CHEM
UNIT - 2Water Chemistry Q1) Define hard water and determine its hardness using EDTA method?A1) Hard water: is water that contains a required quantity of dissolved minerals (like calcium and magnesium) As rainwater falls, it is naturally soft. However, as water flows through the ground and into waterways, it picks up minerals like chalk, lime and mostly calcium and magnesium and becomes hard water. Since hard water contains essential minerals, it is essentially used as drinking water. Not only because of the health benefits, but also the flavour. Water that does not produce lather with soap readily is called hard water. Water hardness is usually measured as calcium hardness in milligrams per litre (mg/l) OR parts per million (ppm) OR in grains per gallon (GPG).For e.g.: sea water, river water, spring water, lake water and well water.Water hardness can be readily determined by titration method with the chelating agent, EDTA (ethylenediaminetetraacetic acid) (Greek χηλή, chelè, meaning claw). This reagent is a weak acid that can lose four H (in bold) on complete neutralization; The four acid oxygen sites and the two nitrogen atoms have unshared pair of electrons, which can form bonds with a metal ion forming a complex ion or coordination compound. The complex that is formed is quite stable, and the conditions of its formation can easily be controlled so that it is ready for selection for a particular metal ion. when a titration to determine the concentration of a metal ion is carried out, the added EDTA quantitatively combines with the cation to form the complex. The endpoint is reached when essentially all of the cation has reacted. In this analysis a solution of EDTA will be standardize by titration against a standard solution made from calcium carbonate, CaCO3. The EDTA solution can then be used to determine the hardness of an unknown water sample. Since both EDTA and Ca2+ are colourless, therefore a special indicator is used to detect the end point of the titration. The indicator mostly used is called Eriochrome Black T, which forms a very stable wine-red complex, MgIn–, with the magnesium ion. A small amount of this complex will be present in the solution during the titration. As EDTA is added, the complex free Ca2+ and Mg2+ ions, leaving the MgIn– complex alone until essentially all of the calcium and magnesium are converted to chelates. At this point EDTA concentration will increase marginally to displace Mg2+ from the complex indicator; the indicator reverts to its uncombined form, which is sky blue, thus establishing the end point of the titration. The titration is carried out at a pH of 10, in a NH3/NH4 + buffer, the equations for the reactions which occur during the titration are: 
Titration reaction: HY3–(aq) + Ca2+(aq) CaY2–(aq) + H+ (aq) (also for Mg2+) End point reaction: HY3–(aq) + MgIn– (aq) MgY2–(aq) + HIn2–(aq) As the indicator requires a trace of Mg2+ to operate properly, a little magnesium ion will be added to each solution. The effect of the added Mg2+ can be subtracted by titrating a blank. A simple way to calculate the hardness in water.A sample of hard water contains 1mg cacl2 and 1 mg mgcl2 per
Q2) Estimate the hardness of water by Alkalinity method?A2) Alkalinity is the measure of all bases present in the water and assumed as the buffering capacity of water, or its ability to resist change in pH. The most common and important base is carbonate. Total alkalinity is expressed as parts per million (ppm) of calcium carbonate (CaCO3) or milligrams per liter (mg/L). Alkalinity can be referred to as "carbonate hardness" or "KH," which is usually measured in degrees (dKH) than in mg/L or ppm. One dKH is equal to 17.9 mg/L or 17.9 ppm.Waters that have moderate to high levels (50 mg/L or greater) of total alkalinity and total hardness usually have a neutral to slightly basic pH. At this stage the pH is more stable and does not change greatly throughout the day because the presence of carbonates and bicarbonates neutralize, or "buffer," the carbon dioxide and other acids in the water.Alkalinity is a measure of the capacity of water to neutralize hydrogen ions or acids. Alkalinity is otherwise to as "Carbonate hardness". Alkalinity acts as a buffer when any changes are made to the water's pH value. The Alkalinity in the water will help keep the pH of water stabilized. The drinking water and all water should be a pH of 7 which neutral. Procedure of Alkalinity TestThe titration tube up is filled up to the 5ml line with water.Then one Bromocresol Green-Methyl Red tablet is added into water. Cap and shake until the tablet disappear. The water might turn blue-green. If the water turns pink, the water doesn't have alkalinity. Fill the direct-reading titration with the Alkalinity Titration Reagent B. Then fill the titration when the plunger is up to the zero-graduation mark. After the plunger is added up to the zero-graduation mark, put the titration in the centre hole of the titration tube cap. Titrate the plunger slowly till the titrate sample which is blue-green colour changes to pink. The amount of Reagent B that is consumed into the titration until it turns pink is the amount of alkalinity in the water. It is measured my parts per million (ppm) out of 200 ppm. Q3) Explain the softening of water by ion exchange method?A3) Ion Exchange ProcessIon exchange process to soften water, using cations or
Titration reaction: HY3–(aq) + Ca2+(aq) CaY2–(aq) + H+ (aq) (also for Mg2+) End point reaction: HY3–(aq) + MgIn– (aq) MgY2–(aq) + HIn2–(aq) As the indicator requires a trace of Mg2+ to operate properly, a little magnesium ion will be added to each solution. The effect of the added Mg2+ can be subtracted by titrating a blank. A simple way to calculate the hardness in water.A sample of hard water contains 1mg cacl2 and 1 mg mgcl2 per litre . calculate the hardness of water in terms of caco3 present in per 106 parts of water. Hardness of water in terms of Caco3= (molar mass of hardness causing substance*molecular weight of Caco3)/ (molecular weight of hardness causing substance. Thus, cacl2 hardness = 1mg*100/111 = 0.900ppm for Mgcl2 Hardness = 1mg*100/95=1.052ppm Total hardness of water in terms of Caco3 = 0.90+1.052= 1.952ppm
