HARDNESS OF WATER

Basic Concenpts:
Definition: If the water produces lather with soap, it is soft water.If the water does not produce lather with
soap, it is called as Hard water. The property is known as hardness. But it will produce a scummy white
precipitate. The hardness is due to Ca2+ , Mg2+, Cl- , CO32-, HCO3 ions and their salts.
Test for hardness: a) Eriochrome Black – T indicator gives wine red colour in hard water.

b) With soap, hard water gives a scummy precipitate.

2C17H35COONa CaCl2 C17H35COO)2 Ca 2NaCl

Types of Hardness:
a) Temporary Carbonate hardness : The hardness due to carbonates and bicarbonates can be removed
by simple boiling. So, they are known as temporary or carbonate or alkaline hardness.

(b). Permanent hardness :( Non-carbonate): The hardness due to chlorides and sulphates cannot be
removed by simple boiling. They need special methods like lime soda process and zeolite process. This
hardness is called as “Permanent hardness or Noncarbonate or non alkaline hardness.

( Lime soda process)

Units of hardness: i) ppm ii) mg/L iii) degree Clarkes iv) Degree French
Hardness is expressed in CaCO3 equivalence.
Because, i) It is the most insoluble and easily precipitaable salt.
ii) Its molecular weight is 100 and equivalent weight is 50.
As these are whole numbers, it is very easy to use them in calculations.

Alkalinity: Alkalinity in water due to the presence of Hydroxide (OH-), Carbonate (CO32-), Bicarbonate
HCO3-) ions. These can be determined titrimetry using standard acid and phenolphthalein and methyl
orange.
Problem:
A water sample contains 204 mgs of CaSO4 and 73 mgs of Mg (HCO3) per litre. What is the total hardness
interms of CaCO3 equivalent?
CaSO4 = (204 × 100) /136 = 150 mgs/ lit
Mg (HCO3) = (73 × 100) /146 = 50 mgs/ lit
Temporary hardness = 50 mgs/ lit
Permanent hardness = 150 mgs/ lit
Total hardness = 50 mgs/ lit+ 150 mgs/ lit = 200 mgs/ lit
Types of impurities in water:
1.Physical impurities:
a). Suspended impurities: Suspended impurities like sand, oil droplets, vegetable and animal matter impart
turbidity to water.
b). Colloidal impurities: Colloidal impurities like finely divided silica, clay and organic waste products
impart colour, odour and taste to water.
2. Chemical impurities:
a). Dissolved salts: Dissolved salts like bicarbonates, sulphates and chlorides of Ca and Mg and carbonates,
bicarbonates of Sodium and Pottassium.
b). Dissolved Gases: Dissolved Gases like O2, CO2, H2S, SO2 make the water acidic. So they accelerate the
rate of corrosion.
3. Bacterial impurities: Bacterial impurities like fungi, bacteria and other microorganism cause disaeses.
 ESTIMATION OF HARDNESS BY EDTA METHOD

1.Aim: To estimate the amount of hardness present in the given water sample.
2. Chemicals required:
a) Standard hard water – 1g CaCO3 + dil. HCl – made up to 1 Lr.
b) EDTA solution – 4 gms of EDTA sodium salt / 1 Lr of water
c) EBT Indicator – 0.5g EBT / 100 ml alcohol
d) Ammonia buffer - 67.5g Ammonium chloride +570ml Ammonia-- made up to 1Lr 3.
Principle: EDTA stands for Ethylene Diamine Tetra Aceticacid. As it is insoluble in water, we use its
disodium salt.
HOOCH2C CH2COOH
H2 H2
N C C N

HOOCH2C CH2COOH
EDTA
By nature, Eriochrome Black T indicator is blue in colour. When EBT indicator is added to water
sample, it forms a wine red coloured unstable Ca-Mg-EBT complex. This reaction is carried out under a
basic PH of 8- 10 using ammonia buffer.
When EDTA is titrated against the complex, EDTA replaces all the EBT and forms a stable Ca /
Mg –EDTA complex. The liberated EBT indicates the end point as steel blue.

pH=8-10
Ca2+ / Mg2+ + EBT [Ca / Mg EBT]

