WATER TECHNOLOGY-II
BOILER TROUBLES
In treatment of water complete elimination of all the impurities is not possible. The impurity that gives rise to certain troubles will be removed to certain extent. In modern pressure boilers and laboratories, water purer than the distilled water is required. Some of the boiler troubles caused by the use of unsuitable water are..
Carryover – Priming and Foaming
Scale formation
Boiler corrosion and
Caustic embrittlement
1. Carryover – Priming and Foaming
Priming: it may defined as the violent or rapid boiling of water occurring in the boiler which results in carrying out of water with steam in in the form of a spray.
When a boiler is producing steam rapidly, some particles of liquid water are carried along with the steam. This process of wet steam formation is called priming.
Priming mainly caused due to very high water level. The presence of large amount of dissolved solids, high steam velocities, sudden increase in steam production rate. Sudden steam demand which leads to sudden boiling, presence of excessive foam filling the foam spare, and due to faulty boiler design.
priming can be controlled by proper boiler design, fitting mechanical steam purifier, avoiding rapid change in steam-rate, proper evaporation and adequate heating surfaces, uniform distribution of fuel and providing anti priming pipes, keeping the water level low and avoid sudden steam demands. Efficient softening and filtration of the boiler feed water.
Foaming: Foaming is the formation of small but stable bubbles above the surface. The main reason for foaming is being presence of fatty acids and other impurities.
Foaming can be controlled by using anti-foaming chemicals, removal of concentrated boiler water and replacing it by fresh feed water. Removing oil from boiler water by adding compounds like sodium aluminate.
Scale formation: Some hard, sticky, adherent deposits formed on the inner surface of the boilers are known as Scales.
Scales are hard, adhering precipitates formed on the inner walls of the boilers. They stick very firmly on to the wall surface and are difficult to remove with chisel and hammer.
Generally scales are formed due to decomposition of calcium bicarbonates, decomposition of calcium sulphates, presence of silica and hydrolysis of magnesium salts.
Scales formation can be prevented by external treatment of boiler feed water, internal treatment of water in boiler and blow-down process.
Boiler Corrosion: The chemical or electro chemical eating away of metal by its environment in a boiler is known as boiler corrosion. The main reason for this problem is the presence of excess of oxygen in water. It can be prevented by mechanical deaerator, pre-heating and chemical treatment.
Caustic Embrittlement: The formation of brittle and incrystalline cracks in the boiler shell is called caustic embrittlement.
It is a type of boiler corrosion and the main reason for this, is the presence of alkali-metal carbonates and bicarbonates in feed water and also the presence of sodium sulphate.
In lime-soda process, it is likely that, some residual Na2CO3 is still present in the softened water. This Na2CO3 decomposes to give NaOH and CO2, due to which the boiler water becomes “Caustic”.
Na2CO3+ H2O à NaOH + CO2
This caustic water flows inside the boiler and causes some minutes hair-cracks, by capillary action.
On evaporation of water, the dissolved caustic soda increases its concentration which attacks the surrounding area, thereby dissolving Iron of boiler as Sodium ferroate.
This causes embrittlement of boiler parts such as bends, joints, reverts etc, due to which the boiler gets fail. Caustic cracking can be explained by considering the following concentration cell:
Iron at bends, + Concentrated Dilute NaOH solution - Iron at plane surfaces
joints, reverts etc, NaOH solution
Therefore, caustic embrittlement can be prevented.
By maintaining the pH value of water and neutralization of alkali.
By using Sodium Phosphate as softening reagents, in the external treatment of boilers.
Caustic embrittlement can also be prevented by adding Tannin or Lignin or Sodium sulphate which prevents the infiltration of caustic-soda solution blocking the hair-cracks.
SLUDGE FORMATION IN BOILERS:
In boilers, because of continuous evaporation of water, the concentration of salts increase progressively and after the saturation point is reached, precipitate form on the inner walls of boiler.
SLUDGE: Sludge is a soft, loose and slimy precipitate formed within the boiler. It is formed at comparatively colder portions of the boiler and collects in the area where flow rate is slow.
These are formed by substances which have greater solubilities in hot water than in cold-water.
Eg. MgCO3, MgCl2, CaCl2, MgSO4.
DIS-ADVANTAGES:
As the sludge’s are poor conductor of heat they cause loss of heat.
The working of the boiler is disturbed because of chocking of pipes by the sludge.
PREVENTION:
By using well softened water.
By drawing off a portion of concentrated water frequently.
SCALES: Scales are hard, adhering precipitates formed on the inner walls of the boilers. They stick very firmly on to the inner wall surface and are difficult to remove with chisel and hammer.
causes of scale formation: Following are the causes
decomposition of calcium bicarbonate:
Ca(HCO3)2 à CaCO3 + H2O + CO2
In low pressure boilers, CaCO3 causes scale formation.
In High pressure boilers, CaCO3 becomes soluble.
CaCO3 + H2O à Ca(OH)2 + CO2
Decomposition of calcium sulphate:
The solubility of CaSO4 in water decreases with rise of Temperature. In super heated water CaSO4 is insoluble.
This is the main cause in high-pressure boilers.
Hydrolysis of Magnesium salts:
Dissolved Magnesium salts undergo hydrolysis forming Mg(OH)2 precipitate.
MgCl2 + 2H2O -à Mg(OH)2 + 2 HCl
Mg(OH)2 so found by hydrolysis of Magnesium salts is a soft type of scale.
Presence of Silica: Silica present in small quantities deposits as silicates like CaSiO3 and MgSiO3. These are very difficult to remove.
Disadvantages:
wastage of fuel: The scale formation causes decreases of heat transfer. As a result over heating is required this causes consumption of fuel.
danger of Explosion: The hot scale cracks because of expansion and water suddenly comes in contact with overheated Iron plates. This causes in formation of large amount o steam suddenly. This results high pressure causing boiler to burst.
PREVENTION:
External treatment: Efficient softening of water is to be carried out.
Internal treatment: Suitable chemicals are added to the boiler water either to precipitate or to convert the scale into compounds.
INTERNAL TREATEMENT:
Internal treatment of boiler water is carried out by adding proper chemicals to precipitate the scale forming impurities in the form of sludge and to convert the scale forming chemicals into compounds which will stay in dissolved form in water. This process is mainly used as a corrective treatment to remove the slight residual hardness and also sometimes to remove the corrosive tendencies in water. Some of the internal treatment methods used for the removed of scale formation in boilers are.
.
COLLODIAL CONDITIONING: The addition of organic substances such as Kerosene, tannin, Gel etc., to the surface in low pressure boilers may prevent the scale formation. These substances gets coated over the scale forming precipitates and gives a loose and non-sticky precipitates which can be removed by using blow-down operation.
PHOSPHATE CONDITIONING: The addition of sodium phosphate in hard water reacts with the hardness causing agents and gives calcium and magnesium phosphates which are soft and non-adhere and can be removed easily by blow-down operation. In this way, scale formation is removed in high-pressure boilers.
3CaCl2 + 2 Na3PO4 à Ca3(PO4)2 + 6NaCl
CARBONATE CONDITIONING: In low-pressure boilers, scale-formation can be avoided by adding sodium carbonate to boiler water, when CaSO4 is converted into calcium carbonate in equilibrium.
CaSO4 + Na2SO4 à CaCO3 + Na2SO4
Consequently, deposition of CaSO4 as scale doesn’t take place and calcium is precipitated as loose sludge of CaCO3 which can be removed by blow-down operation.
CALGON CONDITIONING: Involves in adding calgon to boiler water. it prevents the scale and sludge formation by forming soluble complex compound with CaSO4.
Calgon = Sodium hexa meta phosphate = (NaPO3)6
Na2[Na4(PO3)6] ==== 2Na+ + [Na4P6O18]-2
2CaSO4 [ Na4P6O18]-2à [Ca2P6O18]-2 + 2Na2SO4
SODIUM ALUMINATE: Sodium aluminate gets hydrolyzed yielding NaOH and a gelatinous precipitate of aluminum hydroxide.
