Engineering Chemistry

Water Technology
NOTES
Topics:

1. Characteristics of hard water - hardness, DO, TDS in water and

their determination.
2. Numerical problems in hardness determination by EDTA;
3. Modern techniques of water analysis for industrial use -
Disadvantages of hard water in industries.

Hardness of Water
Que. Explain what is hard water, and what are its types?

Hardness of water is the characteristic of preventing lather formation of water with soap. Generally, salts
like chlorides, bicarbonates and sulfates of Ca2+, Mg2+ and Fe2+ make water hard.
 This hard water on treatment with soap which is stearic or palmitic acid salts of sodium or potassium
causes white precipitate formation of calcium or magnesium stearate or palmitate.

Thus, the cause of hardness is the precipitation of the soap and hence prevents lathering at first. When
the hardness causing ions are removed as insoluble soaps, water becomes soft and forms lather

Hardness of water is of two types: (i) temporary hardness, and (ii) permanent hardness. 1.Temporary
Hardness: This type of hardness is due to presence of soluble bicarbonate salt of calcium and magnesium
(e.g. Ca(HCO3)2, Mg(HCO3)2). Temporary hardness can be removed by boiling as follows:

2. Permanent Hardness: This type of hardness is due to presence of soluble salts of calcium and
magnesium other than bicarbonates (e.g. CaSO4, MgSO4, CaCl2, MgCl2, FeSO4). It does not remove by
boiling of water.

Que. How would you quantify the hardness of water?

Both temporary and permanent harnesses are expressed in ppm as CaCO 3. The choice of CaCO3 is due
to the fact that its mol. wt. is 100 and equivalent weight is 50 and it is the most insoluble salt in water.
Equivalent of CaCO3

This is called
multiplication factor

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Que. A sample hard water contains, 8.1 mg/L Ca(HCO 3)2; 7.5 mg/L Mg(HCO3)2; 13.6 mg/L
CaSO4; 12.0 mg/L MgSO4 and 2.0 mg/L MgCl2. Calculate the total and temporary hardness in
ppm

Temporary hardness, due to Mg(HCO3)2 and Ca(HCO3)2

= (5.136 + 5) mg/L = 10.136 mg/L or 10.136 ppm
Permanent hardness, due to MgCl2 MgSO4 and CaSO4
= (2.105 + 10 + 10) mg/L= 22.105 mg/L or 22.105 ppm
Total hardness = 10.136 + 22.105 = 32.241ppm
Que, Discuss why hardness in water can be a problem.
i)Washing: While using hard water for washing purposes, it does not lather freely with
soap, Moreover, it produces a stick precipitates(of calcium and magnesium soaps) which
adheres on the fabric giving spots .
2 C 17 H 35 COO − Na + + Ca 2+ → (C 17 H 35 COO) 2 Ca + 2 Na +
3
(ii)Bathing: Use of hard water it does not lather freely with soap, making the process
inefficient
(ii)Cooking: As we know, presence of dissolved salts elevates the boiling point
(colligative property), hence,making it fuel inefficient,also hard water has an unpleasant
taste, Moreover, the dissolved salts get deposited as carbonates on walls of the utensil.
(iii)Consumption: Consupto hard water have bad effect on our digestive system.
Moreover, the possibility of forming calcium oxalate in urinary tract is increased.
2. In Industrial Use
(i)Textile Industry: Hard water also hampers the economy by wastage of soap as it does
not form good lather, Moreover, In textile industry hard water causes deposition of insoluble
salts that interfere with the proper dyeing and printing of the fabrics. The stains of iron salts
also are undesirable on fabrics.
(ii)Boilers: For steam generation in boilers, hard water creates many problems like
(i) scale formation
(i) corrosion
(ii) priming and foaming and
(iv) caustic embrittlement.
4

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(iii)Sugar Industry: For sugar industry, the salts responsible for hardness create difficulties
in sugar refining and crystallization of sugar and the sugar becomes deliquescent.
(iv)Paper Industry: Calcium and magnesium salts also interfere with the smooth and glossy
finish of the papers in the paper industry. Iron salts interfere with the colour of the paper.
(v)Laundry: In laundry, hard water causes wastage makes the process inefficient as excess
soap is required and also interferes with the coloration due to the staining of iron salts.
(vi)Concrete making: The hydration of cement and final hardening of cement are affected
by the use of hard water in concrete making.
(vii)pharmaceutical industry: Use of hard water in pharmaceutical industry may give

undesirable results.
Que. Define chemical oxygen demand (COD). What is its significance in effluent treatment.
[4 marks, VIT CAT1 2015]
Que. How do various wastes in water can affect DO? Discuss briefly about the reactions
involve in determining dissolve oxygen of water in laboratory. [5 marks, VIT CAT1 2019]

