Dr V R Kattimani Ph.

D 2022

WATER TECHNOLOGY
Introduction – Water is nature’s most wonderful, abundant and useful compound. Without
food, human can survive for a number of days, but water is such an essential that without it one
cannot survive. Water is not only essential for the lives of animals and plants, but also occupies a
unique position in industries. Probably, it’s most important use as an engineering material is in
the steam generation. Water is also used as coolant in power and chemical plants. Water is
widely distributed in nature. It has been estimated that about 75% matter on the earth’s surface
consists of water. Besides visible water on earth, there is large amount of water under earth to
an average depth of over three kilometres. The air consists 12 to 15% of volume of water vapour.
Water is found in living things. The body of human being consists of about 60% of water. Plants,
fruits and vegetables contain 90 to 95% of water. For long time it was considered as an element
until Cavendish proved that it is a compound of hydrogen and oxygen.

Hard water – Due to presence of certain dissolved salts of calcium and magnesium water
cannot form rich lather with soap this property is called hardness of water.
Soft water - Due to presence of lower concentration of dissolved salts it forms rich leather with
soap.
Temporary Hardness -Temporary Hardness is caused by the presence of dissolved salts 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(HCO3)2 CaCO3 + H2O + CO2
Mg(HCO3)2 MgCO3 + H2O + CO2
Calcium/Magnesium Carbonates thus formed being almost insoluble and 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, iron and other heavy metals
2C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl
Sodium stearate Calcium stearate
(sodium soap) (Insoluble)
Sources of water- (A) Surface water (B) Underground water
Surface water:
Rain water-It is the purest form of naturel water, since it is obtained as a result of
evaporation from the surface water. However, during the journey towards earth, it
dissolves a considerable amount of gases suspended solid particles.
River water-In general, the greater contact that water has with the soil, or the more
soluble the minerals of the soils with which it has come in contact, the greater is the
amount of dissolved impurities in river water. River water thus contains dissolved
minerals of the soil such as chlorides, sulphates, bicarbonates of calcium, magnesium
and iron.
Lake water -It has a more constant composition. It, usually, contains much lesser
amounts of dissolved minerals than even well water, but quality of organic matter
present in it is quite high.
Sea water- It is the most impure form of natural water. It contains, on an average,
about 3.5% of dissolved salts, out of which about 2.6% is sodium chloride. Other salts
present are sulphate of sodium, bicarbonates of potassium, magnesium and calcium etc.
 Dr V R Kattimani Ph.D 2022

Underground water:
A part of the rain water, which reaches the surface of the earth, percolates into the
earth. As this water journeys downwards, it comes in contact with a number of mineral
salts present in the soil. Thus it contains more of the dissolved salts. Thus, water from
these sources contains more hardness. Usually, underground water is of high organic
purity.
Types of impurities in water:
Natural water is contaminated three types of impurities i.e Physical chemical and
biological impurities.
Physical impurities:
Colour- Colour in water is usually caused by metallic substances like salts of iron,
manganese, industrial effluents. Usually, yellowish tinge indicates the presence of
chromium and appreciable amount of organic matter.
Turbidity - It is due to the colloidal, extremely fine suspension such as clay, slit, finely
divided matter etc. Turbidity expresses the optical properties of water, which scatter
light rather than to transmit.
Taste- Bitter taste can be due to the presence of iron, aluminium, manganese sulphate
or excess of lime. Soapy taste can be due to the presence of large amount of sodium
bicarbonate. Brackish taste is due to the presence of unusual amount of salts. Industrial
effluents containing organic substances such as alcohols, aldehydes, phenols etc. flowing
into the water bodies.
Chemical impurities:
Acidity - Surface water and ground water attain acidity from industrial wastes like acid,
liquors, etc. Usually, acidity is caused by the presence of free CO 2, mineral acids (H2SO4)
and weakly dissociated acids.
Gases-All natural waters contain dissolved atmosphere CO 2. It solubility depends upon
temperature, pressure and dissolved mineral content of water.
Dissolved NH3 in water arises from the decomposition of nitrogenous organic matter.
Polluted water and sewage contains nitrogen in the form of nitrogenous organic
compounds and urea, which are partially converted into NH3.
Mineral matter: It has origin from rocks and industrial effluents. These include Ca 2+,
Mg2+, Na+, K+, Fe2+, CO-2, Mn2+, HCO-3 , Cl-, SO24, etc. However, from industrial point of
view, alkalinity and hardness are important.
Biological impurities:
These are algae, pathogenic bacteria, fungi, viruses, pathogens, parasite worms etc. The
source of these contamination is discharge of domestic and sewage wastes, excreta, etc.
Micro-organisms are, usually, abundant in surface water, but their count is often quite
low or even nil in deep-well waters. The common type of micro-organisms from the point
of treatment are algae, fungi and bacteria,
Determination of Hardness - We know that there are two types of hardness of
water, i.e. temporary and permanent hardness. Temporary hardness is due to
bicarbonate of calcium and magnesium and permanent hardness is due to chloride and
 Dr V R Kattimani Ph.D 2022

