DEPARTMENT OF CHEMISTRY
24CYT13-
Chemistry for Electronics & Computer Systems
Dr.A.Geetha
Associate Professor
Department of Chemistry
Kongu Engineering College
Sources of Water
Rain Water - Pure but contaminated with gases
River Water - High dissolved salts moderate organics
Lake Water - Const. composition but high organics
Sea Water - High salinity, pathogens, organics
Well Water - Crystal clear but high dissolved salts and high purity from organics
Classification of Impurities in water
O2, H2S, NH3, )
MAJOR IMPURITIES OF WATER
Ionic and dissolved
Cationic Calcium
Magnesium
Anionic Bicarbonate
Carbonate Hydroxide
Nonionic and undissolved Turbidity, silt, mud, dirt and
other suspended matter
Gases CO2
H2S NH3 CH4 O2
Sodium Potassium Ammonium Iron Manganese
Sulfate Chloride Nitrate Phosphate
Color, Plankton Organic matter, Colloidal silica, Microorganisms, Bacteria
Types of water
Hardness of Water
Temporary Hardness
– Temporary Hardness is caused by the presence of dissolved bicarbonate of calcium, magnesium and other heavy metals and the carbonate of iron. It is mostly destroyed by more boiling of water, when bicarbonates are decomposed yielding insoluble carbonates.
Ca(HCO 3) 2
Calcium bicarbonate
Mg( HCO 3) 2
Heat
CaCO3 + H2O + CO2
Calcium Carbonate
Mg( OH) 2 + 2CO2
Magnesium hydroxide
Magnesium Bicarbonate
Heat
– Calcium/Magnesium Carbonates thus formed being almost insoluble, are deposited as a scale at the bottom of vessel, while carbon dioxide escapes out.
Permanent Hardness
Non Carbonate Hardness is due to the presence of chlorides, sulfates of calcium, Magnesium.
2C17H35COONa + CaCl2
Sodium
(sodium soap)
2C17H35COONa + MgSO4
Sodium
stearate
stearate (sodium soap)
Hardness
Hardness
(C17H35COO)2Ca + 2NaCl
Calcium stearate (Insoluble)
(C17H35COO)2Mg + Na2SO4
Magnesium stearate (Insoluble)
Units of Hardness
CaCO3 equivalent hardness
Calcium carbonate equivalent =
Weight of hardness producing substance
X Molecular weight of Ca CO 3
Molecular weight of hardness producing substances
Problem 1
Calculate the calcium carbonate equivalent hardness of a water sample containing 204mg of CaSO4 per litre
Solution:
Calcium carbonate equivalent hardness =
204 100
136
= 150 mg of Ca CO 3
= 150 ppm
Calcium carbonate equivalence conversion during hardness calculation
Hardness producing substance
Ca(HCO3)2 162
Mg(HCO3)2 146
CaSO4 136
CaCl 111
120
95
100
84
44
61
17
60
CaCO3 Equivalent
The amount of MgSO4 = 240 mg/l
We Know that the molecular weight of MgSO4 = 120
Thus the amount equivalent to CaCO3= 240 x 100 = 200 mg/l
120
Calcium carbonate equivalent =
Weight of hardness producing substance
X Molecular weight of Ca CO 3
Molecular weight of hardness producing substances
Problem
Solution: In order to calculate the hardness of the sample, these salts have to be converted into their equivalents of CaCO3
Temporary Hardness = 20 mg/l
Permanent Hardness = 20 + 20 = 40 mg/l
Total Hardness = (Temp+ Permanent ) hardness = 20 + 40 = 60 mg/l
Molecular Formula | Molecular weight | Quantity Present | Equivalent of CaCO3 |
CaSO4 | 136 | 27.2 | 27.2 x 100 = 20 136 |
Mg(HCO3)2 | 146 | 29.2 | 29.2 x 100 = 20 146 |
MgSO4 | 120 | 24 | 24 X 100 = 20 120 |
1. A water sample on analysis gives the following data:
Mg(HCO3) = 292 mg/l, Ca(HCO3)= 162 mg/l, CaSO4= 136 mg/l, MgCl2= 190 mg/l and
CaCl2=222 mg/l calculate the temporary, permanent and Total hardness of the water sample.
