, which is due to the  Pharmaceutical industry: Hard water may form some undesirable products whileformation of insoluble soap of calcium & magnesium. For example a typical soap (Sodium preparation of pharmaceutical products.Sterate) reactions with salts like calcium chloride & magnesium sulphate are depicted asfollowed.  Concrete making: Chlorides and sulphates present in hard water will affect the hydration of cement and the final strength of the hardened concrete. 2C17H35COONa + CaCl2 → (C17H35COO)2Ca↓ + 2NaCl Sodium Sterate Hardness Insoluble PPT.  Finally in industries where steam is employed, if hard water is used in steamOr production, the troubles like corrosion, scale & sludge formation, priming & foaming & caustic embrittlement are seen. 2C17H35COO¯ + Ca 2+ → (C17H35COO)2Ca↓ 2C17H35COOMg + MgSO4 → (C17H35COO)2Mg↓ + Na 2SO4 Expression of Concentration of Hardness: Sodium Sterate Hardness Insoluble PPT.Or The concentration of hardness as well as non–hardness constituting ions is 2C17H35COO¯ + Mg2+ → (C17H35COO)2Mg↓ expressed in terms of an equivalent amount of CaCO3. The equivalence of CaCO3 is usedHardness of water can be classified as because it permits the addition & subtraction of concentration, when required. The choice  Temporary or Carbonate Hardness: of CaCO3 in particular is due to its molecular weight is 100. Not only that, it is the mostTemporary hardness of water is due to the presence of dissolved bicarbonates of calcium, insoluble salt that can be precipitated in the water treatment easily.magnesium and other heavy metals or carbonate of iron. It is easily destroyed by mereboiling of water. When the water is boiled, the bicarbonates are decomposed yielding The equivalents of CaCO3 can be defined as:insoluble carbonates or hydroxides which are deposited as a crust or scales at the bottomof the vessel, while carbon dioxide escapes out. 3 Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑ = Soluble insoluble3 Narayana Reddy 4 Narayana Reddy
  3. 3. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistryor commonly into 1000ml standard flask. Make up the solution to the mark with the distilled water and shake the flask well for uniform concentration. = EDTA solution: Dissolve 3.72 gms of EDTA crystals in in one litre of distilled waterUnits of Expression of Hardness of Water: Indicator: Dissolve 0.5gms of Eriochrome Black – T in 100ml of ethanol / methanol.Normally the hardness of water is represented in ppm. ie. Parts per million. Or mg/L Buffer solution: Add 67.5gms of NH4Cl to 570ml of Conc. Ammonia solution and dilute1mg/L = 1ppm. with distilled water to one litre.1.ppm: It is the number of parts by weight of CaCO3 equivalent hardness present per Standardization of EDTA solution:million (106) parts by water. Rinse and then fill the burette with EDTA solution up to the mark and fix it to the buretteie. 1ppm=1part of CaCO3 equivalent hardness in 106 parts of water. stand. Pipette out 20ml of standard hard water solution into a conical flask, add 2ml of2. mg/L: It is the number of milligrams of calcium carbonate equivalent present per liter of buffer solution. Add 2 – 3 drops of EBT indicator, the solution turns to wine red. Thewater.ie. 1mg/L = 1 mg of CaCO3 equivalent hardness per liter of water. contents of conical flask are titrated against EDTA. The end point is achieved with theBut, it is known that 1lit = 1kg = 1000 g = 1000 x 1000 mg = 106mg. turning of wine red solution to deep blue coloured solution. The volume of EDTA rundown==> 1 mg/L = 1 mg of CaCO3 equivalent hardness per 106 mg of water. is noted as V1 ml. = 1 part of CaCO3 equivalent hardness per 106 mg of water. Standardization of Hard water: = 1 ppm. Rinse and then fill the burette with EDTA solution up to the mark and fix it to the buretteHardness is also expressed in the following ways. stand. Pipette out 20ml of hard water sample into a conical flask, add 5ml of buffer3.Degree clark (oCl): 1 oCl = 1 part of CaCO3 equivalent hardness per 70,000 parts of water. solution. Add 2 – 3drops of EBT indicator, the solution turns to wine red. The contents ofi.e. 1ppm = 0.07oCl. conical flask are titrated against EDTA. The end point is achieved with the