sulphates of calcium and magnesium. The hardness of water can be determined by

complexometric titration by using ethylene diamine tetra acetic acid [EDTA] method.

EDTA method – It is the most important and more accurate method to determine the
hardness of water. EDTA has limited solubility in water, Hence disodium salt of EDTA is
used which is soluble in water.

Principle – EDTA can form stable complex with Ca++ and Mg++ which are present in hard
water, Hence, it is known as complexometric titration, Calcium and Magnesium ion
present in hard water sample in presence of buffer solution reacts with Erichrome Black T
to forms unstable wine red colour complex. When this unstable complex is titrated with
EDTA solution the metal ions present in hard water gives a stable complex with EDTA
and releases the free indicator which gives blue colour.

EDTA Disodium salt of EDTA

Required solutions

1) Standard of hard water - 1.0 gram of pure CaCO3 dissolved in minimum quantity of
concentrated HCl and dilute to a one litre with distilled water, each ml contain 1 mg
CaCO3.
2) EDTA solution- 4 gram of pure EDTA (disodium salt) is dissolved in 1 litre of water.
3) Erichrome Black-T indicator
4) Buffer solution

Estimation of total hardness

1) 50ml of standard hard water is pipetted out in a clean 250 ml conical flask, add 2-3 ml
buffer pinch of EBT indicator. The colour of solution turns to wine red.
2) Fill the burette with EDTA solution and titrate against standard hard water in flask.
Let volume of EDTA required is A ml when colour changes from wine red to blue.
3) Pipette out 50 ml of hard water sample, add 3 ml buffer and pinch of EBT titrate
against EDTA till to get blue colour let burette reading is B ml.
 Dr V R Kattimani Ph.D 2022

Calculations

Since standard hard water contains 1 mg/ml of CaCO 3 hardness equivalent,

50 ml of standard hard water  50mg of CaCO3 hardness
50 ml of standard hard water requires  A ml of EDTA
 ‘A’ ml of EDTA  50 mg of CaCO3 hardness
50
 Each ml of EDTA  mg of CaCO3 hardness
A
50 ml of water sample requires ‘B’ ml of EDTA solution
50
 B× mg of CaCO3 hardness
A
50
(1 ml of EDTA = mg of CaCO3 hardness equivalent)
A
 1000 ml of water sample
50 1000
 B× × mg of CaCO3 hardness equivalent.
A 50
B
Total Hardness  ×1000 mg of CaCO3
A
50 ml of water sample after boiling requires C ml of EDTA
50
 C× mg of CaCO3 hardness equivalent
A
50
(1 ml of EDTA = mg of CaCO3 hardness equivalent)
A
 1000 ml water sample after boiling
50 1000
 C× × mg of CaCO3 hardness equivalent
A 50
C
 1000 × mg of CaCO3 hardness equivalent.
A
C
Permanent hardness = ×1000 mg of CaCO3
A
Temporary hardness = Total hardness – Permanent hardness
B C
= ×1000− ×1000
A A

= 1000( B−C
A ) mg of CaCO3

Degree of hardness - The unit in which hardness is usually expressed, known as

degree of hardness. Degree of hardness is expressed in terms of calcium carbonate
(CaCO3) equivalent because CaCO3 have molecular weight 100 and it is easily precipitate.
Degree of hardness may be expressed as follows
 Dr V R Kattimani Ph.D 2022