Thus NaAlO2 + 2H2O à NaOH + Al(OH)3
The sodium hydroxide, so-formed, precipitates some of the magnesium as Mg(OH)2, i.e.,
MgCl2 + 2NaOH à Mg(OH)2 + 2NaCl
The flocculent precipitate of Mg(OH)2 plus aluminum hydroxide, produced inside the boiler, entraps finely suspended and colloidal impurities, including oil drops and silica. The loose precipitate can be removed by pre-determined blow-down operation.
.
Water used for industrial purposes such as for steam generation, should be sufficiently pure.
It should, therefore be freed from hardness-producing salts before put to use.
The process of removing hardness-producing salts from water is known as softening of water.
In industry main three external methods employed for softening of water are.
Lime-Soda process
Zeolite process
Ion-Exchange process
LIME-SODA PROCESS:
In this method, the soluble calcium and magnesium salts in water are chemically converted in to insoluble compounds, by adding calculated amount of Lime and Soda. CaCO3 and Mg(OH)2 so precipitated, these precipitates are filtered off.
Lime soda process mainly two types, they are
Cold Lime-soda process
Hot Lime-soda process.
COLD LIME SODA PROCESS
In this method, calculated quantity of chemical like lime and soda are mixed with water at
room temperature.At room temperature, the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily.
Consequently, it is essential to add small amounts of coagulants like alum, aluminum sulphate, sodium aluminate, etc. Which hydrolyze to flocculent, gelatinousprecipitate of aluminum hydroxide, and entraps fine precipitates.
Use of sodium aluminate as coagulant also helps the removal of silica as well as oil,
if present in water. Cold L-S process provides water, containing a residual hardness of 50 to 60 ppm.
NaAlO2 + 2H2O à NaOH + Al(OH)3
Al2(SO4)3 + 3Ca(HCO3)2 à 2Al(OH)3 + 3CaSO4 + 6CO2
METHOD: Raw water and calculated quantities of chemicals ( Lime + soda + Coagulants) are fed from the top into the inner vertical circular chamber, fitted with a vertical rotating shat carrying a number of paddles. As the raw water and chemicals flow down, there is a vigorous stirring and continuous mixing, whereby softening of water reaches up. The softened water comes into the outer co-axial chamber, it rises upwards. The heavy sludge or precipitated floc settles down the outer chamber by the time the softened water reaches up. The softened water then passes through a filtering media this is usually made of wood fibres to ensure complete removal of sludge. Filtered soft water finally flows out continuously through the outlet at the top. Sludge settling at the bottom of the outer chamber is drawn of occasionally.
HOT LIME-SODA PROCESS:
The reaction proceeds faster
The softening capacity f hot process is increased to many fold
The precipitate and sludge formed settle down rapidly and hence, no coagulants are needed
Much of the dissolved gases such as CO2 and air driven out of the water
Viscosity of softened water is lower, so filtration of water becomes much easier. this in-turn increases the filtering capacity of filters, and
Hot lime-soda process produces water of comparatively lower residual hardness of 15 to 30 ppm.
Hot lime-soda plant consists essentially of three parts
A ‘Reaction tank’ in which raw water, chemicals and steam are thoroughly mixed.
A ‘Conical sedimentation vessel’ in which sludge settles down, and
A ‘sand filter’ which ensures completes removal of sludge from the softened water.
ADVANTAGES OF LIME-SODA PROCESS:
It is very economical
If this process is combined with sedimentation with coagulation, lesser amounts of coagulants shall be needed.
The process increases the pH value of the treated-water; thereby corrosion of the distribution pipes is reduced.
Besides the removal of harness, the quantity of minerals in the water is reduced.
To certain extent, iron and manganese are also removed from the water.
Due to alkaline nature of treated-water, amount of pathogenic bacteria in water is considerably reduced.
DIS-ADVANTAGES OF LIME-SODA PROCESS:
For efficient and economical softening, careful operation and skilled supervision is required.
Disposal of large amounts of sludge or insoluble precipitates poses a problem. However, the sludge may be disposed off in raising low-lying areas of the city.
This can remove hardness only upto 15 ppm, which is not good for boilers.
ION- EXCHANGE PROCESS:
Ion exchange process also known as demineralization or de-ionization process.
Ion-Exchange resins are insoluble. Cross linked long chain organic polymers with a micro porous structure, and the “functional Groups” attached to the chains are responsible for the ion-exchanging properties.
In De-ionization process all the ions present in water are eliminated by using ion-exchange resins. Basically resins with acidic functional group are capable of exchanging H+ ions with other cations. Resins with functional groups are capable of exchanging OH- ions with other anions.
Resins are classified as
1. Cation Exchange Resins
2. Anion Exchange Resins
1. Cation Exchange Resins: These are mainly styrene divinyl benzene co-polymers, which on sulphonation or carboxylation. These are capable of exchanging their hydrogen ions with cations in water.
2. Anion Exchange Resins: Anion exchange resins are styrene-divinyl benzene or amine-formaldehyde copolymers, which contains amino, quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an internal parts of the resin matrix. These after treatment with dilute NaOH solution. Become capable of exchanging their OH- ions with anions in water.
In ion-exchange process, hard water is allowed to pass through cation exchange resins, which remove Ca+2 and Mg+2 ions and exchange equivalent amount of H+ ions. Anions exchange resins remove bicarbonates, chlorides and sulphates from water exchange equivalent amount of Oh ions. Thus by passing hard water through cation hardness is observed by the following reactions.
Cation Exchange Resins
2RH+ + Ca+2 -à R2Ca+2 + 2H+
2RH+ + Mg+2 à R2Mg+2 + 2H+ (RH+ = cation exchange resin)
Anion Exchange Resins
R’OH + Cl+2 à R’Cl- + OH-
2R’OH- + SO4-2 à R2SO4-2 -2 + 2OH-
2R’OH + CO-32 à R’2 CO-32 + 2OH- (R’OH = anion exchange resin)
H- And OH- ions, thus released in water from respective cation and anion exchange columns, get combined to produce water molecules.
H+ + OH- à H2O
The water coming out from the exchanger is ion free i.e., free from anions and cations. Thus water of zero hardness is obtained.
REGENERATION:
When cation exchanger losses capacity of producing H+ ions and exchanger losses capacity of producing OH- ions, they are said to be exhausted. The exhausted cation exchanger is regenerated by passing it through dilute sulphruric acid.
R2Ca+2 + 2H+ à 2RH+ + Ca+2
The exhausted anion exchanger is regenerated by passing a dilute solution of NaOH.
R2SO4-2 + 2OH à 2R’OH- + SO
Merits of Ion-exchange process:
a. The process can be used to soften highly acidic or alkaline water.
b. It produces water of very low hardness (2 ppm)
c. So it is very good for treating water for use in high-pressure boilers.
Demerits of Ion-exchange process:
The equipment is costly and more expensive chemicals are needed.
If water contains turbidity, the output of the process is reduced. The turbidity must be below 10 ppm; else it has to be removed by coagulation and filtration.
ZEOLITE PROCESS:
Zeolites are also known as Permutits;
Zeolites are mainly 2 types:
1. Natural zeolites are non-porous, eg. Natrolite- Na2O.Al2O3.4SiO2.2H2O.
2. Synthetic Zeolites are porous and possess gel structure. Sodium zeolites are generally used for softening of water and are simply represented as Na2Ze, where ‘Ze’ stands for insoluble zeolite. In the process, when hard water is passed through a bed of zeolite placed in an closed cylinder, the hardness causing ions like Ca+2 and Mg+2 ions are taken up by the zeolite. Sodium salts are released during the reaction, as byproduct.
Process: For softening of water by zeolite process, hard water is percolated at a specified rate through a bed of zeolite, kept in a cylinder. The hardness causing ions like Ca+2, Mg+2 are retained by the zeolite as CaZe and MgZe; while the outgoing water contains sodium salts.