2. Calculate the carbonate and non-carbonate hardness of a water sample containing
Mg(HCO3) = 9.3 mg/l, Ca(HCO3) = 20.50 mg/l, CaSO4= 26 mg/l, MgCl2=10 mg/l and NaCl=60mg/l.
Estimation of Hardness of Water by EDTA Method
Burette solution - Unknown EDTA solution
Pipette solution - 20 ml of Standard Hard water
Condition - Room Temp.
Reagents to
be added - 5 ml of buffer solution
Indicator - 2 drops of EBT (Eriochrome Black-T)
End point - Colour change from wine red to steel blue
Let the volume of EDTA consumed be V1 ml.
V1 ml of EDTA consumes 20 ml of
std. hard water = 20 * 1 mg of CaCO3 eq. hardness
1ml of EDTA consumes = 20/V1 mg of CaCO3 eq. hardness
(since 1ml of standard hard water = 1ml of CaCO3)
(ii) Estimation of Total Hardness:
Calculation
Burette solution - Standardised EDTA solution
Pipette solution - 20 ml of Sample Hard water
Condition - Room Temp.
Reagents to
be added - 5 ml of buffer solution
Indicator - 2 drops of EBT (Eriochrome Black-T)
End point - Colour change from wine red to steel blue
(iii) Estimation of Permanent Hardness:
Burette solution - Standardised EDTA solution
Pipette solution - 20 ml of Boiled Hard water
Condition - Room Temp.
Reagents to
be added - 5 ml of buffer solution
Indicator - 2 drops of EBT (Eriochrome Black-T)
End point - Colour change from wine red to steel blue
Alkalinity in water analysis:
In water analysis it is often desirable to know the kinds and amounts of the various of alkalinity present in water.
The major portion of alkalinity in natural water is caused by presence of bicarbonates.
CaCO3 + CO2+H2O→Ca(HCO3)2
Classification:
1. Bicarbonate alkalinity
2. Carbonate alkalinity
3. Hydroxide alkalinity
Experimental Determination:
Principle
OH- + H+ → H2O (phenolphthalein end point)
CO32- + H+ →HCO3- (phenolphthalein end point)
HCO3- + H+ → H2O + CO2
The results are summarized in table from which the amount of OH-,CO32-, HCO3- present in water sample can be calculated
Alkalinity | OH- (ppm) | CO32- (ppm) | HCO3- (ppm) |
P=0 | 0 | 0 | M |
P=1/2M | 0 | 2P | 0 |
P<1/2 M | 0 | 2P | (M-2P) |
P>1/2 M | (2P-M) | 2(M-P) | 0 |
P=M | M | 0 | 0 |
Short Procedure
Standardisation of acid
Burette solution - Unknown H2SO4
Pipette solution - 20 ml of standard sodium hydroxide
Condition - Room Temp.
Indicator - 2 drops of phenolphthalein
End Point - colour change from pink to colourless
All the three ions cannot exists together. OH- and HCO3 cannot be present at the same time together, because
OH- + HCO3- → CO32- + H2O
Procedure :
Burette solution - standard H2SO4
Pipette solution - 20 ml of water sample
Condition - Room Temp.
Indicator - 2 drops of phenolphthalein and 2 drops of methyl orange
End Point - pink to colourless (V1 ml) and yellow to reddish orange (V2 ml).
Calculation
Volume of acid used up to phenolphthalein end point = V1 ml
Normality of acid = N1
phenolphthalein alkalinity(P) in terms of calcium carbonate
equivalent =V1 X N1 /20 X 50 X 1000 mg/l
Additional volume of acid used up to methyl orange end point = V2 ml
Normality of acid = N1
methyl orange alkalinity (M) in terms of CaCO3 Equation = (V1+V2) X N1/20 X 50 X1000 mg/l
Then the calculation of OH-, CO2-3, HCO3- is made with the help of above table.