Equivalent of CaCO3 =
[strength of substance producing hardness(mglit )]×[chemical equvalent of CaCO 3]
[chemical equivalent of hardness producing substance ]
Unit of hardness :
1) Parts per million (ppm): It is the number of equivalent part CaCO 3 present per
million (106 ) part of water by weight.
2) Milligram per litre (mg/lit) : It is the number of milligram of CaCO 3 present in one litre of
water.
3) Degree Clarke (oCl): it is the number of equivalent part of CaCO 3 present per 70,000 part of
water.
4) Degree French (oFr): it is the number of equivalent part of CaCO 3 present per 105 part of
water.
Correlation between ppm, mg/lit, oCl and oFr: [1ppm= 1mg/lit = 0.07 oCl = 0.1 oFr]
Problems on Hardness Calculations
Problem 2.1: 50 ml of standard and hard water containing 1 mg of pure CaCO 3 per ml
consumed 10 ml of EDTA solution. 50 ml of the given water sample required 10 ml of
same EDTA solution. Calculate the total hardness of water sample in ppm.
Solution:
50 ml standard hard water  10 ml of EDTA solution
50
 1 ml of EDTA solution  ml std hard water
10
50
 mg of CaCO3
10
 5 mg of CaCO3
50 ml of water sample  10 ml of EDTA solution
 10 X 5 mg of CaCO3
 50 mg of CaCO3
50 ml of water sample  50 mg of CaCO3
1000
1000 ml of water sample  50 ×
50
 1000 mg CaCO3
Hardness of water sample  1000 ppm.
Problem 2 : In the determination of hardness by EDTA method, 50ml of standard hard
water (containing 1 mg of CaCO 3 hardness per ml of solution) required 30 ml of EDTA
solution while 50 ml of the sample of hard water consumed 20 ml of EDTA solution. After
boiling 50 ml of the same sample required 10 ml of EDTA solution. Calculate the various
hardnesses in ppm.
Solution: 1 ml of std. hard water  1 mg of CaCO3
50 ml of std. hard water  50 mg CaCO3
50 ml of std. hard water  30 ml of EDTA
 50 mg of CaCO3
50
 1 ml of EDTA  mg of CaCO3
30
Now 50 ml of sample water  20 ml of EDTA solution
 Dr V R Kattimani Ph.D 2022

50
 20 × mg of CaCO3
30
50
1000 ml of sample water  20 × × 20 mg of CaCO3
30
Total Hardness  664 mg of CaCO3
 664 ppm.
50 ml of boiled water sample  10 ml of EDTA solution
50
 10 × mg of CaCO3
30
50
1000 ml of boiled water sample  10 × × 20 mg of CaCO3
30
 332 mg of CaCO3
i.e. Permanent hardness  332 ppm.
i.e. Temporary hardness  Total – Permanent
 664 – 332 = 332 ppm.

Problem 3: 50 ml of SHW containing 1 mg of pure CaCO 3 per ml consumed 20 ml of

EDTA. 50 ml of water sample consuming 25 ml of Same EDTA solution using buffer and
Eriochrome black T indicator calculate total hardness of water sample
Problem 4: 1 gram of CaCO3 was dissolved in dilute HCl and the solution is diluted to 1
litre. 50 ml of this requires 45 ml of EDTA solution, 50 ml of hard water requires 18 ml of
EDTA solution during titration using buffer and EBT indicator. On the other hand 50 ml of
boiled water sample requires 9ml of EDTA solution under same condition calculate each
type of hardness.
Problem 5 : 0.3 gram of CaCO3 was dissolved in HCl and solution was made up to 1 litre
with distilled water ,100 ml of the above solution required 30 ml of EDTA solution on
titration . 100ml of hard sample requires 35 ml of same EDTA solution on titration .After
boiling 100ml of this water cooling filtering and then titration requires 20 ml EDTA,
Calculate all types of hardness
Problem 6 : 20 ml of SHE (containing 1.5 gram CaCO3 per litre) requires 25 ml of EDTA
solution for end point , 100 ml water sample requires 18 ml of same EDTA solution , while
same water after boiling requires 12 ml of EDTA solution calculate Carbonate and
noncarbonated hardness of the water sample.
Problem 7: 0.28 gram of CaCO3 were dissolved in Hcl and the solution was made up to
one litre with distilled water, 100 ml of above solution requires 28 ml of EDTA solution on
titration 100ml of hard water sample requires 33 ml of same EDTA 100 ml of this water
sample after boiling and cooling and filtering requires 10 ml of same EDTA calculate all
type of hardness.
Problem 8 : 1.53 gram of CaCO3 was dissolved in HCI and the solution made up to 1000
ml with distilled water. 50 ml of the solution required 32 ml of EDTA solution for titration.
50 ml of hard water sample required 16 ml of EDTA and after boiling and filtering
required 9 ml of EDTA solution. Calculate temporary, permanent and total hardness
of water.
 Dr V R Kattimani Ph.D 2022