The various reactions taking place may be
Na2Ze + Ca(HCO3)2 à CaZe + 2NaHCO3
Na2Ze + Mg(HCO3)2 à MgZe + 2NaHCO3
Na2Ze + MgCl2 à MgZe + 2NaCl
Na2Ze + CaCl2 à CaZe + 2NaCl
Na2Ze + MgSO4à MgZe + 2Na2SO4
Na2Ze + CaSO4à CaZe + 2Na2SO4
Hence zeolite process removes the hardness of water effectively.
Regeneration: After some time, the Zeolite is completely converted into calcium and magnesium zeolites and it ceases to soften water, i.e., it gets exhausted. At this stage, the supply of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a concentrated Brine solution
(10% NaCl).
CaZe + 2 NaCl à Na2Ze + CaCl2
MgZe + 2 NaCl à Na2Ze + MgCl2
Advantages:
It removes the hardness completely and water of about 10 ppm hardness is produced.
The equipment used is compact, occupying a small space.
No impurities are precipitated, so there is no danger of sludge formation in the treated water at a later stage.
The process automatically adjusts itself for variation in hardness of incoming water.
It is quite clean
It requires less time for softening.
It requires less skill for maintenance as well as operation.
Disadvantages:
The treated water contains more sodium salts than in lime-soda process.
This method causes caustic embrittlement.
High turbidity water cannot be treated efficiently by this method.
Limitations:
1. If the supply of water is turbid, the suspended matter must be removed, before the water is admitted to the zeolite bed. Otherwise the turbidity will clog the pores of zeolite bed, thereby making it inactive.
2. If water contains large quantities of coloured ions such as Mn+2 and fe+2 they must be removed first, because these ions produce magnesium and Ferrous zeolites. This cannot be easily regenerated.
3. Mineral acids, if present in water, destroy the zeolite bed and therefore, they must be neutralized with soda, before admitting the water to the zeolite softening plant.
DIFERENCES
S.NO
LIME-SODA PROCESS
PERMUTIT/ZEOLITE
ION-EXCHANGE/RESIN
1
Water treatment plant occupies more area or place.
Water treatment plant occupies less area.
Water treatment plant occupies less area.
2
Water after treatment has lesser dissolved solids.
Water after treatment has much more dissolved solids.
Water after treatment free from dissolved solids
3
This method of water treatment plants is not much expensive and material used is cheap.
This method of water treatment plants is more expensive and material used for softening is expensive.
This method of water treatment plants is more expensive and material used for softening is expensive.
4
Operation expenses are higher
Operation expenses are low
Operation expenses are higher
5
The cost incurred on softening of water is high.
The cost incurred on softening of water is low.
The cost incurred on softening of water is high
6
It cannot operate in under pressure.
It can even operate in under pressure.
It can even operate in under pressure.
7
It can be used for treating acidic water also.
This method of softening the water is not used for treating acidic water.
It can be used for treating acidic and alkaline water also.
8
There is a problem of settling, coagulation and removal of sludge.
There is no problem of settling, coagulation and removal of sludge.
There is problem of turbidity.
9
It is not possible.
This method can be made automatic.
This method can be made automatic.
10
In order to meet the changing hardness of incoming water, frequent control and adjustment of reagents is needed.
Control test comprises only in checking the hardness of treated-water.
Control test comprises only in checking the hardness of treated-water.
11
Residual hardness is low about 15 to 50 ppm
Residual hardness is low about 10 to 15 ppm
Residual hardness is low about 0 to 2 ppm
12
It is not good for boilers
It is not good for boilers
It is very good for treating water for use in high pressure boilers.
13
Skilled persons required
It required less skill for maintenance as well as operation
It required less skill for maintenance as well as operation
Wednesday, April 2, 2008
engineering chemistry notes(water technology)
WATER TECHNOLOGY-I
1. Introduction
Water is nature’s most wonderful, abundant and useful compound.
Water is not only essential for the lives of animals and plants. But also occupies a unique position in industries.
Water is also used a coolant in power and chemical plants.
It is widely used in drinking, bathing, sanitary, washing, irrigation, fire-fights, air-conditioning and also production of industrial materials.
2. Effect of water on rocks and minerals:
Dissolution: some mineral constituents of rocks such as NaCl and CuSO4.2H2O readily dissolve in water.
Hydration: Some minerals are easily hydrated with the consequent increase in volume leading to disintegration of the rocks in which these minerals are present. Eg.
CaSO4 Hydration à CaSO4.2H2O (Gypsum)
Mg2SiO4 àMg2SiO4.XH2O (Serpentine)
Effect of dissolved oxygen: This leads to oxidation and Hydration
Fe3O4 àFe2O3 à3 Fe2O3.2H2O(Limonite)
2FeS2 + 7O2 + 2H2O à2FeSO4 + 2H2SO4
Effect of dissolved CO2: Water containing dissolved CO2 converts the insoluble carbonate of calcium, Magnesium and iron into their relatively soluble bicarbonates.
CaCO3 + H2O + CO2 --------------- Ca(HCO3)2
3. Types of Impurities present in water:
The natural water is usually contaminated by different types of impurities.
They are mainly 3 types..
Physical impurities
Chemical impurities
Biological impurities
1. PHYSICAL IMPURITIES:
a. Colour: Colour in water is caused by metallic substances like salts.
b. Turbidity: is due to the colloidal, extremely fine suspensions such as insoluble substances like clay, slit, and micro-organisms.
c. Taste: presence of dissolved minerals in water produces taste. Bitter taste can be due to the presence of Fe, Al, Mn, Sulphates, lime. Soap taste can be due to the presence of large amount of sodium bicarbonate.
d. Odour: In water is undesirable for domestic as well as industrial purpose.
2. CHEMICAL IMPURITIES:
a. In-organic-organic chemicals:
Inorganic chemicals:
1. Cations: Ca+2, Mg+2, Na+, K+, Fe+2, Al+3, Zn+2, Cu+2
2. Anions: Cl-, SO-2, NO3-, HCO3-, F-, NO2-
Organic chemicals: dyes, paints, petroleum products, pesticides, detergents, drugs textile materials, other organic related materials.
Acidity: surface and ground water attains acidity from industrial wastes like acid, mine, drainage, pickling liquors, usually acidity caused by the presence of free CO2, mineral acids, and weakly dissociated acids.
b. Gases: All natural waters contains dissolved atmosphere CO2, O2, NH3 gases, pollutant and sewage water contains nitrogen in the form of nitrogenous organic compounds and urea , which are partially converted into NH3.
c. Mineral matters: have organs from rocks and industrial effluents. These include mineral acids, Ca+2, Mg+2, Na+, K+, Fe+2, Al+3, Zn+2, Cu+2, Mn+2, Cl-, SO-2, NO3-, HCO3-, F-, NO3-, SiO2
3.BIOLOGICAL IMPURITES:
Biological impurities are Algae, pathogenic bacteria, fungi, viruses, pathogens, parasite-worms, etc. the source of these contamination is discharge of domestic and sewage wastes, excreta, etc.
A. Micro-organisms: algae, fungi, viruses, etc.
B. Water bodies in water include 1. Bacteria, organisms’ inhabitating the bottom sludge, and 2. Organisms and planktons developed at the water surfaces. These are inhibitated by different groups of worms like flat worms, hair worms, tiny roundworms, oligochetes, etc.
HARDNESS OF WATER
Hardness of water defined as which prevent the lathering of soap.This is due to presence of in water of certain salts of Ca+2, Mg+2 and other Heavy metals dissolved in it. Soaps are Sodium or Potassium salts of higher fatty acids like Oleic acid or Palmitic acid or stearic acids. C17H35COONaTemporary Hardness is mainly caused by Bicarbonates of Calcium, Magnesium and other heavy metals.