turning of wine4.Degree French (oFr): 1oFr = 1 part of CaCO3 equivalent hardness per 105 parts of water. red solution to deep blue coloured solution. The volume of EDTA rundown is noted as V2 ml.i.e. 1ppm =0.1 oFr. Standardization of Permanent hardness of water: Rinse and then fill the burette with EDTA solution up to the mark and fix it to the buretteDetermination of Hardness of water: stand. Pipette out 100ml of hard water sample into a beaker and boil the water till theEstimation of Hardness in water by EDTA : volume reduces to 50ml. (all the bicarbonates are dissociated in to carbonates orPrinciple: Ethylene diamine tetra acetic acid (EDTA) in the form of sodium salt yield the hydroxides). Cool the solution and filter the water into a conical flask. Wash the beaker andanion which forms stable complex ions with Ca2+ or Mg2+. the filter paper twice and add the filtrate to conical flask. Make up the filtrate up to 250ml. pipette 20ml of the sample add 5ml of buffer solution. Add 2 – 3 drops of EBT indicator, the solution turns to wine red. The contents of conical flask are titrated against EDTA. The end point is achieved with the turning of wine red solution to deep blue coloured solution. The volume of EDTA rundown is noted as V3 ml. With the volumes of the solutions utilized are known, the hardness can be estimated by the following procedure:In order to determine the equivalent point, indicator Eriochrome black – T (EBT) is used.When EBT is added to hard water at a pH 9- 10, using Ammonical buffer solution (NH4OH- Volume of standard hard water used = 20 ml. Volume of EDTA used for Standardization of Standard hard water = V1 ml.NH4Cl),a wine red unstable complex with Ca2+ or Mg2+ ions is formed. When this solution is Volume of EDTA used for Standardization of water sample = V2 ml.titrated with EDTA, at the endpoint wine red colour changes to deep blue colour solution Volume of EDTA used for Standardization of water sample subjected to boiling = V3 ml.i.e. the EDTA has formed stable complexes. M2+ + EBT  M-EBT + EDTA  M-EDTA + EBT Calculations: Unstable wine Red Stable deep blue 1ml of standard hard water = 1mg of CaCO3Experimental procedure: 20ml of standard hard water =V1 ml of EDTA consumed.Preparation of standard solutions:Standard Hard Water: i.e. 20 x 1mg of CaCO3 = V1 ml of the EDTA consumed.Dissolve one gram of pure, dry CaCO3 in little quantity of dil. HCl and evaporate the solution 1ml of EDTA= mg of CaCO3 eq. hardnessto dryness on the water bath. Dissolve the residue in small amount of water and transfer it Now for 20ml of hard water sample given:5 Narayana Reddy 6 Narayana Reddy
  4. 4. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry20ml of hard water sample ≡ V 2 ml of EDTA consumed. microorganisms or other pathogens is known as potable water. Sterilization is done in the20ml of standard hard water : V 1 ml of EDTA : : 50ml of sample water : V2ml of EDTA following ways:1ml of sample water contain mg of CaCO3 eq. hardness 1.Boiling: Boiling water for 15 – 20 minutes, will kill the bacteria in water. Yet this method is not suitable for municipal supply of water. This is adopted for domestic purpose.Since the hardness is estimated per 1 litre (1000 ml) the equivalent hardness will be 2.Aeration or UV treatment: Exposing water to UV rays or pumping water through X1000mg of CaCO equivalent  mg of CaCO equivalent nozzles into air in broad day light will kill the germs and microorganisms in water.Total Hardness of water = mg/ L or ppm 3.Ozonisation: Ozone passed into water acts as disinfectant. Ozone is an unstable isotopeSimilarly 1000ml of the boiled water = 1000ml of sample water of oxygen, produces nascent oxygen which is a powerful disinfectant.Mg/ L or ppm O3  O2 + [O] Nascent oxygenPermanent Hardness of water = mg/ L or ppm This treatment is expensive and ozone cannot be stored for longtime due to its instability.Temporary hardness = Total Hardness – Permanent Hardness 4.Chlorination: Chlorine is also a powerful disinfectant. The use of chlorine for disinfecting 1000[ - ] 1000( ) ppm or mg/L water is known as chlorination. The chlorination is done in the following ways:Methods of water treatment: (i) By adding Bleaching