Problem 9: 0.30 gm of CaCO3 was dissolved in HCl and the solution made up to 1000
ml with distilled water. 100 ml of the solution required 30 ml of EDTA solution for
titration. 100 ml of hard water sample required 35 ml of EDTA and after boiling and
filtering required 12 ml of EDTA solution. Calculate temporary hardness of water.
Problem 10: 0.25 gm of CaCO3 was dissolved in HCI and the solution made up to 250
ml with distilled water. 50 ml of the solution required 20 ml of EDTA solution for titration.
50 ml of hard water sample required 18 ml of EDTA and after boiling and filtering
required 10 ml of EDTA solution. Calculate temporary hardness of water.
Boilers - In all industries boilers are used to generate steam, boiler feed water is the
water required for the generation of steam and with the safety, economy and efficiency
concerns, It should be of very good quality. Depending upon the operating pressure,
boilers are classified into low pressure (10-15 kg/cm 2) medium pressure (15-35 kg/cm 2),
High pressure (50140kg/cm2) very high pressure (150-225kg/cm2), and super critical
boilers (>225kg/cm2)
Boiler consists of two principal parts: the furnace, which provides heat, usually by
burning a fuel, and the boiler proper, a device in which the heat changes water into
steam. The steam or hot fluid is then recirculated out of the boiler for use in various
processes in heating applications including water heating, central heating, boiler-based
power generation, cooking, and sanitation.
Proper treatment of boiler feed water is an important part of operating and maintaining a
boiler system. As steam is produced, dissolved solids become concentrated and form
deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of
the boiler. A big boiler in these days may produce 5,00,000kg of steam per hour and if
water containing as low as 20 mg/l of impurities is fed to boiler, 10 kg of deposits will be
produced every hour or over one and half tonnes deposits per week, such situation is not
allowed to occurs in actual practices
Depending upon the quality of the feed water so many problems may arises in the
boilers some of them are scale and sludge formation, Priming and foaming ,Boiler
corrosion and caustic embrittlement.
Boiler Feed water is water that undergoes purification or preheating and is then supplied
to boilers for hot water and steam production,
SCALE AND SLUDGE FORMATION IN BOILERS - In boilers, water evaporates
continuously and the concentration of the dissolved salts increases progressively. When
their concentrations reaches saturation point, they are thrown out of water in the form of
precipitates which stick to the inner walls of the boiler. If the precipitation takes place in
the form of loose or slimy precipitate it is called sludge. On the other hand, if the
precipitated matter forms a hard adhering crust/coating on the inner walls of the boiler, it
is a scale.
Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge can be
easily scrapped off with a wire brush. It is formed at comparatively colder portions of the
boiler and collects in areas of the system, where the flow rate is slow at bends. Sludges
are formed by substances which have greater solubilities in hot water than in cold water.
Examples are MgCO3, MgCl2, CaCl2, MgSO4 etc.
 Dr V R Kattimani Ph.D 2022

Disadvantages of sludge formation :

I Sludge are poor conductors of heat hence there is wastage of fuel.
II. If sludge are formed along with scales, the former get entrapped in the later and
both get deposited as scale
III. Excessive sludge formation disturbs the working of the boilers. It settles in the
region of poor water circulation such as pipe connection, plug opening, gauge-
glass
connection etc thereby causing even chocking of pipes
IV. The sludge formation may decrease the efficiency of boilers.
Prevention of sludge formation:
I. By using well softened water.
II. By a frequent blow down operation, i.e., replacement of concentrated water with
fresh water.
Formation of scales may be due to
Decomposition of calcium bicarbonate
Ca(HCO3)2 → CaCO3 + H2O + CO2
However, scale composed chiefly of calcium carbonate is soft and is the main
cause of scale formation in low pressure boilers. But in high pressure boilers
CaCO3 is soluble.
CaCO3 + H2O → Ca(OH)2 + CO2
Decomposition of calcium sulphate- Calcium sulphate is soluble in cold water, but
almost completely insoluble in superheated water. Consequently calcium sulphate gets
precipitated as hard scale on the heated portions of the boiler. This is the main cause in
the high pressure boilers.
Hydrolysis of magnesium salts - Dissolved magnesium salts undergo hydrolysis
at prevailing high temperatures in the boiler forming magnesium hydroxide
precipitate, which forms a soft type of scale.
MgCl 2 + 2H2O → Mg(OH)2 + 2HCl
Presence of silica: SiO2 even if present in small quantities, deposits as calcium
silicate (CaSiO3) and/ or magnesium silicate (MgSiO 3). These deposits stick very
firmly to the inner walls of the boiler surface and are very difficult for removal. One
important source of silica in water is the sand filter used.
Disadvantages of scale formation :
Wastage of fuels - The scales are bad conductors of heat, therefore, their effect is like
that of insulating material. This results in reduction in the rate of heat transfer in order to
 Dr V R Kattimani Ph.D 2022