Industrial use:
(i) Textile industry: Hard water causes much of the soap (used in washing yarn, fabric etc.) to go as waste, because hard water cannot produce good quality of lather. Moreover, precipitated of calcium and magnesium soaps adhere to the fabrics. These fabrics, when dyed latter on, do not produce exact shades of color. Iron and manganese salts-containing water may cause coloured spots on fabrics, thereby spoiling their beauty.
(ii) Sugar industry: Water containing sulphates, nitrates, alkali carbonated, etc., if used in sugar refining, causes difficulties in the crystallization of sugar. Moreover, the sugar so-produced may be deliquescent.
(iii) Dyeing industry: The dissolved calcium, magnesium and iron salts in hard water may react with costly dyes, forming undesirable precipitated, which yields impure shades and give spots on the fabrics being dyed.
(iv) Paper industry: Calcium and magnesium salts tend to react with chemicals and other materials employed to provide a smooth and glossy (i.e., shining) finish to paper. Moreover, iron salts may even affect the colour of the paper being produced.
(v) Laundry: Hard water, 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.
(vi) 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.
(vii) Pharmaceutical industry: Hard water, if used for preparing pharmaceutical products (like drugs, injections, ointments, etc.) may produce certain undesirable products in them.
Disadvantages of using hard water in Industries
Boiler Feed Water
Essential requirements of Boiler Feed Water :- It should be free from
Boiler Troubles due to Hard Water
1.Sludge
Slimy loose precipitate called sludge suspended in water
water
Boiler wall
Sludge is a soft, loose and slimy precipitate formed within the boiler. It can be easily scrapped off with a wire brush.
It is formed at comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends.
It is formed by substances which have greater solubility's in hot water than in cold water, e. g. MgCO 3, MgCl2, CaCl2, MgSO 4 etc.,
Remedy: Sludges can be removed using wire brush or mild acid
1. Scale
Hard adherent coating on inner walls of boiler
water
Boiler wall
Scales are hard substances which sticks very firmly to the inner surfaces of the boiler wall.
Scales are difficult to remove even with the help of a hammer and chisel.
Examples: Ca SO 4, CaCO 3, Mg( OH) 2
Sludge | Scale |
1. Sludges are soft and non- adherent deposits. | 1. Scales are hard deposits which stick very firmly to the inner surface of boiler. |
2. Sludges can be removed easily. | 2. Scales are very difficult to remove. |
3. Sludges can transfer heat to some extent and is less dangerous. | 3. Scales are bad conductors of heat and are more dangerous. |
4. Sludges are formed by substances like MgCl2 and CaCl2. | 4. Scales are formed by substances like CaSO4 and Mg(OH)2. |
Reasons for formation of scale
1. Presence of Ca(HCO 3) 2 in low pressure boilers
Ca(HCO 3) 2 Ca CO 3 + H2O + CO2
Calcium bicarbonate Calcium Carbonate (scale)
Low pressure boilers but in high pressure boilers it is soluble by forming Ca(OH) 2
2. Presence of Ca SO 4 in high pressure boilers
| ToC 15 230 320 Cold water Super heated (scale) | | Solubility of CaSO 4 3200 ppm 15 ppm 27 ppm soluble water Insoluble | | |
3. | Presence | of | Mg Cl 2 in high temperature | boilers | |
Mg Cl2 + 2 H 2O
Magnesium chloride
Mg (OH)2
scale
+ 2HCl
Mg(OH)2 can also be generated by thermally decomposing Mg(HCO 3)2
4. Presence of SiO 2
It forms insoluble hard adherent CaSiO3 and MgSiO 3 as scales
Disadvantages of scale formation
1. Fuel wastage – scales have low thermal conductivity
2. Degradation of boiler material and increases of risk of accident
Remedies: Removal of scale
IV. Boiler corrosion
Degradation or destruction of boiler materials ( Fe) due to the chemical or electrochemical attack of dissolved gases or salts are called boiler corrosion
Boiler corrosion is of three types
1. Corrosion due to dissolved O 2
2. Corrosion due to dissolved CO 2
3. Corrosion due to acids formed by dissolved salts
1. Corrosion due to dissolved oxygen ( DO)
2 Fe + 2 H2O + O2 2 Fe(OH)2
4 Fe(OH)2 + O2
Ferrous hydroxide
2 [Fe2O3.2H2O]
Rust
Removal of Dissolved Oxygen (DO)
1. By the addition of chemicals
The dissolved oxygen present in the boiler feed water can be removed by the addition of sodium sulphite or hydrazine and the reactions can be written as below
2 Na2SO3 + O2 2 Na2SO4
Na2S + 2O2
N2H4 + O2
Na2SO4
N2 + 2H2O
Sodium sulphite
DO
Sodium sulphate
Hydrazine
Nitrogen
2. By mechanical deaeration
I t comprises of a tall stainless tower with different layers capped with baffles to facilitate multiple equilibration.