SOAP with HARDWATER REACTIONS
2C17H35COONa+CaCl2---- à(C17H35COO)2Ca+2NaCl
2C17H35COONa+MgSO4--à(C17H35COO)2Mg+Na2SO4
Hardness of water is mainly TWO types1.Temporary Hardness2. Permanent Hardness
1. Temporary Hardness mainly caused by the presence of dissolved Bicarbonates of Calcium, Magnesium and other Heavy metals and the carbonate of Iron.
Temporary hardness of water mainly responsible salts are…
1. Calcium bicarbonate Ca(HCO3)2
2. Magnesium bicarbonate Mg(HCO3)2
When bicarbonates are decomposed a, yielding insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel.
Temporary Hardness can be largely removed by mere Boiling of water.
Temporary Hardness is also known as….
Carbonate Hardness Alkaline Hardness
2. Permanent harness: It is due to the presence of dissolved Chlorides and sulphates of Calcium, Magnesium, Iron and other metals Permanent hardness responsible salts are….
CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3
Permanent Hardness cannot removed by boiling but it can be removed by the use of chemical agents.
Permanent Hardness also known as….
Non-Carbonate HardnessNon-Alkaline
Heat
• Ca(HCO3) 2 ---àCaCO3+H2O+CO2
Calcium bicarbonate
Heat
• Mg(HCO3)2 ----àMg(OH)2+2CO2
Magnesium bicarbonate
Explain the terms Temporary and permanent hardness of water.
Write a short note on hardness of water.
Hardness Distinguish between Temporary and Permanent Hardness of water.
DISADVANTAGES OF HARDWATER:
1. In Domestic use:
WASHING: hardwater, when used for washing purposes, does not producing lather freely with soap. As a result cleansing quality of soap is decreased and a lot of it is wasted. Hardwater reacts with soap it produces sticky precipitates of calcium and magnesium soaps. These are insoluble formations.
Bathing: hardwater does not lather freely with soap solution, but produces sticky scum on the bath-tub and body. Thus, the cleansing quality of soap is depressed and a lot o it is wasted.
Cooking: the boiling point of water is increased because of presence of salts. Hence more fuel and time are required for cooking.
Drinking: hardwater causes bad effects on our digestive system. Moreover, the possibility of forming calciumoxalate crystals in urinary tracks is increased.
2. INDUSTRIAL USE:
Textile Industry: Hardwater causes wastage of soap. Precipitates of calcium and magnesium soaps adhere to the fabrics and cause problem.
Sugar Industry: water containing sulphates, nitrates, alkali carbonates. Etc. if used in sugar refining, causes difficulties in the crystallization of sugar. Moreover, the sugar so produced may be de-liquescent.
Dyeing Industry: The dissolved salts in hardwater may reacts with costly dyes forming precipitates.
Paper Industry: Calcium, magnesium, Iron salts in water may affect the quality of paper.
Pharmaceutical Industry: Hardwater may cause some undesirable products while preparation of pharmaceutical products.
Concrete Making: Water containing chlorides and sulphates, if used for concrete making, affects the hydration of cement and the final strength of the hardened concrete.
Laundry: Hardwater, if used in laundry, causes much of the soap used in washing to go as waste Iron salts may even cause coloration of the clothes.
3. IN STEAM GENERATION IN BOILERS:
For steam generation, boilers are almost invariably employed. If the hardwater is fed directly to the boilers, there arise many troubles such as..
Scale and sludge formation: the hardness of water fed to the boiler may cause scale and sludge formation.
Corrosion: Hardwater may cause caustic embrittlement which is a type of boiler corrosion.
Priming and Foaming: Hardwater used in boiler causes priming and foaming which results in the formation of wet stream.
Caustic embrittlement
ESTIMATION OF HARDNESS BY EDTA METHOD:
The hardness of water is not a pollution parameter but indicate the eater quality in terms of Calcium and magnesium expressed in terms of CaCO3.
The analysis is done by complex metric titration using standard EDTA and EBT as an indicator.
EDTA is Ethylene diamaine tetra acetic acid whose structural formulae is
EDTA can form 4 to 6 dative bonds with cations like Calcium Magnesium.
PRINCIPLE: In this complex metric Titration, the water sample is titrated with standard solution of Di sodium salt of EDTA using EBT indicator.
PROCEDURE: To 50 mL of sample add 10-15 mL buffer solutions adjust the pH about 10 and then warm the solution. Add 2 drops o Eriochrome Block-T and it is titrated with 0.01 M EDTA solution changes its colour from red-wine to blue.
REACTIONS INVOLVED: EBT indicator when added to hard water at pH = 10, forms weak complexes with calcium and magnesium present in hard water. It results in the formation of Ca-EBT or Mg-EBT complexes which is wine-red, these are unstable.
During titration with EDTA, calcium first reacts to form relatively stable followed by magnesium to give Mg-EDTA+2 complex releasing the free indicator(blue) the colour changes from wine-red to blue at the endpoint.
Various steps involved in this method are…
Preparation of standard hard water: dissolve 1g of pure, dry CaCO3 in minimum quantity of dil.HCl and then evaporate the solution to dryness on a water bath. Dissolve the residue in distilled water to make 1 Litre solution. Each mL of this solution thus contains 1mg of CaCO3 equalent hardness.
1 mL hard water solution = 1mg of CaCO3 equalent hardness.
Preparation of EDTA solution: Dissolve 4 g of pure EDTA crystals + 0.lg MgCl2 in 1 Litre of distilled water.
Preparation of Indicator (EBT): Dissolve 0.5 g of Eriochrome Black-T in 100mL alcohol.
Preparation of Buffer solution: add 67.5g of NH4Cl to 570 mL of Con. Ammonia solution and then dilute with distilled water to 1 Litre.
Standardization of EDTA solution: Rinse and fill the burette with EDTA solution. Pipette out 50 mL of standard hard water in a conical flask. Add 10-15 mL of buffer solution and 4 to 5 drops indicator. Titrate with EDTA solution till wine-red colour changes to clear blue. Let volume used by V1 mL.
Titration of Unknown Hard water: Titrate 50 mL of water sample just in Step-5. let volume used be V2 mL.
Titration of Permanent hardness: take 250 mL of the water sample in a large beaker. Boil it. Till the volume is reduced to about 50 mL, filter it, wash the precipitate with distilled water, collecting filtrate and washing in a 250 mL measuring flask. Finally makeup the volume to 250 mL with distilled water. Then, titrate 50 mL of boiled water sample just as in Step (5). Let volume used by V3 mL.
CALCULATION:
V1 mL of EDTA is consumed by 50 mL standard hardwater.
50 mL of standard hard water = V1 mL of EDTA
50 * 1 mg of CaCO3 = V1 mL of EDTA
1 mL of EDTA =50/V1 mg of CaCO3 eq.
Now 50 mL of given/ unknown hard water =V2 mL of EDTA
V2*50/V1 mg of CaCO3 eq.
1 Lt or 1000 mL of given hard water = 1000*V2/V1 mg of CaCO3 eq.
Total hardness of water = 1000*V2/V1 mg/L
1 mg/L = 1ppm so,
= 1000*V2/V1 ppm
Now 50 mL of boiled water = V3 mL of EDTA
= V3*50/V1 mg of CaCO3 eq.
1000 mL of boiled water = 1000*V3/V1 mg of CaCO3 eq.
= 1000*V3/V1 ppm
So,
Permanent hardness = 1000*V3/V1 ppm
Temporary hardness = [Total hardness – Permanent hardness]
= 1000*V2/V1 – 1000*V3/V1
= 1000[(V2-V3) V1] ppm
Total hardness of water = 1000*V2/V1 ppm
Permanent hardness = 1000*V3/V1 ppm
Temporary hardness = 1000[(V2-V3) V1] ppm
Advantages of EDTA method:
This method is definitely prep arable to the other methods, because of the
1. Greater accuracy
2. Convenience and 3. More rapid procedure
Analysis of water
1. ALKALINITY: Alkalinity of water is due to the presence of carbonates and bicarbonates of calcium and magnesium. So the carbonate and bicarbonates in water can be determined by titrating the solution with standard acid like N/50 H2SO4 using first Phenolphthalein and later methyl orange as an indicator conversion of carbonates into bicarbonates occur.