Wine red - Weak complex

[Ca / Mg EBT] EDTA pH=8-10 [Ca / Mg EDTA] EBT
Wine red - Weak complex Stable complex- steel blue
Calculation:
Hence, total hardness = V2/V1 X 1000 ppm
Hence, permanent hardness = V3/V1 X 1000 ppm
Tempory hardness= total hardness-permanent hardness
Standardization of EDTA:
1ml of Std. hard water = 1 mg of CaCO3
50 ml of Std. hard water = 50 mgs of CaC03
50 ml of Std. hard water consumes = V1 ml of EDTA
V1 ml of EDTA = 50 mgs of CaC03
1 ml of EDTA = 50 / V1 mgs of CaC03
Estimation of Total hardness:
50 ml of given hard water consumes = V2 ml of EDTA = V2 X ( 50 / V1) mgs of CaC03
1ml of EDTA = (V2 /50) X ( 50 / V1) mgs of CaC03
1000 ml of EDTA = (V2 /50) X ( 50 / V1) X 1000 ppm= (V2/V1) X 1000 ppm
Estimation of Permanent hardness:
50 ml of hard water after boiling consumes = V3 ml of EDTA = V3 X ( 50 / V1) mgs of CaC03
1ml of EDTA = (V3 /50) X ( 50 / V1) mgs of CaC03
1000 ml of EDTA = (V3 /50) X ( 50 / V1) X 1000 ppm= (V3/V1) X 1000 ppm
Estimation of Tempory hardness:
Tempory hardness = Total hardness-permanent hardness= (V2 - V3)/V1 X 1000 ppm
 Boiler feed water
Definition: The setup used to generate steam is known as ‘Boiler’. Water is fed to the boiler and
heated to produce steam. The water fed into the boiler is known as “Boiler feed water”.
Requirements of Boiler Feed Water
Hardness - ≤ 0.2 ppm
Soda alkalinity - 0.15-1.0 ppm
Caustic alkalinity - 0.15-0.45 ppm
Excess soda ash - 0.3-0.55 ppm
Dissolved gases like O2, CO2 - 0 ppm

Boiler troubles
Sludge, scale, priming and foaming, caustic embrittlement, boiler corrosion are collectively
known as boiler troubles.
1. Sludge and scale :
As water evaporates continuously in boiler for steam production, the concentration of
dissolved salts increases and after reached their saturation points salts are precipitated on
the inner walls of the boiler.

Sludge Scale

Sludge Scale
Loose, slim , non-adherent precipitate Hard, thick , strong adherent precipitate
Due to salts like MgSO4 , MgCl2 Due to salts like CaSO4 , Ca(HCO3)2
Due to poor conductance, they decrease Due to poor conductance, they decrease the
the boiler efficiency to lesser extent and
boiler efficiency to maximum extent, cause
causing chocking in the pipelines. reduced fuel economy , improper boiling,
boiler explosion etc.,
It can be prevented by periodical It can be prevented by special methods like
replacement of concentrated hard water i)external treatment of ion exchange ,
by fresh water. This process is known as ii)Internal carbonate, phosphate, Calgon
“blow down” method. conditioning iii)Mechanical hard scrubbing
methods.
2.Caustic Embrittlement: (Inter crystalline cracking of boiler metal) : It is the intercrystalline
cracking of boiler due to Na2CO3. In high pressure, Na2CO3 undergoes hydrolysis to produce
NaOH. This makes water caustic. The NaOH contenting water flows into the minute hair-cracks.
Na2CO3 + H2O 2 NaOH + CO2
Prevention of caustic embrittlement: 1. As softening agent, we can use sodium
phosphate instead of sodium carbonate. 2. cracks can be sealed by waxy materials like
Tannin and Lignin.

3. Priming and Foaming: ) Foaming and priming are collectively known as ‘ Carry over”. Due
to rapid boiling, the steam may carry some water droplets along with it. This is called wet steam
.The process of wet steam production is called Priming. It can reduce the heat of the steam and
cause corrosion in the pipelines. Priming is due to: a) Improper design of boiler b) High water
level c) High velocity of steam d) Uneven boiling. Priming can be controlled by i)Proper boiler
design ii)Maintaining proper water level iii)Proper boiling

4. Foaming : If oils and greases are present, they produce stable bubbles on the water surface.
This will increase the wet steam production. This is known as “Foaming”. Foaming is prevented
by adding i) Anti foaming agents (e.g.) synthetic poly amides , castor oil. ii) Coagulants (e.g.)
Aluminium hydroxide