The entire chamber is vacuumized and also maintained at high tempt using perforated heating plates on the walls.
Water feed
To vacuum
Steam jacket
Perforated plate
Deaerate d water
2. Corrosion due to dissolved CO2
Presence of bicarbonate salts of either magnesium or calcium also causes the release of CO 2 inside the boiler apart from the dissolved CO 2
Mg( HCO 3 )2
CO 2 + H 2O
Mg (OH)2 + H 2O + CO 2
H 2 CO3 (causes slow corrosion)
1. I t can be removed by the addition of ammonia
2 NH 4OH + CO 2 (NH 4 ) 2CO 3 + H 2 O
3 . Corrosion due to dissolved salts
MgCl2 + 2 H2O Mg(OH)2 + 2HCl
Fe + 2 HCl FeCl2 + H2
FeCl2 + 2 H2O Fe(OH)2 + 2HCl
Removal
Softening of water
Methods of softening
“Process of removing hardness”
“Hardness is due to the presence of calcium, magnesium ions”
Internal treatment process
40
Colloidal conditioning
Phosphate conditioning
Carbonate conditioning
Calgon conditioning
41
Carbonate conditioning
Scale
Loose sludge
Calgon conditioning
External treatment process
42
Lime soda process
Zeolite process
Ion-Exchange (or) Demineralisation
Ion-Exchange (or) Demineralisation (or) Deionization process
Ion exchange resins are insoluble, cross linked, long chain organic polymers with a microporous structure, and the functional groups attached to the chain is responsible for the “ion-exchange” properties.
Cation exchange Resin
Resin after treatment
In general the resins containing acidic functional groups (-COOH, -SO3H etc) are capable of exchanging their H+ ions with other cations, which comes in their contact; whereas those containing basic functional groups ( -NH2, =NH as hydrochlorides) are capable of exchanging their anions with other ions, which comes in their contact.
Based on the above fact the resins are classified into two types
Strongly acidic (SO3-H+) and weakly acidic (COO-H+) cation exchange resins
Strongly basic (R4N+OH-) and weakly basic (RNH2+OH-) anion exchange resins
Continued… next slide
Resins..
Structure of Cation and Anoin resins
R = CH3
Cation exchange resin
Anion exchange resin
Ion exchange purifier (or) softener
Cation exchange Resin
Anion exchange Resin
Gravel bed
Hard water
Injector
Injector
Alkaline solution for regeneration of resin
Wastages to sink
Wastages to sink
Acid
solution for
regeneratio n of resin
pump
Soft water
-
Reactions occurring at Cation exchange resin
Reactions occurring at Anion exchange resin
At the end of the process
Process or Ion-exchange mechanism involved in water softening
Regeneration of ion exchange resins
Advantages
Disadvantages
Regeneration of Cation exchange resin
Regeneration of Anion exchange resin