provide steady supply of heat to water, over heating is done and this causes wastage of
fuel.

Thickness of scale in 0.325 0.625 1.25 2.5 12

mm
Wastage of fuel 10% 15% 50% 80% 150%
Lowering of boiler safety-Due to scale formation, over-heating of the boiler has to be
done in order to maintain a constant supply of steam. The overheating of the boiler tube
makes the boiler material softer and weaker and this causes distortion of the boiler tube
and makes the boiler tube unsafe to bear the pressure of the steam especially in high-
pressure boilers
Decrease in efficiency- Scales may sometimes get deposited in the valves and
condensers
of the boiler and choke them partially or totally. This results in decrease the
efficiency of the boiler.
Danger of explosion- When thick scales crack due to uneven expansion, the water
comes in contact with the overheated iron plates. This causes a release of a large
amount of steam suddenly, developing a high pressure, which may cause explosion in
the boiler.
Removal of scales :
With the help of scraper or piece of wood or wire brush, if they are loosely adhering.
By giving thermal shocks like heating the boiler and suddenly cooling it with cold
water.
I. Dissolving scales by adding suitable chemicals, if they are adherent and hard.
Thus calcium carbonate scales can be dissolved by the addition of 5% HCl.
Calcium sulphate scales can be dissolved by the addition of EDTA (ethylene
diamine tetra acetic acid), with which they form complexes .
II. By frequent blow down operation, if the scales are loosely adhering.
BOILER CORROSION-Degradation or destruction of boiler materials (Fe) due to the
chemical or electrochemical attack of dissolved gases or salts is called boiler corrosion
Boiler corrosion is of three types - Corrosion due to dissolved O2
Corrosion due to dissolved CO2
Corrosion due to acids formed by dissolved salts

Corrosion due to dissolved oxygen (DO) – Water usually contains 8 mg of dissolved

oxygen per liter at room temperature. Dissolved oxygen in water in the presence of
prevailing high temperature of the boiler, attacks the boiler material as
 Dr V R Kattimani Ph.D 2022

Removal of Dissolved Oxygen (DO) - The dissolved oxygen present in the boiler feed
water can be removed by the addition of sodium sulphite or hydrazine and the reactions

2 𝑵𝒂𝟐SO3 + O2
can be written as below
2 Na2SO4
Na2S + 2O2 Na2SO4
N2H4+ O2 N2 + 2H2O

Corrosion due to dissolved CO2 - Carbon dioxide dissolved in water forming carbonic
acid, has a slow corrosive effect on the boiler material.
CO2 + H2O H2CO3
Presence of bicarbonate salts of either magnesium or calcium also causes the release
Mg(HCO3)2 MgCO3 + H2O + CO2
CO2 + H2O H2CO3 (causes slow corrosion)
Removal of dissolved carbon dioxide -The carbon dioxide can be removed by
adding
calculated amount of ammonia
2NH 4OH + CO2 → (NH4)2CO3 + + H2O
But if the ammonium hydroxide is used is more than the calculated amount, It may
attack the condenser tube made of copper
Corrosion due to dissolved salts- Water containing dissolved salts of magnesium can
liberates acids on hydrolysis. MgCl2 + H2O → Mg(OH)2 + 2 HCl
The liberated acid reacts with the iron material of the boiler in chain like processes,
producing HCl again and again.
Carry over - As Steam rises from the surface of the boiling water in the boiler. It my
be associated with small droplets of water such steam containing liquid water is called
wet steam. These droplets of water naturally carry with them some suspended and
dissolved impurities present in the boiler water. This phenomenon of carrying of water
by steam along with the impurities is called carry over. This is mainly due to priming
and foaming
PRIMING AND FOAMING - When a boiler is producing steam rapidly, some particles of
the condensed liquid water are carried along with the steam. The process of wet steam
formation is called priming.
Priming is caused by :
I. The presence of large amounts of dissolved solids
II. Steam velocities high enough to carry droplets of water into the steam pipe.
III. Presence of excessive foam on the surface of the water which substantially fills
the foam space
IV. Sudden boiling
V. Improper boiler design
VI. Sudden increase in the steam production rate.
Disadvantages of Priming
I. Dissolved salt in boiler water are carried out by the wet steam to turbine blades.
which reduces their efficiency.
II. Dissolved salts may enter the parts of other machinery may decrease the life of
the machinery.
 Dr V R Kattimani Ph.D 2022