In the II step to the solution using methyl orange as indicator (pH 4.2 to 5.4) bicarbonates present in the water and bicarbonates formed from carbonates gets neutralized.
The alkalinity of water is attributed to the presence of the
Caustic alkalinity (due to hydroxide & carbonate ions)
Temporary hardness (due to Bicarbonate ions)
PROCEDURE: Pipette out 100 mL of the water sample in a clean titration flask. Add to it 2 to 3 drops of a phenolphthalein indicator. Run in N/50 H2SO4 from burette. Till the pink colour is just dis-charged. Then to the same pink colour reappears.
CALCULATION:
let volume of acid used to phenolphthalein endpoint = V1 mL
And
Extra volume of acid used to Methyl orange endpoint = V2 mL
Phenolphthalein alkalinity P= V1 x 50 x 1000000 = 10 V1 ppm
50 x 100 x 1000
Methyl orange alkalinity M = (V1+V2) x 50 x 1000000 = 10(V1+V2) ppm
50 x 100 x 1000
CALCULATION OF ALKALINITY OF WATER:
ALKALINITY
OH- (ppm)
CARBONATE ion (ppm)
Bicarbonate ion (ppm)
P=0
P=1/2 M
P<1/2 M
P>1/2 M
P=M
0
0
0
(2P-M)
P=M
0
2P
2P
2(M-P)
0
M
O
(M-2P)
0
0
2.ESTIMATION OF CHLORIDES IN WATER
The chloride ions are present in water in the form of one or more of the compounds like CaCl2, MgCl2, NaCl etc. The estimation of chloride ions is generally made by titrating the water sample against a standard solution of silver nitrate using potassiumchromate as indicator. The added silvernitrate precipitate chloride ions as white ppt of silverchoride, as per the following chemical reactions..
NaCl + AgNO3 ------à AgCl + NaNO3
Cl- + Ag+ ------à AgCl
When all the chlorine ions are removed as AgCl ppt, the excess drop of silvernitrate reacts with potassiumchromate forming silverchromate, which is red in colour.
K2CrO4 + 2AgNO3 ----à Ag2CrO4 + 2KNO3
CrO4-2 + 2Ag+ ----à Ag2CrO4
The endpoint is therefore the change in colour from bright yellow due to brick red colour due to formation of Ag2CrO4 indicator. Then titrate it against N/50 AgNO3 solution till the colour changes from yellow to permanent brick-red.
Calculation:
Let volume of N/50 AgNO3 used = V mL
50 x Normality of free chlorine = V x [N/50]
Strength of free chlorine = V x 35.5 x 1000000
25 x 100 x 1000
= 14.2 x V ppm.
3. DISSOLVED OXYGEN:
The oxidation of KI by dissolved Oxygen determines the amount of dissolved Oxygen in water. Iodine is titrated with standard sodiumthiosulphate solution using starch as final indicator. The dissolved molecular Oxygen in water does not react with KI. So, an oxygen carrier is used to bring about the reaction between Oxygen and KI. Hence manganesehydroxide is produced as a result of the reaction of KOH and MnSO4.
2KOH + MnSO4 ---à Mn(OH)2 + K2SO4
2Mn(OH)2 + O2 ---à 2 MnO(OH)2
MnO(OH)2 + H2SO4 ---à MnSO4 + 2H2O + [O]
2KI + H2SO4 + [O] ---à K2SO4 + H2O + I2
2Na2S2O3 + I2 ------à 2NaI + Na2S4O6
The presence of sulphates, nitrates, etc. in rain water gives wrong results in determination of dissolved Oxygen in water because these ions also liberates Iodine from KI. In order to prevent the liberation of Iodine solution from nitrate, If present in water, sodium-azide (NaN3) is added to the alkaline Iodide solution. This reacts with nitrates and gives..
2NaN3 + H2SO4 --à 2HN3 + Na2SO4
HNO2 + HN3 -----à N20 + N2 + H2O
PROCEDURE: 250 mL of water is taken in bottle preventing it to contact with air. 2 mL of Manganoussulphate solution is added by means of pipette, and also 2 mL of alkalineiodine solution is also added. Shake the bottle thoroughly, and repeat the process thrice. The ppt is allowed to settle half-way and mix again. Then add to it Conc. H2SO4 insert the stopper and shake the bottle again allow the yellow solution to stand for 5 min. Withdraw 100 mL of solution and titrate it against N/100 solution (Na2S2O3 solution) using freshly prepared starch as indicator. The endpoint will be the disappearance of blue- colour. When the volume of Thiosulphate is V mL then dissolved oxygen content in water is 8 V ppm.
Methods of Treatment of Water for Domestic Purposes-
SEDIMENTATION: Sedimentation is the process of removing large suspended particles at the bottom of the reservoir. Which are collected due to gravity.
Sedimentation is the process of allowing to stand undisturbed in big tanks, about 5M deep, when most of the suspended particles settle down at the bottom, due to the force of gravity. The clear supernatant water is then drawn from tank with the help of pumps. The retention period in a sedimentation tank ranges from 2-6 hours.
In order to carry out the sedimentation process successfully, some of the chemicals are added to water before sedimentation. These chemicals are called Coagulants.
The coagulants when added to water, form an insoluble gelatinous, flocculent precipitates, which descends through the water and mixes-up into very fine suspended impurities forming bigger impurities forming bigger impurities or flocs, which easily settles down.
Some of the coagulants used for sedimentation process are alum, ferrous sulphate, sodium aluminate etc.
Chemical coagulants:
Alum: K2SO4. Al2(SO4)3. 24H2O
Alum is the most widely used in water treatment plants.
Al2(SO4)3 + Ca(HCO3)2 à 2Al(OH)3 + 3CaSO4 + 6CO2
NaAlO2 + 2H2O à Al(OH)3 + NaOH
FeSO4 + Mg(HCO3)2 à Fe(OH)2 + MgCO3 + CO2
2) FILTRATION: It is the process of removing colloidal matter and most of the bacteria, micro-organisms etc, by passing water through a bed of fine sand and other proper-sized granular materials. Filtration is carried out by using Sand-filter.
Sand filter is used for removing suspended particles, micro-organisms, bacteria etc., by passing through a finely graded sand bed. All the colloidal matter and sediments gets accumulated and water is purified.
OPERATION OF SAND FILTER: In sand filtration process, water is passed through different sized sand beds like fine, coarse and gravel beds. The suspended particles are unable to pass through the gaps in the sand bed, because of their size. Water is first passed through a fine sand-bed. The suspended particles are first collected and gets clogged in fine sand and then passed through sand-bed. Here colloidal matter gets collected and the water is free from sediments. After passing of water from coarse sand bed it enters through a bed of medium sized stones called gravel or gravel-bed. This is the last bed where water is filtered completely and the filtered wate is collected at the filter outlet. During the filtration process the fine pores of sand bed gets seized and clogged. In order to continue the filtration process, 2-3 cm of fine sand at the top is scrapped, replaced and leveled with makeup sand.
3) REMOVAL OF MICROORGANISMS:
Water after passing through Sedimentation, Coagulation and Filtration operations still contains a small quantity of Pathogenic bacteria.
Disinfection: The process of destroying/killing the disease producing Bacteria, microorganisms, etc. from the water and making it safe for use, is called disinfection.
Disinfectants: the chemicals or substances which are added to eater for killing the Bacteria.
The disinfection of water can be carried out by following methods:
A). BOILING: Water for 10-15 minutes, all the disease-producing bacteria are killed and water becomes safe for use.