power: Bleaching power contains 80% chlorine. When it is used asWater treatment for domestic purpose: The water obtained from natural sources like disinfectant, it is called hypo – chlorination because the disinfection is due to the formationrivers, lakes and ponds will have impurities. Water for drinking and other domestic uses of Hypochlorous acid formed during the process.should be free from disease producing bacteria, chemicals or other substances. Hence CaOCl2 + H2OCa(OH)2 + Cl2water is treated before using for drinking or other domestic purpose. Bleaching powderThere are mainly two stages involved in water treatment for domestic purpose: Cl2 + H2O  HOCl + HClI. Removal of suspended impurities. DisinfectantII. Disinfection or Sterilization. HOCl  [O] + HClVarious impurities of water are removed in the following stages: (ii)Passing chlorine: Directly administering the chlorine gas along with water in a1.Screening: The process of removing floating matter from the water is known as chlorinator will be an effective process. The passed chlorine react with water and generatesscreening. In this process, water is passed through a screen, which arrests all the floating hypochlorous acid, which acts as disinfectant.matter. Cl2 + H2O  HOCl + HCl2.Plain sedimentation: This is the process of removing big sized suspended solid particles Hypochlorus acidform water. In this process, the water is stored in huge tanks for several hours. 70% of Chlorine is a powerful disinfectant than chloramines and bleaching powder.solid particles settle down due to the force of gravity. Periodically the accumulation of the Calculated amount must be added to water because in excess will be toxic and tastes bad.debris is scraped away and the floating impurities are continuously cleaned using screens. The amount of chlorine required to kill bacteria and to remove organic matter is called3.Sedimentation by Coagulation: This is the process of removal of suspended colloidal break point of chlorination.impurities by using coagulants like Alum (K2SO4.Al2(SO4)3.24H2O),Ferrous Sulphate (FeSO4),Sodium Aluminate (NaAlO2). When a coagulant is added to water, floc formation takes place Water for Industrial use:due to hydroxide [Al(OH)3]formation which can gather tiny particles together to form Water plays a key role in industry. Without it no industry runs. The major use is inbigger particles and settle down quickly. production of steam and as a coolant. The water used for making steam should be free from Al2(SO4)3 + 6H2O2Al(OH)3 + 3H2SO4 dissolved salts, gases, oil, silica and suspended impurities. The steam is used in the NaAlO2 + 2H2ONaOH + Al(OH)3 production of electricity, as a heat source where temperature is an important criteria (like4. Filtration: This process helps in removal of the colloidal and suspended impurities not food processing plants) etc.removed by sedimentation. Usually the water is passed through filters or suitable porous Boiler troubles:material so as to remove the suspended impurities and some microorganisms. Normally If the concentration of salts is more in water that is used for boilers, the following troublesfilters made of three layers like sand, gravel and charcoal is employed. Yet the Indian may arise:Cotton saree four fold will work as ultra filter. (Proved by the NASA testing, reported in  Carry over: Priming and foamingNews papers).  Scale and Sludge formationII. Sterilisation or Disinfection: The process of killing pathogens or other microorganisms  Caustic Embrittlementin the water is known as Sterilization or Disinfection. The water which is free from  Boiler Corrosion Carry over:7 Narayana Reddy 8 Narayana Reddy