Wastewater from sewage treatment plants often contains organic materials that are decomposed by
microorganisms, which use oxygen in the process. (The amount of oxygen consumed by these
organisms in breaking down the waste is known as the biochemical oxygen demand or BOD. A
discussion of BOD and how to monitor it is included at the end of this section.) Other sources of
oxygen-consuming waste include stormwater runoff from farmland or urban streets, feedlots, and
failing septic systems.
DO is determined by the titrimetric method developed by Winkler.
1. Dissolved molecular oxygen in water is not capable of reacting with KI, therefore an oxygen
carrier such as manganese hydroxide is used. Mn(OH) 2 is produced by the action of KOH on
MnSO4.
2. Mn(OH)2 so obtained reacts with dissolved molecular oxygen to form a brown precipitate
of basic manganic oxide, MnO(OH)2.
3. MnO(OH)2 then reacts with concentrated sulphuric acid to liberate nascent oxygen.
4. Nascent oxygen results in oxidation of KI to I 2.
5. This liberated iodine is then titrated against standard sodium thiosulphate solution using
starch as an indicator.
6. Thiosulphate reduces iodine to iodide ions and itself gets oxidized to tetrathionate ion.

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Que. Explain how hardness of water can be estimated using EDTA method.

When the indicator EBT (Eriochrome black-T) is added to the water sample

the indicator forms a weak indicator-metal complex with Ca 2+ and Mg2+ ions

giving a wine red color to it. When this is titrated against EDTA, latter forms a

complex with the remaining Ca2+, Mg2+ ions of water sample. Near the end

point the EDTA abstracts the Ca2+ and Mg2+ ions from the weak indictor-metal

complex thus releasing the free indicator. This results in a steel blue color.

EDTA-metal complex is found to be stable at pH 8-10

EDTA is found to behave like a dicarboxylic acid, i.e. two of its carboxyl group
is found to be strongly acidic, the other two hydrogens are released during its complex
formation
Reagents Required
EDTA (0.01M) Dissolve 4 g disodium salt of EDTA in one litre distilled water
Standard hard water Dissolve 1 g of calcium carbonate with a small quantity of HCl and make
it up to
one litre using distilled water
EBT Dissolve 0.5 g of indicator in 100 ml of water

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Ammonium chloride-ammonium hydroxide buffer (pH) Dissolve 67.5 g of pure ammonium
chloride in 570 ml of pre-cooled conc. ammonium hydroxide and make it up to one litre with
distilled water
Sample and boiled water
Ground water available in the area
Ground water boiled for 30 min
Procedure
Standardization of EDTA 20 ml of standard hard water (1 ml of this solution contains
1 mg of CaCO3) is pipetted out into a clean conical flask. 5 ml of ammonium buffer
and a few drops of EBT indicator are added. This then titrated against EDTA until
the wine red color changes to steel blue color. The titration is repeated till a concordant
value is obtained
Let the volume of EDTA consumed be V1 ml
Estimation of total hardness 20 ml of a given sample water is pipetted into a clean concial
flask . 5 ml of ammonium buffer and a few drops of EBT is added and titrated against EDTA
until the wine red color changes to steel blue color. The titration is repeated till the concordant
Value is obtained.
Let the volume of EDTA consmed be V2 ml
Estimation of permanent hardness 20 ml of given sample water is pipetted into a clean
conical Flask and boiled. The boiled sample water is filtered and the filtrate is collected in
another
conical flask. 5 ml of ammonia buffer and a few drops EBT indicator are added. This is then
titrated against EDTA until the wine red color changes to steel blue color. The titration is
repeated till the concordant value is obtained

Let the volume of E DTA

consmed be V3 ml

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 Que. 50 mL of a sample water consume 15 mL of 0.01 M EDTA before boiling
And 5 mL of the same EDTA after boiling.
Calculate the degree of hardness, permanent hardness & temporary hardness

1 mole of CaCO3 = 1 mole of EDTA

1.1 M CaCO3 = 0.01 M EDTA

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 {NOTE: This formula won’t be applicable when the concentration of standard hard water
is not 1g CaCO3 per ml, see the next question to get a better idea}
Que.

[5 Marks, VIT CAT1 2019]

Que. Explain what is boiler and how hardwater is a problem to it.

A boiler is a closed vessel in which water or other fluid is heated.
The heated or vaporized fluid exits the boiler for use in various processes or
heating applications.
Problems caused by hardwater are:
1. Sludge and Scale Formation
2. Caustic embrittlement
3. Corrosion
4. Priming and Foaming
Que. Explain Scale and Sludge Formation in Boilers.
Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge can
easily be scrapped off with a wire brush. It is formed at comparatively colder portion
of the boiler and collects in areas of the system, where the flow rate is slow or at

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bends. Sludge are formed by substances which have greater solubility in hot water
than in cold water, e.g., MgCO3, MgCl2, CaCl2, MgSO4, etc.

Scales are hard deposits, which stick very firmly to the inner surfaces of the boiler.
Scales are difficult to remove, even with the help of hammer and chisel. Scales are
the main source of boiler troubles. Formation of scales may be due to:

1.Decomposition of calcium bicarbonate:

2. However, scale composed chiefly of calcium carbonate is soft and is the main
cause if scale formation in low pressure boilers. But in high pressure boilers,
CaCO3 is soluble.