B) BLEACHING POWER: It is used to purify the drinking water from micro organisms. The purification process is achieved by dissolving 1 kg of bleaching powder in 1000 kiloliters of water. This dissolved water solution is left undisturbed for many hours. When bleaching powder is mixed with water, the result of chemical reaction produces a powerful Germicide called Hypochlorous acid. The presence of chlorine in the bleaching powder produces disinfecting action, kills germs and purifies the drinking water effectively.
CaOCl2 + H2O -à Ca(OH)2 + Cl2
H2O + Cl2 à HCl + HOCl
HOCl + Germs à Germs are killed à water purified.
C) CHLORINE: Chlorination is the process of purifying the drinking water by producing a powerful Germicide like Hypochlorous acid. When this Chlorine is mixed with water it produces Hypochlorous acid which kills the germs present in water.
H2O + Cl2 à HOCl + HCl
Chlorine is basic (means pH value is more than 7) disinfectant and is much effective over the germs. Hence Chlorine is widely used all over the world as a powerful disinfectant. Chlorinator is an apparatus, which is used to purify the water by chlorination process.
BREAK-POINT CHLORINATION:
Break-point chlorination is a controlled process. In this process suitable amount of Chlorine is added to water. In order to kill all the bacteria present in water, to oxidize the entire organic matter and to react with free ammonia the chlorine required should be appropriate. Break-point determines whether chlorine is further added or not. By Chlorination, organic matter and disease producing Bacteria are completely eliminated which are responsible for bad taste and bad odour in water.
When certain amount of Chlorine is added to the water, it leads to the formation of Chloro-organic compounds and chloramines. Addition of some more chlorine leads to destruction of chloro-organic compounds and chloramines. The point at which free residual chlorine begins to appear is terms as “Break-point”.
OZONISATION: Ozone is powerful disinfectant and is readily dissolved in water. Ozone being unstable decomposes by giving Nascent Oxygen which is capable of destroying the Bacteria. This Nascent Oxygen removes the colour and taste of water and oxidizes the organic matter present in water. O3 à O2 + [O]
1. Introduction
Water is nature’s most wonderful, abundant and useful compound.
Water is not only essential for the lives of animals and plants. But also occupies a unique position in industries.
Water is also used a coolant in power and chemical plants.
It is widely used in drinking, bathing, sanitary, washing, irrigation, fire-fights, air-conditioning and also production of industrial materials.
2. Effect of water on rocks and minerals:
Dissolution: some mineral constituents of rocks such as NaCl and CuSO4.2H2O readily dissolve in water.
Hydration: Some minerals are easily hydrated with the consequent increase in volume leading to disintegration of the rocks in which these minerals are present. Eg.
CaSO4 Hydration à CaSO4.2H2O (Gypsum)
Mg2SiO4 àMg2SiO4.XH2O (Serpentine)
Effect of dissolved oxygen: This leads to oxidation and Hydration
Fe3O4 àFe2O3 à3 Fe2O3.2H2O(Limonite)
2FeS2 + 7O2 + 2H2O à2FeSO4 + 2H2SO4
Effect of dissolved CO2: Water containing dissolved CO2 converts the insoluble carbonate of calcium, Magnesium and iron into their relatively soluble bicarbonates.
CaCO3 + H2O + CO2 --------------- Ca(HCO3)2
3. Types of Impurities present in water:
The natural water is usually contaminated by different types of impurities.
They are mainly 3 types..
Physical impurities
Chemical impurities
Biological impurities
1. PHYSICAL IMPURITIES:
a. Colour: Colour in water is caused by metallic substances like salts.
b. Turbidity: is due to the colloidal, extremely fine suspensions such as insoluble substances like clay, slit, and micro-organisms.
c. Taste: presence of dissolved minerals in water produces taste. Bitter taste can be due to the presence of Fe, Al, Mn, Sulphates, lime. Soap taste can be due to the presence of large amount of sodium bicarbonate.
d. Odour: In water is undesirable for domestic as well as industrial purpose.
2. CHEMICAL IMPURITIES:
a. In-organic-organic chemicals:
Inorganic chemicals:
1. Cations: Ca+2, Mg+2, Na+, K+, Fe+2, Al+3, Zn+2, Cu+2
2. Anions: Cl-, SO-2, NO3-, HCO3-, F-, NO2-
Organic chemicals: dyes, paints, petroleum products, pesticides, detergents, drugs textile materials, other organic related materials.
Acidity: surface and ground water attains acidity from industrial wastes like acid, mine, drainage, pickling liquors, usually acidity caused by the presence of free CO2, mineral acids, and weakly dissociated acids.
b. Gases: All natural waters contains dissolved atmosphere CO2, O2, NH3 gases, pollutant and sewage water contains nitrogen in the form of nitrogenous organic compounds and urea , which are partially converted into NH3.
c. Mineral matters: have organs from rocks and industrial effluents. These include mineral acids, Ca+2, Mg+2, Na+, K+, Fe+2, Al+3, Zn+2, Cu+2, Mn+2, Cl-, SO-2, NO3-, HCO3-, F-, NO3-, SiO2
3.BIOLOGICAL IMPURITES:
Biological impurities are Algae, pathogenic bacteria, fungi, viruses, pathogens, parasite-worms, etc. the source of these contamination is discharge of domestic and sewage wastes, excreta, etc.
A. Micro-organisms: algae, fungi, viruses, etc.
B. Water bodies in water include 1. Bacteria, organisms’ inhabitating the bottom sludge, and 2. Organisms and planktons developed at the water surfaces. These are inhibitated by different groups of worms like flat worms, hair worms, tiny roundworms, oligochetes, etc.
HARDNESS OF WATER
Hardness of water defined as which prevent the lathering of soap.This is due to presence of in water of certain salts of Ca+2, Mg+2 and other Heavy metals dissolved in it. Soaps are Sodium or Potassium salts of higher fatty acids like Oleic acid or Palmitic acid or stearic acids. C17H35COONaTemporary Hardness is mainly caused by Bicarbonates of Calcium, Magnesium and other heavy metals.
SOAP with HARDWATER REACTIONS
2C17H35COONa+CaCl2---- à(C17H35COO)2Ca+2NaCl
2C17H35COONa+MgSO4--à(C17H35COO)2Mg+Na2SO4
Hardness of water is mainly TWO types1.Temporary Hardness2. Permanent Hardness
1. Temporary Hardness mainly caused by the presence of dissolved Bicarbonates of Calcium, Magnesium and other Heavy metals and the carbonate of Iron.
Temporary hardness of water mainly responsible salts are…
1. Calcium bicarbonate Ca(HCO3)2
2. Magnesium bicarbonate Mg(HCO3)2
When bicarbonates are decomposed a, yielding insoluble carbonates or hydroxides, which are deposited as a crust at the bottom of vessel.
Temporary Hardness can be largely removed by mere Boiling of water.
Temporary Hardness is also known as….
Carbonate Hardness Alkaline Hardness
2. Permanent harness: It is due to the presence of dissolved Chlorides and sulphates of Calcium, Magnesium, Iron and other metals Permanent hardness responsible salts are….
CaCl2, MgCl2, CaSO4, MgSO4, FeSO4, Al2(SO4)3
Permanent Hardness cannot removed by boiling but it can be removed by the use of chemical agents.
Permanent Hardness also known as….
Non-Carbonate HardnessNon-Alkaline
Heat
• Ca(HCO3) 2 ---àCaCO3+H2O+CO2
Calcium bicarbonate
Heat
• Mg(HCO3)2 ----àMg(OH)2+2CO2
Magnesium bicarbonate
Explain the terms Temporary and permanent hardness of water.
Write a short note on hardness of water.
Hardness Distinguish between Temporary and Permanent Hardness of water.
DISADVANTAGES OF HARDWATER:
1. In Domestic use:
WASHING: hardwater, when used for washing purposes, does not producing lather freely with soap. As a result cleansing quality of soap is decreased and a lot of it is wasted. Hardwater reacts with soap it produces sticky precipitates of calcium and magnesium soaps. These are insoluble formations.