  5. 5. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. ChemistryThe phenomenon of carrying of water by steam along with the impurities is called carry  Water concentrated with dissolved salts may deposit on the parts of the machinery,over. This is mainly due to priming and foaming. As the steam rises from the surface of the which cause corrosion.boiling water in the boiler, it may be associated with small droplets of water. Such steam, Scale & sludge formation:containing liquid water, is called wet steam. These droplets of water naturally carry with In the boiler the continuous steam production results in the concentration of the dissolvedthem some suspended and dissolved impurities present in the boiler water. impurities. Then the salts start separating out from the solution in order of their solubility,Priming: the least soluble one separating out first. The concentrations of the solids that separate inPriming is the phenomenon of carrying out droplets of water with steam in boilers. the liquid and form soft and muddy deposits or form suspension are known as sludges.Because of very rapid and violent boiling of water inside the boiler, the water particles mix Where as some of the solids deposit on the surface form sticky and coherent deposits calledup with steam and pass out of the boiler. This process of wet steam generation is due to the scales.following reasons. The most commonest solids that separate from boiler water are the sparingly soluble  Presence of large quantity of suspended organic matter, oily matter and alkalis. calcium salts e.g. CaSO4, Ca(OH)2, CaCO3, Ca 3(PO4) and magnesium compounds.  Very high steam velocity. Disadvantages of Scale formation:  Sudden boiling. 1. Scale effect like an insulator coating on the metal, which results in the reduced rate of  Defective boiler design. heat transfer hindering the boiler efficiency.  High water level in boiler. 2. Scale formation on boiler acts as insulator for water and over heats the metal, making it  Sudden increase in steam – production rate. soft and weak. This is unsafe for boiler at high pressures.Priming can be avoided in the following ways: 3. The overheating of boiler metal and rapid reaction between water and iron at high  Using mechanical purifiers or purifying the water by effecting softening and temperatures, cause additional thinning of the walls. filtration process. 4. Under high pressure of steam, the metal expands and causes the cracks in the scales.  Avoiding rapid change in steam rate. Sudden entry of water through these cracks to the very hot metal causes sudden cooling of  Maintaining the low water level. the boiler in the metal with the simultaneous conversion of water into steam. The sudden increase in the pressure of steam may lead to explosion.  Using well designed boiler providing for proper evaporation of water with uniform heat distribution and adequate heating surfaces. Prevention of scale formation: External treatment:  Minimizing foaming. This involves removal of hardness causing impurities and silica before the water fed into  Blow down of the boiler (replacing the water concentrated with impurities with boiler. fresh water).Foaming is the phenomenon of formation small but persistent bubbles on the surface of Internal treatment:water inside the boiler. Foams are generally formed when there is a difference in The internal treatment consists of adding chemicals directly to the water in the boilers forconcentration of solute or suspended matter between the surface film and the bulk of the removing scales and sludges.liquid. 1. Colloidal conditioning: organic substances like kerosene and agar-agar are added toCauses: water which prevent the scales.  Presence of oily or soapy substances. 2. Phosphate conditioning: Phosphates react with water and a loose sludge is formed  Presence of dissolved impurities and suspended matter in high concentrations. which can be easily removed by blow-down operation.Preventive measures: 3CaCl2 + 2Na3PO4  Ca3(PO4)2↓ + 6 NaCl  Adding anti foaming agents like castor oil. (It should be noted that excess castor oil 3. Carbonate conditioning: In low pressure boilers, scale formation can be avoided by also produces foam and the amount of castor oil required differs from boiler to adding sodium carbonate to the boiler water. boiler) CaSO4 +Na2CO3  CaSO4 + Na2SO4  Use of coagulants like sodium aluminate for sedimenting the suspended oily 4. Calgon conditioning: Sodium hexa meta phosphate(calgon) is added to boiler water to impurities before feeding the water to boiler. prevent scale formation by formation of soluble complex.  Blow down of the boiler. 2CaSO4 + Na (Na4(PO3)6) Na2(Ca2(PO3)6) + 2Na2SO4 2Disadvantages of Priming & Foaming: Priming and foaming causes the following boiler 5. Sodium aluminate conditioning: Sodium aluminate gets hydrolysed yielding NaOH andtroubles: aluminium hydroxide precipitate. The sodium hydroxide reacts with magnesium salts to  The actual height of water in the boiler can not be judged. magnesium hydroxide. These hydroxides entrap finely divided particles and neutralize the  Wastage of heat results in decrease of steam pressure and efficiency of boiler. charge on colloidal particles, and the loose precipitate can be removed by the blow-down operation.9 Narayana Reddy 10 Narayana Reddy