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anions. This is done by exchanging cations or anions with the calcium and magnesium ions in hard water. This process involves a reversible chemical reaction. However, we can use this technique only in dilute solutions. The equipment that we use for this purpose is ion exchangers.Types There are two types;Cation exchangers – use zeolite, greensand, sulfonated coal, etc. as the exchanging material. Anion exchangers – uses metallic oxides, synthetic resins, etc. The materials used in cation exchangers include either weak acids or strong acids. Strong acid cation exchangers mainly contain sulfate functional groups. Weak acid cation exchangers mainly contain carboxyl groups. The materials that are used in anion exchangers include either weak bases or strong bases. Moreover, there are several categories of ion exchange process used for water softening, Dealkalization and demineralization. The ions that are part of the exchange process (the ions that exchange with the calcium and magnesium cations in hard water) include sodium ions, hydrogen cations, chloride anions and hydroxyl anions.Zeolite process
Zeolite process is a process of softening hard water thru ion exchange technique using a chemical compound zeolite. It possesses a chemical compound that has hydrated sodium aluminosilicate. Thus, the name of the process is called as zeolite process. Zeolite compound can exchange its sodium cations reversibly with calcium and magnesium ions in the water softening process.There are two types of zeolite used in this process they include natural and synthetic zeolite. The natural form is found to be porous and synthetic form is a non-porous zeolite. however synthetic form possesses a high exchange capacity per unit weight than the natural form.
Cylinder Containing the Zeolite BedProcessIn the water softening process, hard water is passed through a bed of zeolite (inside a cylinder) at a specified rate. Then the cations that cause the water-hardening will remain on the zeolite bed because these cations exchange with the sodium cations of zeolite. Therefore, the water coming out of this cylinder contains sodium cations rather than calcium and magnesium cations.After some time, the zeolite bed gets exhausted, the water flow is stopped and treat the bed is treated with concentrated brine solution (10%) in order to regenerate the zeolite. The bed is treated with a brine solution, as it washes away all the calcium and magnesium ions, by exchanging them with sodium ions in a brine solution. hence, this treatment regenerates the zeolite. Q4) What do you understand by Breakpoint chlorination?A4) Breakpoint ChlorinationBreakpoint chlorination is defined as the point where sufficient chlorine has been added to a quantity of water to enable disinfection. In other words, it is at this point that all the undesirable contaminants are removed from the water. It is at this breakpoint that, all the chlorine that is added to the solution gets consumed by the chemical reactions of the contaminants, that results in water having no free availed chlorine (FAC).In waste water treatment, breakpoint chlorination involves removing of ammonia from a solution, that changes to an oxidised volatile form. Increased amounts of chlorine residue are produced by adding chlorine to water that contains ammonia or organic matter that contains nitrogen.This process involves the continuous addition of chlorine to water, till a point where chlorine brings about a suppression in the rate of corrosion successfully., that makes the diffusion process difficult due to rust. Excessive amount of chlorine causes detrimentation to the human tissues. In a reaction Chlorine can undergo strong oxidation causing hydrogen to separate from water, this hydrogen combines with chlorine to form hydrogen chloride and nascent oxygen, that causes damage in tissues.Generally, in swimming pools, breakpoint chlorination has a certain amount of free chlorine that is used to fully remove combined chlorine from the pool water. It is necessary to avoid the unsuccessful termination Q5) Explain Reverse osmosis?A5) Reverse osmosis is a process that involves water softening, the principle is that this process uses an applied pressure gradient present across a semipermeable membrane to overcome the osmotic pressure and also to remove water molecules that have hardness ions in the solution. The semi permeable membrane has large pores to allow water molecules to pass through, Ions of Ca2+ and Mg2+ that cause in hardness in water do not pass through the membrane. The resulting water is soft and free of hardness ions and does not show the presence of any other ions. Membranes are a type of water filter that require cleaning at regular intervals and replacement.Distillation and reverse osmosis are the most widely used two non-chemical methods of water softening.To remove the temporary hardness from 5 L5 L of H2OHX2O 5.6 mg5.6 mg of lime is required. Calculate the degree of hardness due to Ca (HCO3)2Ca (HCOX3) X2 present in water