Bathing: hardwater does not lather freely with soap solution, but produces sticky scum on the bath-tub and body. Thus, the cleansing quality of soap is depressed and a lot o it is wasted.
Cooking: the boiling point of water is increased because of presence of salts. Hence more fuel and time are required for cooking.
Drinking: hardwater causes bad effects on our digestive system. Moreover, the possibility of forming calciumoxalate crystals in urinary tracks is increased.
2. INDUSTRIAL USE:
Textile Industry: Hardwater causes wastage of soap. Precipitates of calcium and magnesium soaps adhere to the fabrics and cause problem.
Sugar Industry: water containing sulphates, nitrates, alkali carbonates. Etc. if used in sugar refining, causes difficulties in the crystallization of sugar. Moreover, the sugar so produced may be de-liquescent.
Dyeing Industry: The dissolved salts in hardwater may reacts with costly dyes forming precipitates.
Paper Industry: Calcium, magnesium, Iron salts in water may affect the quality of paper.
Pharmaceutical Industry: Hardwater may cause some undesirable products while preparation of pharmaceutical products.
Concrete Making: Water containing chlorides and sulphates, if used for concrete making, affects the hydration of cement and the final strength of the hardened concrete.
Laundry: Hardwater, if used in laundry, causes much of the soap used in washing to go as waste Iron salts may even cause coloration of the clothes.
3. IN STEAM GENERATION IN BOILERS:
For steam generation, boilers are almost invariably employed. If the hardwater is fed directly to the boilers, there arise many troubles such as..
Scale and sludge formation: the hardness of water fed to the boiler may cause scale and sludge formation.
Corrosion: Hardwater may cause caustic embrittlement which is a type of boiler corrosion.
Priming and Foaming: Hardwater used in boiler causes priming and foaming which results in the formation of wet stream.
Caustic embrittlement
ESTIMATION OF HARDNESS BY EDTA METHOD:
The hardness of water is not a pollution parameter but indicate the eater quality in terms of Calcium and magnesium expressed in terms of CaCO3.
The analysis is done by complex metric titration using standard EDTA and EBT as an indicator.
EDTA is Ethylene diamaine tetra acetic acid whose structural formulae is
EDTA can form 4 to 6 dative bonds with cations like Calcium Magnesium.
PRINCIPLE: In this complex metric Titration, the water sample is titrated with standard solution of Di sodium salt of EDTA using EBT indicator.
PROCEDURE: To 50 mL of sample add 10-15 mL buffer solutions adjust the pH about 10 and then warm the solution. Add 2 drops o Eriochrome Block-T and it is titrated with 0.01 M EDTA solution changes its colour from red-wine to blue.
REACTIONS INVOLVED: EBT indicator when added to hard water at pH = 10, forms weak complexes with calcium and magnesium present in hard water. It results in the formation of Ca-EBT or Mg-EBT complexes which is wine-red, these are unstable.
During titration with EDTA, calcium first reacts to form relatively stable followed by magnesium to give Mg-EDTA+2 complex releasing the free indicator(blue) the colour changes from wine-red to blue at the endpoint.
Various steps involved in this method are…
Preparation of standard hard water: dissolve 1g of pure, dry CaCO3 in minimum quantity of dil.HCl and then evaporate the solution to dryness on a water bath. Dissolve the residue in distilled water to make 1 Litre solution. Each mL of this solution thus contains 1mg of CaCO3 equalent hardness.
1 mL hard water solution = 1mg of CaCO3 equalent hardness.
Preparation of EDTA solution: Dissolve 4 g of pure EDTA crystals + 0.lg MgCl2 in 1 Litre of distilled water.
Preparation of Indicator (EBT): Dissolve 0.5 g of Eriochrome Black-T in 100mL alcohol.
Preparation of Buffer solution: add 67.5g of NH4Cl to 570 mL of Con. Ammonia solution and then dilute with distilled water to 1 Litre.
Standardization of EDTA solution: Rinse and fill the burette with EDTA solution. Pipette out 50 mL of standard hard water in a conical flask. Add 10-15 mL of buffer solution and 4 to 5 drops indicator. Titrate with EDTA solution till wine-red colour changes to clear blue. Let volume used by V1 mL.
Titration of Unknown Hard water: Titrate 50 mL of water sample just in Step-5. let volume used be V2 mL.
Titration of Permanent hardness: take 250 mL of the water sample in a large beaker. Boil it. Till the volume is reduced to about 50 mL, filter it, wash the precipitate with distilled water, collecting filtrate and washing in a 250 mL measuring flask. Finally makeup the volume to 250 mL with distilled water. Then, titrate 50 mL of boiled water sample just as in Step (5). Let volume used by V3 mL.
CALCULATION:
V1 mL of EDTA is consumed by 50 mL standard hardwater.
50 mL of standard hard water = V1 mL of EDTA
50 * 1 mg of CaCO3 = V1 mL of EDTA
1 mL of EDTA =50/V1 mg of CaCO3 eq.
Now 50 mL of given/ unknown hard water =V2 mL of EDTA
V2*50/V1 mg of CaCO3 eq.
1 Lt or 1000 mL of given hard water = 1000*V2/V1 mg of CaCO3 eq.
Total hardness of water = 1000*V2/V1 mg/L
1 mg/L = 1ppm so,
= 1000*V2/V1 ppm
Now 50 mL of boiled water = V3 mL of EDTA
= V3*50/V1 mg of CaCO3 eq.
1000 mL of boiled water = 1000*V3/V1 mg of CaCO3 eq.
= 1000*V3/V1 ppm
So,
Permanent hardness = 1000*V3/V1 ppm
Temporary hardness = [Total hardness – Permanent hardness]
= 1000*V2/V1 – 1000*V3/V1
= 1000[(V2-V3) V1] ppm
Total hardness of water = 1000*V2/V1 ppm
Permanent hardness = 1000*V3/V1 ppm
Temporary hardness = 1000[(V2-V3) V1] ppm
Advantages of EDTA method:
This method is definitely prep arable to the other methods, because of the
1. Greater accuracy
2. Convenience and 3. More rapid procedure
Analysis of water
1. ALKALINITY: Alkalinity of water is due to the presence of carbonates and bicarbonates of calcium and magnesium. So the carbonate and bicarbonates in water can be determined by titrating the solution with standard acid like N/50 H2SO4 using first Phenolphthalein and later methyl orange as an indicator conversion of carbonates into bicarbonates occur.
In the II step to the solution using methyl orange as indicator (pH 4.2 to 5.4) bicarbonates present in the water and bicarbonates formed from carbonates gets neutralized.
The alkalinity of water is attributed to the presence of the
Caustic alkalinity (due to hydroxide & carbonate ions)
Temporary hardness (due to Bicarbonate ions)
PROCEDURE: Pipette out 100 mL of the water sample in a clean titration flask. Add to it 2 to 3 drops of a phenolphthalein indicator. Run in N/50 H2SO4 from burette. Till the pink colour is just dis-charged. Then to the same pink colour reappears.
CALCULATION:
let volume of acid used to phenolphthalein endpoint = V1 mL
And
Extra volume of acid used to Methyl orange endpoint = V2 mL
Phenolphthalein alkalinity P= V1 x 50 x 1000000 = 10 V1 ppm
50 x 100 x 1000
Methyl orange alkalinity M = (V1+V2) x 50 x 1000000 = 10(V1+V2) ppm
50 x 100 x 1000
CALCULATION OF ALKALINITY OF WATER:
ALKALINITY
OH- (ppm)
CARBONATE ion (ppm)
Bicarbonate ion (ppm)
P=0
P=1/2 M
P<1/2 M
P>1/2 M
P=M
0
0
0
(2P-M)
P=M
0
2P
2P
2(M-P)
0
M
O
(M-2P)
0
0
2.ESTIMATION OF CHLORIDES IN WATER
The chloride ions are present in water in the form of one or more of the compounds like CaCl2, MgCl2, NaCl etc. The estimation of chloride ions is generally made by titrating the water sample against a standard solution of silver nitrate using potassiumchromate as indicator. The added silvernitrate precipitate chloride ions as white ppt of silverchoride, as per the following chemical reactions..