  6. 6. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry NaAlO2 + 2H2O  NaOH + Al(OH)3↓ NH2NH2 + O2  N2 + 2H2OCaustic Embrittlement: 3. A polyvalent organic compound under the trade name A2 aming 8001-RD is employed toCaustic embrittlement is a form of corrosion caused by high concentration of sodium degasify water now-a-days.hydroxide in the boiler water. It is characterized by the formation of irregular cracks on the 4. Mechanical deaeration is another method of degasification. Water is sprayed through aboiler metal, particularly at places of high local stress, such as riveted seams, bends and perforated plate, fitted in the degasification tower, heated from sides and connected tojoints. The caustic embrittlement occurs in boilers at high pressures, where NaOH is vacuum pump as shown in the figure. High temperature, low pressure and large exposedproduced in the boiler by the hydrolysis of Na2CO3. surface reduce dissolved oxygen and other gases in water. Na2CO3 + H2O 2NaOH + CO2 Dissolved CO2:The extent of the hydrolysis increases with temperature and may reach even 90% of the Dissolved Carbon dioxide has a slow corrosive effect on the materials of boiler plate. Sourcecarbonate present. The rate and extent of corrosion by caustic embrittlement increases of carbon dioxide into water is the boiler feed water which contains bicarbonates. Underwith the concentration of NaOH and the temperature and hence with increasing operating high temperature and pressure, maintained in the boiler the bicarbonates decompose topressure produce CO2.Preventing of caustic embrittlement: Ca( HCO3)2  CaCO3 + CO2+H2O1. By using sodium phosphate as softening reagent instead of sodium carbonate. Disodium Mg(HCO3 )2  Mg(OH)2 + 2CO2hydrogen phosphate is the best softening reagent because it not only forms complex with The carbon dioxide is removed by adding addition of calculated quantity of ammonia.Ca2+ and Mg2+ resulting the softening of water but also maintains pH of water at 9-10. The NH4OH + CO2  (NH4)2CO3 + H2Oother phosphates used are trisodium phosphate and sodium dihydrogen phosphate etc. Carbon dioxide is also removed by the using mechanical deareator.2. By adding tannin or lignin to the boiler water which block the hair cracks and pits that Action of Acids in boiler:are present on the surface of the boiler plate thus preventing the infiltration of caustic soda The acids are produced due to the hydrolysis of magnesium salts in the boiler.solution. MgCl2 + 2H2O  Mg(OH)2 + HCl3. By adding sodium sulphate to boiler water, which also blocks the hair cracks and pits The acids produced react with iron of the boiler and decay the metal.present on the surface of the boiler plate, preventing the infiltration of the caustic soda Fe + 2HCl  FeCl2 + HClsolution. The amount of sodium sulphate added to boiler water should be in the ratio 2FeCl2 + H2O 2Fe(OH)2 + 2HCl [ ] kept as 1:1, 2:1 and 3:1 in boilers working as pressures upto 10, 20 2Fe(OH)2 + O2  2[Fe2O3.3H2O] rustand above 30 atmospheres respectively. Consequently even a small amount of MgCl2 can cause corrosion to a large extent.Boiler Corrosion: The decay of boiler material by chemical or electrochemical attack of its Preventive measures:environment is called boiler corrosion. The prime reasons of boiler corrosion are  Water softened before fed into boiler.  Dissolved oxygen  Frequent blow – down of the boiler.  