Q6) Explain the theories of corrosion?A6) Theories of CorrosionDry or Chemical theory of corrosion: The direct reaction of Atmospheric gases (like halogens, oxygen, oxides of nitrogen and sulphur, fumes of chemicals and with metals and hydrogen sulphide), on the surface of the metal causes corrosion. Oxygen is the most responsible for corrosion of metals than any other gases or chemicals. This process occurs in the absence of moisture and affects the metal surface directly. Corrosion products are formed at the site of corrosion. The three types of corrosion are Oxidation Corrosion, Liquid metal corrosion and Corrosion by other gases.Can be used where there is no power Lower initial cost Less supervision required Comparatively simple installation and additional anodes can easily be added if the initial installation proves to be inadequate Sacrificial anodes are used to protect:Hulls of ships Water heaters Pipelines Distribution systems Above-ground tanks Underground tanks Refineries The anodes in sacrificial anode cathodic protection systems must be periodically inspected and replaced when consumed. Q9) What do you understand by surface coating?A9) Surface CoatingThis is the most economical method and simple wherein the membrane surface is coated by a layer. This method can be carried out on a large scale in industries.The most common coating materials include tin, lead, aluminium etc. The principle in this method includes formation of a transition layer from the various alloy compositions. To the substrate inter-metallic compounds of the two metals are normally present and on the exterior are alloys that consist of solid solution alloys that predominantly form the coating materials. Surface coating is a process where a substance is applied to other materials bringing about changes like gloss, colour, resistance to chemical attacks or permeability, however the bulk properties of the material remains unchanged. Production of surface coating by any method depends primarily on two factors: the cohesion between the film forming substances and the adhesion between the film and the substrate. The development of science and technology revolutionized the surface coating industry.Q10) Explain Hot dipping and Galvanization?A10) Hot-dip galvanizing is a form of galvanization where the iron or steel is immersed in a bath that consists of molten zinc at a temperature around 449o C (840 °F), this helps in producing a multi-layer coating of zinc-iron alloy and zinc metal. The layer is also corrosion resistant, when steel is immersed in zinc, a reaction which is metallurgical occurs between the iron and the molten zinc.Galvanizing, it is a process that involves the protection of steel and iron from exposure to the atmosphere leading to rusting, this is however prevented by the application of zinc coating. Proper coating by galvanization can protect the material from the atmosphere and corrosion for nearly 15 to 30 years, however is pores develop in the coating galvanic or electrolytic action occurs.
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Ca (HCO3)2+CaO⟶2CaCO3+H2O Since 5.6 mg5.6 mg of lime is required, amount of calcium bicarbonate present is 16.2×10−316.2×10−3. Molarity = 16.2×10−3/ (162×5) =0.002 M=0.002 M. This is equivalent to the concentration of calcium carbonate. ppm of calcium carbonate = 0.002×10−2×100×106×10−3=2000.002×10−2×100×106×10−3=200 Therefore, degree of hardness is 200 ppm |
2. Wet or Electrochemical theory of Corrosion: This corrosion occurs in metals that come in direct contact with a conducting liquid of two different metals are dissolved in a solution partly. There is formation of galvanic cell on the metal, as part of the metal acts as an anode and the rest is the cathode, the chemical in the solution along with the humidity acts as the electrolyte. Oxidation takes place in such conditions resulting in corrosion at the anode surface and reduction occurs at the cathode surface of the metal. In this case corrosion occurs at the anode but rust gets deposited on the cathode.
Q7) Define waterline corrosion?A7) This is a kind of corrosion that involves an oxidation process to materials that come in contact with water. Waterline corrosion occurs when one portion of the material is exposed to air and the other portion is immersed in water. Therefore, a corrosion reaction occurs as there is a differential amount of oxygen in contact with the material's surface above and below the waterline and results in a corrosive reaction.This mainly occurs when there is a difference in the oxygen concentration between the water and the atmosphere. The material that is more exposed to air forms the cathode and the portion that is less exposed to oxygen (the portion dipped in water) forms the anode. The presence of anode and cathode allows an oxidation reaction to occur. This ultimately causes the area of the substrate submersed in water to become oxidized and corrode Q8) Explain the concept of sacrificial anode?A8) It is a method where an easily eroded material is deliberately placed in a tank or a pipe to be sacrificed for corrosion and the rest of the system remains free from corrosion.A sacrificial anode is also known as a galvanic anodeThe mechanism that occurs in this process is very similar to that of an electrochemical system. In this technique the metal that is protected is placed on the cathode side and then a more reactive metal or alloy (having a larger potential difference than the protected metal) is chosen and connected to the protected metal as an anode. The redox reaction will proceed spontaneously. As the reaction proceeds the sacrificial metal gets consumed as oxidation reaction occurs at the anode, simultaneously reduction reaction occurs on the cathode, thus preventing the metal from erosion. Thus, corrosion on the protected metal is successfully shifted to the anode, protecting the metal.Sacrificial anodes are normally supplied with either lead wires or cast-m straps to facilitate their connection to the structure being protected. The lead wires may be attached to the structure by welding or mechanical connThe materials used for sacrificial anodes are either relatively pure active metals, such as zinc or magnesium, or are magnesium or aluminium alloys that have been specifically developed for use as sacrificial anodes.Advantages of using sacrificial anodes:
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