3. Deposition of calcium sulphate: CaSO4 is soluble in cold water, but almost

completely insoluble in super-heated water. Consequently, CaSO4 gets
precipitated as hard scale on the heated portion of boiler. This is the main cause
of scale in the high- pressure boilers.

Note: Calcium sulphate scale is quite adherent and difficult to remove, even with the
help of hammer and chisel.
1. Hydrolysis of magnesium salts: Dissolved magnesium salts undergo hydrolysis
(at prevailing high temperature inside the boiler) forming magnesium hydroxide
precipitate, which forms a soft type scale, e.g.,

2. Presence of silica: (SiO2), even present in small quantities, deposits as calcium

silicate (CaSiO3) and/ or magnesium silicate (MgSiO3). These deposits stick very
firmly on the inner side of the boiler surface and are very difficult to remove.
One important source of silica in water is the sand filter.

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Que, what are the disadvantages of sludge formation and how can it be prevented?
Disadvantages:
Sludges are poor conductor of heat, so they tend to waste a portion of heat.
Excessive sludge formation disturbs the working of the boiler. It settles at the bends thereby causing
blocking
Prevention:
By using well softened water and by frequently blow down operation

Que. What are the disadvantage of scale formation?

Low thermal conductivity (poor conductor)
Hinders the flow of heat from the source to the water

Lowering boiler safety

- Reducing the life of the boilers

Decrease in efficiency:

- tubes of the boilers may be clogged by scales

Corrode away the tubes

MgCl2 + Fe + 2H2O ---- Mg(OH)2 + FeCl2 + H2

Que. How can scales be removed?

1.By giving thermal Shock if they are brittle
(heating the boiler and then suddenly cooling with cold water)

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2.If they are adherent and hard, dissolving them with help of chemicals.
(chemical treatment)
Calcium carbonate scales can dissolved by using 5-10% HCl.
- Calcium Sulphate scales can be dissolved by adding EDTA (ethylene diaminetetraacetic acid) with
which they form soluble complex.

3. Frequent blow down operation - If they scale are loosely adhering

Que. Explain how scales can be prevented.

1.External Treatment: By feeding soft water
2.Internal Treatment:

Que. Define
Caustic
embrittlement in
Boiler. How is it
removed in
boiler? [4 marks,
VIT CAT1 2015]
Or
Que. Highly
alkaline water in
the boiler leads
to boiler
corrosion. Justify
your answer
with the
reactions
involved
Development of certain types of cracks resulting from excessive stress and chemical
attack leading to boiler failures is called embrittlement. Chemicals, like NaOH are
believed to responsible for embrittlement in a steam boiler operation. These cracks
appear like a brittle fracture and hence called caustic embrittlement.

Na2CO3 + H2O 2 NaOH + CO2

This NaOH flows into the minute hairline cracks present on the boiler material by capillary action and
dissolves the surrounding area of iron as Sodium ferroate, Na2FeO2

Fe + 2NaOH Na2FeO2 + H2
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It is characterized by the formation of irregular intergranular cracks on the boiler metal, particularly at
places of high local stress such as bends and joints
This type of electrochemical corrosion occurs when the
concentration of NaOH is above 100 ppm

Que. Explain how boiler happens due to dissolved oxygen and how it can be removed?
Dissolved oxygen in water at high temperature attack boiler material.
2Fe + 2H2O + O2 2Fe(OH)2

4Fe(OH)2 + O2 2[Fe2O3.2H2O]
(Ferrous hydroxide) (Rust)

Removal of dissolved oxygen

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By mechanical de-aeration: Water spraying in a perforated plate-fitted tower, heated from sides and
connected to Vacuum pump. High temperature, low pressure and large exposed surface reduces dissolved
oxygen in water

Que. Explain how boiler happens due to dissolved Carbon dioxide and how it can be removed?

Que. How can dissolved salts corrode boiler

Que, What is priming and foaming, how it is caused, and how can it be prevented?
Stream sometimes may be associated with small droplets of water. Such steam
containing liquid water is called wet steam. These droplets of water carry with them
some dissolved salts and sludge materials present in water. This phenomenon is

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called carry over. This occurs mainly due to priming and foaming
Priming: Formation of wet steam by rapid boiling of the water at the heating surfaces
is called priming. It may be caused by
• very high water level
• high steam velocity
• sudden steam demands leading to sudden boiling
• improper boiler design
Prevented by
• keeping the water level lower
• good boiler design with a mechanical steam purifier
• avoiding rapid changes in steam rate
• using treated water
Foaming:
The formation of stable bubbles above the surface of water
Caused by
• the presence of oil or grease in water
• fine sludge particles
Prevented by
• Removing the foaming stabilizing agents such as soluble salts, clay, and organic
matter from water by using antifoaming chemicals like synthetic polyamides
• Adding coagulants such as sodium aluminate, ferrous sulphate etc. to remove
sludge particles

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