Dissolved carbon dioxide  Addition of inhibitors like Sodium silicate, sodium phosphate and sodium chromate  Formation of Acids from dissolved salts to slow down correction. The inhibitors form a thin film on the surface of the boilerDissolved oxygen: Among the dissolved gases oxygen is the most corroding impurity. At and protect the metal from the attack of acid.room temperature water contains 8g. of oxygen per liter. The dissolved oxygen at hightemperatures attacks the boiler plate creating serious corrosion problem. 2Fe + 2H2O + O2  2Fe(OH)2↓ Estimation of Dissolved oxygen: 4Fe(OH)2 + O2  2[Fe 2O3.2H2O] A good amount of oxygen is dissolved in water at room temperature and pressure i.e. 8 rust mg/L. It is essential for sustaining of aquatic life and also serves as an indicator for theRemoval of dissolved oxygen: extent of water pollution. Hence, determination of dissolved oxygen content is of1. Addition of sodium sulphite or sodium sulphide removes O2 by converting O2 to sodium significance both in environmental and industrial point of view. Dissolved oxygen in watersulphate. (Na2SO3 or Na2S should be added in very little quantity [5-10ppm] as the residual is determined by Winkler’s method.sodium sulphite may decompose to SO2 under high pressure, which finally appear assulphurous acid) Principle: The determination of dissolved oxygen is based on the “oxidation of potassium 2Na2SO3 + O2  2Na 2SO4 iodide by dissolved oxygen”. The liberated iodine is titrated against a standard solution of Na2S + 2O2 Na 2SO4 sodium thiosulphate (hypo) using starch indicator. As the dissolved oxygen is non reactive2. Addition of hydrazine (NH2NH2) is an ideal reagent added to boiler to remove dissolved with KI, the oxygen carrier Manganese hydroxide is used to bring the reaction between KIoxygen as H2O. and Oxygen.11 Narayana Reddy 12 Narayana Reddy
  7. 7. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry MnSO4 + 2KOH => Mn(OH)2 + K2SO4 Standardisation of water sample: 2Mn(OH)2 + O2 => 2MnO(OH)2 The burette is rinsed and filled with hypo solution. 100 ml of water sample is taken in a Oxygen carrier Basic manganic oxide 250ml iodine flask (care should be taken to avoid air bubbles) and stoppered immediately. Then add 0.2 ml of manganese sulphate with the help of pipette, dipping the end well below 2MnO(OH)2 + H2SO4 => MnSO4 + 2H2O + [O] the water surface. Add 2m of alkaline iodide azide solution to the flask, stopper the flask 2KI + H2SO4 + [O] => K2SO4 + I2 + H2O and shaked thoroughly. Allow the precipitate to settle half way and mix again. Repeat the Na2S2O3 + I2 => Na 2S4O6 + 2NaI process of shaking and settling at least three times. Then add 10ml of concentrated H2SO4. Stopper the flask and shake the bottle again. Keep the flask in dark for 5 minutes, and Sodium tetrathionate titrate the yellow coloured solution and continue the titration with hypo till the blue colourNote: Some times, water (like rain water) contains nitrites, sulphites etc. The nitrates of the solution disappears. Repeat the titration to get concurrent values. Let the titer valuepresent in water interfere with the estimation of dissolved oxygen of water, since these be y ml.ions liberate iodine form KI. Sodium azide is added to water in alkaline solution todecompose nitrates. Calculations:Preparation of standard solutions: Volume of hypo (titer value with water sample (y ml).= V 1 mlStandard K2Cr2O7 solution: 0.49 g. of Potassium dichromate is weighed accurately into clean Normality of hypo = N1100ml standard flask. The content of standard flask is dissolved in little distilled water and N1V1 Normality of dissolved oxygen =the solution is made up to the mark by adding distilled water. The solution is shaked well 100for uniform concentration. Then the amount of dissolved oxygen in the water sample is given by: V1X N1X 8X10 ppmSodium thiosulphate solution: 2.48g. of sodium thiosulphate(hypo) is weighed accuratelyinto 1000ml standard flask. Dissolve the hypo with little water and add the distilled water External Water Treatment:up to the mark. Shake the solution well to get uniform concentration. Softening of water: The process of removing hardness causing salts from water is known as softening of water.MnSO4 solution: Dissolve 40 g. of MnSO4 in 100ml of distilled water and stir the solution Generally water is softened by the following methods.well. Temporary hardness is removed by prolonged heating of water. The