NaCl + AgNO3 ------à AgCl + NaNO3
Cl- + Ag+ ------à AgCl
When all the chlorine ions are removed as AgCl ppt, the excess drop of silvernitrate reacts with potassiumchromate forming silverchromate, which is red in colour.
K2CrO4 + 2AgNO3 ----à Ag2CrO4 + 2KNO3
CrO4-2 + 2Ag+ ----à Ag2CrO4
The endpoint is therefore the change in colour from bright yellow due to brick red colour due to formation of Ag2CrO4 indicator. Then titrate it against N/50 AgNO3 solution till the colour changes from yellow to permanent brick-red.
Calculation:
Let volume of N/50 AgNO3 used = V mL
50 x Normality of free chlorine = V x [N/50]
Strength of free chlorine = V x 35.5 x 1000000
25 x 100 x 1000
= 14.2 x V ppm.
3. DISSOLVED OXYGEN:
The oxidation of KI by dissolved Oxygen determines the amount of dissolved Oxygen in water. Iodine is titrated with standard sodiumthiosulphate solution using starch as final indicator. The dissolved molecular Oxygen in water does not react with KI. So, an oxygen carrier is used to bring about the reaction between Oxygen and KI. Hence manganesehydroxide is produced as a result of the reaction of KOH and MnSO4.
2KOH + MnSO4 ---à Mn(OH)2 + K2SO4
2Mn(OH)2 + O2 ---à 2 MnO(OH)2
MnO(OH)2 + H2SO4 ---à MnSO4 + 2H2O + [O]
2KI + H2SO4 + [O] ---à K2SO4 + H2O + I2
2Na2S2O3 + I2 ------à 2NaI + Na2S4O6
The presence of sulphates, nitrates, etc. in rain water gives wrong results in determination of dissolved Oxygen in water because these ions also liberates Iodine from KI. In order to prevent the liberation of Iodine solution from nitrate, If present in water, sodium-azide (NaN3) is added to the alkaline Iodide solution. This reacts with nitrates and gives..
2NaN3 + H2SO4 --à 2HN3 + Na2SO4
HNO2 + HN3 -----à N20 + N2 + H2O
PROCEDURE: 250 mL of water is taken in bottle preventing it to contact with air. 2 mL of Manganoussulphate solution is added by means of pipette, and also 2 mL of alkalineiodine solution is also added. Shake the bottle thoroughly, and repeat the process thrice. The ppt is allowed to settle half-way and mix again. Then add to it Conc. H2SO4 insert the stopper and shake the bottle again allow the yellow solution to stand for 5 min. Withdraw 100 mL of solution and titrate it against N/100 solution (Na2S2O3 solution) using freshly prepared starch as indicator. The endpoint will be the disappearance of blue- colour. When the volume of Thiosulphate is V mL then dissolved oxygen content in water is 8 V ppm.
Methods of Treatment of Water for Domestic Purposes-
SEDIMENTATION: Sedimentation is the process of removing large suspended particles at the bottom of the reservoir. Which are collected due to gravity.
Sedimentation is the process of allowing to stand undisturbed in big tanks, about 5M deep, when most of the suspended particles settle down at the bottom, due to the force of gravity. The clear supernatant water is then drawn from tank with the help of pumps. The retention period in a sedimentation tank ranges from 2-6 hours.
In order to carry out the sedimentation process successfully, some of the chemicals are added to water before sedimentation. These chemicals are called Coagulants.
The coagulants when added to water, form an insoluble gelatinous, flocculent precipitates, which descends through the water and mixes-up into very fine suspended impurities forming bigger impurities forming bigger impurities or flocs, which easily settles down.
Some of the coagulants used for sedimentation process are alum, ferrous sulphate, sodium aluminate etc.
Chemical coagulants:
Alum: K2SO4. Al2(SO4)3. 24H2O
Alum is the most widely used in water treatment plants.
Al2(SO4)3 + Ca(HCO3)2 à 2Al(OH)3 + 3CaSO4 + 6CO2
NaAlO2 + 2H2O à Al(OH)3 + NaOH
FeSO4 + Mg(HCO3)2 à Fe(OH)2 + MgCO3 + CO2
2) FILTRATION: It is the process of removing colloidal matter and most of the bacteria, micro-organisms etc, by passing water through a bed of fine sand and other proper-sized granular materials. Filtration is carried out by using Sand-filter.
Sand filter is used for removing suspended particles, micro-organisms, bacteria etc., by passing through a finely graded sand bed. All the colloidal matter and sediments gets accumulated and water is purified.
OPERATION OF SAND FILTER: In sand filtration process, water is passed through different sized sand beds like fine, coarse and gravel beds. The suspended particles are unable to pass through the gaps in the sand bed, because of their size. Water is first passed through a fine sand-bed. The suspended particles are first collected and gets clogged in fine sand and then passed through sand-bed. Here colloidal matter gets collected and the water is free from sediments. After passing of water from coarse sand bed it enters through a bed of medium sized stones called gravel or gravel-bed. This is the last bed where water is filtered completely and the filtered wate is collected at the filter outlet. During the filtration process the fine pores of sand bed gets seized and clogged. In order to continue the filtration process, 2-3 cm of fine sand at the top is scrapped, replaced and leveled with makeup sand.
3) REMOVAL OF MICROORGANISMS:
Water after passing through Sedimentation, Coagulation and Filtration operations still contains a small quantity of Pathogenic bacteria.
Disinfection: The process of destroying/killing the disease producing Bacteria, microorganisms, etc. from the water and making it safe for use, is called disinfection.
Disinfectants: the chemicals or substances which are added to eater for killing the Bacteria.
The disinfection of water can be carried out by following methods:
A). BOILING: Water for 10-15 minutes, all the disease-producing bacteria are killed and water becomes safe for use.
B) BLEACHING POWER: It is used to purify the drinking water from micro organisms. The purification process is achieved by dissolving 1 kg of bleaching powder in 1000 kiloliters of water. This dissolved water solution is left undisturbed for many hours. When bleaching powder is mixed with water, the result of chemical reaction produces a powerful Germicide called Hypochlorous acid. The presence of chlorine in the bleaching powder produces disinfecting action, kills germs and purifies the drinking water effectively.
CaOCl2 + H2O -à Ca(OH)2 + Cl2
H2O + Cl2 à HCl + HOCl
HOCl + Germs à Germs are killed à water purified.
C) CHLORINE: Chlorination is the process of purifying the drinking water by producing a powerful Germicide like Hypochlorous acid. When this Chlorine is mixed with water it produces Hypochlorous acid which kills the germs present in water.
H2O + Cl2 à HOCl + HCl
Chlorine is basic (means pH value is more than 7) disinfectant and is much effective over the germs. Hence Chlorine is widely used all over the world as a powerful disinfectant. Chlorinator is an apparatus, which is used to purify the water by chlorination process.
BREAK-POINT CHLORINATION:
Break-point chlorination is a controlled process. In this process suitable amount of Chlorine is added to water. In order to kill all the bacteria present in water, to oxidize the entire organic matter and to react with free ammonia the chlorine required should be appropriate. Break-point determines whether chlorine is further added or not. By Chlorination, organic matter and disease producing Bacteria are completely eliminated which are responsible for bad taste and bad odour in water.
When certain amount of Chlorine is added to the water, it leads to the formation of Chloro-organic compounds and chloramines. Addition of some more chlorine leads to destruction of chloro-organic compounds and chloramines. The point at which free residual chlorine begins to appear is terms as “Break-point”.
OZONISATION: Ozone is powerful disinfectant and is readily dissolved in water. Ozone being unstable decomposes by giving Nascent Oxygen which is capable of destroying the Bacteria. This Nascent Oxygen removes the colour and taste of water and oxidizes the organic matter present in water. O3 à O2 + [O]
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