III. Actual height of the water column cannot be judge properly, There by making the
maintenance of the boiler pressure becomes difficult.
Prevention of Priming
I. By improving boiler design.
II. By fitting mechanical steam purifiers.
III. By maintaining low water level in boilers
IV. By using soft water.
V. By decreasing the amount of dissolved salts.
VI. Avoiding rapid changes in the steaming rate caused by sudden steam demands

Foaming- Foaming is the formation of persistent foam or stable bubbles in boilers,

which do not break easily. Foaming is due to the concentration difference of suspended
solid between the film and the bulk of water. Foaming is also due to the presence of
substances like oils and alkalies in boiler feed water, Oily substances react with alkalies
to form soaps which reduce the surface tension of water. Priming and foaming usually
occur together. They have to be eliminated.
Causes of Foaming :
I. It is due to the presence of oily substances in water.
II. Low level of water in boiler.
III. The presence of dissolved salts in water.
IV. Sudden increase in steam production rate
Disadvantages of foaming :
I. Actual height of the water column cannot be judge.
II. Dissolved salts in water carried by the wet steam may damage turbine
blades or machinery parts.
III. Boiler pressure cannot be maintained.
Prevention of Foaming :
I. By the addition of anti-foaming chemicals like castor oil, Gallic acid, tannic acid etc.
II. Removing oil from boiler water by adding compounds like sodium aluminate.
CHARACTERISTICS OF POTABLE WATER - The water which is fit for human
consumption is known as potable water. Municipalities have to supply potable water, i.e.,
water which is safe to human consumption should satisfy the following essential
requirments. As per World Health Organisation and by Indian Council of Medical Research
(ICMR), the following are the important characteristics of potable water.

I. It should be clear, colorless and odorless.

II. It should be cool and pleasant to taste.
III. It should be free from harmful bacteria and suspended impurities.
IV. It should be free from dissolved gases like CO2, H2S, NH3, etc., and poisonous
minerals like lead, arsenic, manganese, etc.,
V. Hardness should be less than 300 ppm.
VI. Alkalinity should not exceeds 600mg/l
VII. Chloride & sulphate ions should be less than 250 ppm.
 Dr V R Kattimani Ph.D 2022

VIII. Fluoride ion content should be less than 1.5 ppm.

IX. Total Dissolved Solids (TDS) content should be less than 500 ppm.
X. Turbidity should be less than 10 ppm
XI. pH of the potable water should be about 7-8
Estimation of Chlorides in water- Chloride is major inorganic anion present in
natural and waste water. A high chloride content accelerate the corrosion process of
metallic pipes and structures. Further it has harmful effect on agriculture. When present
of concentration above 250 mg/L.it imparts an objectionable taste to water The chloride
content is an important factor in determining the type of desalting apparatus to be used.
The chlorides are estimated by titrating the water sample with standard solution of silver
nitrate using potassium chromate as indicator (Mohr's method).
Ag+ + Cl- AgCl
White ppt
2 Ag+ + CrO42− Ag2CrO4
Red ppt
Procedure- Pipette out 100 mL of given Cl- solution in conical flask add 1 ml of K 2CrO4
indicator. Slowly add (N/50)AgNO3 solution from the burette. Initially, white ppt is
obtained. Continue the addition of silver nitrate till permanent reddish brown colour is
obtained. Let V2 mL of AgNO3 gets consumed till end point.
Calculations: N1V1 = N2V2