dissolved gases like O2 and CO2 are also removed simultaneously.Alkaline Iodie-Azide solution: 15g. of KI, 50 g. of NaOH and 2 g. of NaN3 is dissolved in 100ml Ion-exchange process of water softening:of distilled water in a standard flask. Stopper the flask and shake well for uniform This is a reversible exchange of ions between a liquid phase and a solid phase. Materialsconcentration. capable of exchange of cations are called cation exchangers and those which are capable of exchanging anions are called anion exchangers. In the Ion Exchange process the cations areStarch indicator: 1g. of starch is added to little water and 100ml of boiling water is added to exchanged with H+ and anions are exchanged with OH¯ ions. For this two types of ion-the contents. The contents are stirred to get colourless, colloidal solution of starch. exchangers are commonly employed, which are insoluble, cross linked long chain organicExperimental procedure: polymers with microscopic structure. The cation exchangers are represented with general formula RH. These are mainly styrene-Standardisation of sodium thiosulphate solution: divinyl benzene copolymers containing the functional groups –COOH or SO3 H. In RH, R is the general structure of the resin and H is exchangeable with cation. Anion exchangers areA burette is rinsed first and then filled with hypo solution. A 250ml conical flask / Iodine phenol-formaldehyde or amine formaldehyde copolymer resins which exchange their OH¯flask is taken; in to it 20ml of KI, spatula of NaHCO3 and 5ml of Conc. HCl is added. 20ml of ion with any anion present in the dissolved salts. The anion exchangers are represented byK2Cr2O7 solution is pippetted out into conical flask. Stopper the flask, shake it well and keep the formula R`OH. R` is the general structure of the resin and OH- is the exchangeable anion.it in the dark for 5 minutes. We will get dark red coloured solution. The contents of conicalflask are titrated against hypo. The endpoint is attained with the appearance of greenishyellow colour in the conical flask. Now 1ml of starch indication is added. The solution turnsinto blue. Again the solution is titrated against hypo till the light green colour endpoint isobtained. The titrations are repeated to get the concurrent values. Let the titre value be xml.13 Narayana Reddy 14 Narayana Reddy
  8. 8. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry exhausted anion exchanger is treated with dil. NaOH solution. The regeneration can be represented as R2SO4 + NaOH  2R`OH + Na 2SO4 (Washings) R`Cl + NaOH R`OH + NaCl (Washings) R` 2HCO3 + NaOH 2R`OH + NaHCO3 (Washings) R`NO3 + NaOH R`OH + NaCl (Washings) The washings are discarded into sink. The regenerated ion-exchangers are used for softening. Thus deionization and regeneration are the alternate process. Advantages:Principle: 1. Highly acidic or alkaline water samples can be purified by this process.When raw water is first passed through cation exchanger, the exchange of cations takes 2. The hardness possessed by the deionised water is 2 ppm.place as shown below. 3. The deionised water is most suitable for high pressure boilers. 2RH + Ca(HCO3)2  R2Ca + 2H2 CO3 Disadvantages: 2RH + Mg(HCO3)2  R2Mg + 2H2CO3  The ion exchanging resins are expensive; hence the cost of purification is high. 2RH + CaCl2  R2Ca + 2HCl  Raw water should contain turbidy below 10 ppm. Otherwise pores in the resin will 2RH + CaSO4 R2Ca + 2H2SO4 be blocked and output of the process is reduced. 2RH + Ca(NO3)2 R2Ca + 2HNO3 2RH + MgCl2  R2Mg + 2HCl Desalination of Brackish water: 2RH + MgSO4  R2Mg + 2H2SO4 Water containing high concentrations of dissolved solids with a peculiar salty or brackish 2RH + Mg(NO3)2  R2 Mg + 2HNO3 taste is called brackish water containing about 3.5% of dissolved salts. This water cannotThe Ca2+ and Mg2+ are retained by the cation exchangers as Ca2R and Mg2R releasing H+ into be used for domestic and industrial applications unless the dissolved salts are removed bywater. Thus the water coming out of the resin is highly acidic because the H+ released by desalination.the exchange combines with anion of the dissolved salt to produce the corresponding acids. The common desalination