( Chloride solution) (Silver Nitrate Solution)

50𝑥100
N1 = x V2

Strength of Cl ions
-
= N1 X 35.5 gm/L
= N 1 X 35.5 x 1000 mg/L
 Note – The pH of the solution should be in between 7-8 because at higher pH
silver ions get precipitated as AgOH

Ag+ + OH− AgOH

If pH is less than 7-8 then the chromate ions are converted to dichromate ions

Determination of Dissolved Oxygen by Winkler’s method- Much of

the DO in water comes from the atmosphere due to wind action. Algae and rooted
aquatic plants also give out oxygen into water through photosynthesis. The oxygen
content of natural water varies with temperature, salinity, the photosynthetic activity
of algae and higher plants and the atmospheric pressure. The reduction in DO levels
causes anaerobic condition in water and adversely affects the aquatic biota.
Principle: This method is based on the fact that Oxygen (Dissolved in water)
oxidises pottassium iodide to iodine. The liberated iodine is titrated against a standard
 Dr V R Kattimani Ph.D 2022

hypo solution. However since dissolved oxygen in water is in molecular form It cannot
oxidise pottassium iodide as such. therefore an oxygen carrier such as manganese
hydroxide is used to bring about the reaction between KI and oxygen. manganese
hydroxide in turn is obtained by the reaction of KOH on manganese sulphate

The liberated oxygen reacts with KI and liberates iodine equivalent to that of dissolved
oxygen originally present in the sample. The liberated iodine is titrated with standard
solution of sodium thiosulphate using starch as the indicator.
Procedure-
a) Pipette exactly 300 ml of water sample into a glass stoppered bottle. Add
3ml of manganous sulphate solution
b) Add 3ml of alkaline KI solution. (KI + NaN 3 + KOH). Stopper the bottle and
shake well and allowed to settle down the precipitate. Add 1ml of conc. Sulphuric
acid and shake well until the ppt dissolves completely.
c) 100ml of above prepared solution is pipetted out into a clean conical flask
and titrated against 0.02N sodium thiosulphate solution using few drops of
starch as indicator near the end point. Record the volume of sodium thiosulphate
solution consumed.
Calculation: N1V1 = N2V2
(Oxygen solution = ( Na2 S2 O3solution )
N2
N1 = x Vol. of Na2 S2 O3
V1
Amount Of DO = N1 X 8 gm/L
= N 1 x 8 X 1000 mg/L or ppm

SOFTENING METHODS- Water used for industrial purposes (such as for steam
generation) should be sufficiently pure.it should, therefore made free from hardness-
producing salts before it is being put to use. The process of removing hardness-
producing salts from water is known as softening of water. In industry three methods are
mainly employed for softening of water.

REVERSE OSMOSIS
When two solutions of unequal concentrations are separated by a semi permeable
membrane, flow of solvent takes place from dilute to concentrate sides, due to osmosis.
If however a hydrostatic pressure in excess to osmotic pressure is applied on the
concentrated side, the solvent flow is reversed, i.e, solvent is forced to move from
concentrated side to dilute side across the membrane. This is the principle of reverse
osmosis.(RO)
 Dr V R Kattimani Ph.D 2022

Thus in reverse osmosis method, pure solvent is separated from its contaminants, rather
than removing contaminants from the water. The membrane filtration is sometimes also
called super- filtration or hyper filtration.
METHOD - In this process, pressure is applied to the sea water or impure water to force
the pure water content of it out the semi-permeable membrane, leaving behind the
dissolve solids. The principle of reverse osmosis as applied for treating saline/sea water
The membrane consists of very thin film of cellulose acetate, affixed to either side of a
perforated tube. However, more recently superior membranes made of polymethyl
methacrylate and polyamide polymers have come into use.
ADVANTAGES

I. Reverse osmosis possesses distinct advantages of removing ionic as well as non-

ionic, colloidal and high molecular weight organic matter.
II. It removes colloidal silica, which is not removed by demineralization.
III. The maintenance cost is almost entirely on the replacement of the semi permeable
membrane.
IV. The life time of membrane is quite high, about 2 years,
V. The membrane can be replaced within a few minutes, thereby providing nearly
uninterrupted water supply.
VI. Due to low capital cost, simplicity, low operating cost and high reliability, the
reverse osmosis is gaining grounds at present for converting sea water into
drinking water and for obtaining water for very high –pressure boilers.