of Brackish water areThen the water is passed through anion exchanger where the anions of the acids present in  Reverse osmosis methodthe water are removed by the exchanger releasing OH¯ into water. The H+ and OH¯ released  Electrodialysis methodfrom exchangers get combined and produce H2 O. Reverse Osmosis: R`OH + HCl R`Cl + H2O When two solutions of unequal concentration are separated by a semi-permeable 2R`OH + H2SO4  R`2 SO4 + H2O membrane which does not permit the passage of dissolved solute particles, (i.e., molecules R`OH + H2CO3  R`HCO3 + H2O and ions) flow of solvent takes place from the dilute solution to concentrates solution. This R`OH + HNO3 R`NO3 + H2O is called Osmosis. If a hydrostatic pressure in excess of osmotic pressure is applied on theThus the water coming out from exchanger is free from all ions known as deionised or concentrated side the solvent, the solvent is forced to move from higher concentration todematerialised water. When water sample is completely deionised, it has the tendency to lower concentrated side across. Thus the solvent flow is reversed hence this method isabsorb gases like CO2, O2 etc, from atmosphere which cause boiler corrosion. Hence called reverse osmosis. Thus in reverse osmosis, pure water is separated fromdeionization must be followed by degasification. contaminated water. This membrane filtration is also called super filtration or hyperRegeneration: After the deionization of certain amount of raw water the cation and anion filtration. The reverse osmosis cell is as shown as below.exchangers will be exhausted. Regeneration of cation exchanger is carried out by passingdil. HCl or H2SO4 solution into the bed. The H+ ions of the acid are exchanged with thecations (Ca2+ and Mg2+) present in the cation exchanger regenerating it the following way. R2Ca + HCl  2RH + CaCl2 R2Mg + H2SO4  2RH + MgSO4The washings containing CaCl2 and MgSO4 etc, were passed to sink or drain. Similarly the15 Narayana Reddy 16 Narayana Reddy
  9. 9. SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. Chemistry SIDDARTHA INSTITUTE OF SCIENCE AND TECHNOLOGY PUTTUR Engg. ChemistryThe reverse osmosis cell consists of a chamber fitted with a semi-permeable membrane,above which sea water. Impure water is taken and a pressure of 15 to 10 kg/cm2 is appliedon the seawater/impure water. The pure water is forced through the semi-permeablemembrane. This is made of very thin films of cellulose acetate. However superiormembrane made of polymethyl methaacrylate and polyamide polymers have come to use.Advantages:  Both ionic and non-ionic, colloidal and high molecule weight organic matter is removed from water sample.  Cost of purification of water is less and maintenance cost is less.  This water can be used for high pressure boilers.Electrodialysis : Electrodialysis is based on the principle that the ions present in salinewater migrate towards their respective electrodes through ion selective membrane, underthe influence of applied e.m.f. The electrodialysis unit consists of a chamber, two electrodesa cathode and an anode. The chamber is divided into three compartments with the help ofthin, rigid, ion-selective membranes which are permeable to either cation or anion. Theanode is placed near anion selective membrane. While the cathode placed near cationselective membrane. The anion selective membrane is containing positively chargedfunctional groups such as Na+ and is permeable to anions only. The cation selectivemembrane consists of negatively charged functional groups. Such as Cl- and is permeable to www.pvnreddy.blogspot.comcations only. Under the influence of applied e.m.f. across the electrodes the cations movetowards anode through the membrane. The net result is depletion of ions in the centralcompartment while it increases in the cathodic and anodic compartments. Desalinatedwater is periodically drawn from the central compartment while concentrated brackishwater is replaced with fresh sample. pvnr.svu@gmail.comAdvantages: The unit is compact.The process is economical as for as capital cost and operational expenses are concerned. ΩΩΩΩΩΩ17 Narayana Reddy 18 Narayana Reddy

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