5. Boiler Corrosion : Boiler corrosion is decay of boiler material by chemical or

electrochemical attack on its environment. It may be due to three major reasons:
i) Dissolved Oxygen ii) Dissolved CO2 iii) Dissolved salts like MgCl2 Corrosion
a. dissolved oxygen : Dissolved oxygen in presence of water, causes corrosion and form rust.

b. Chemical method : i)Adding Sodium Sulphite: Na2SO3 OR Hydrazine:

c. Mechanical deaeration method: This is based on the principle that at high temperature , low
pressure and high exposed area, the solubility of gases in water is decreased. So, the gases can be
expelled easily. Here, 1. The water is fed into the mechanical deaerator which is provided with
vacuum pump, heaters and perforated plates. 2. The out coming water will be free from
dissolved gases.
Aeration: Mechanical aerator that Removes Oxygen, Carbon-dioxide , toxic gases, Fe, Mn salts
Corrosion due to CO2 Salts like Calcium bicarbonate on heating produces CO2 . CO2 dissolves
in water to form carbonic acid which corrodes the boiler metal.
 EXTERNAL TREATMENT METHODS:
Demineralisation (Ion exchange method)

Ion Exchange or Deionisation : Deionisation process removes all the anions and cations present
in the hard water. TDS is as low as 10 ppm and the water can be used even in high pressure boilers.
Demineralisation of water is done in an ion exchanger.
Working:
1. Here all the cations and anions are completely removed. It uses two column of cation
exchange column and anion exchange lolumn filled with resins.
2. Resins are long chain, insoluble, cross linked, organic polymers.
Cation exchange resins: Materials capable of exchanging cations are called cation exchange
Process: The hard water is first passed through a cation exchange column, which removes all the
cations like Calcium, Magnesium from it and equivalent amount of H+ ions are released. resins, –
RH+ (e.g) Sulphonated coals , RSO3H
Anion exchange resins: Materials capable of exchanging anions are called anion exchange resins.
The cation free water is then passed through an anion exchange column, which removes all the
anions like chlorides, sulphates etc. and equivalent amount of OH▬ ions are released from the
column. R’OH- (e.g) Ureaformaldehyde, Amines R-NH2 . The water is fed into cylinder –I where
all the cations are replaced by RH2 Resins.
Process: The water is fed into Cation exchange resins which absorbs all cations like Na+, Mg2+
where all the cations are replaced by RH2 Resins.

The cation free water is fed to ANOIN exchange resins which absorbs ANOINS, where all the
anions are replaced. So, the resultant water is free from all types of ion
Regeneration:
On prolonged use, as all the resins are exhausted,
there will be no H+ or OH – ions to exchange the unwanted ions. So, they have to be regenerated.
Cation resins are regenerated by HCl

Anion resins are regenerated by NaOH.

Advantages of Ion exchange method: i) Can be used for high pressure boilers also. ii) It can
treat highly acidic or alkaline water. iii) We can get pure water as hardness as low of 2 ppm.
Drawbacks of Ion exchange method: i) Expensive ii) Fe, Mn cannot be removed as they form
complexes with resins iii) Cannot be used for turbid water as they clog the resins.

INTERNAL TREATMENT BY BOILER COMPOUNDS

The residual salts that are not removed by external methods can be removed by adding some
chemicals directly into the boiler water. These chemicals are known as ‘Boiler compounds’. This
method is known as ‘Internal treatment’.
a) Carbonate conditioning: Used for low pressure boilers. Here the salts like CaSO4 are
converted to easlity removable CaCO3. But some times it produces NaOH, CO2 and
hence Carbonic acid. So it is less preferred.

b) Phosphate conditioning: Used for high pressure boiler. No risk of CO2 liberation.

c) Calgon conditioning: Calgon is the trade name of sodium hexa meta phosphate- Na2 [
Na4 (PO3)6]. With calcium ions it forms a soluble complex and prevents scale and sludge
formation. It is used for high and low pressure boilers.
 Desalination
Removal of common salt (NaCl) from water is called ‘ Desalination’. Various methods: Reverse
Osmosis, Distillation, Electro dialysis, Freezing, Solar distillation, etc.,
Osmosis: When two different concentrated solutions are separated by a semi permeable
membrane, due to osmotic pressure, low concentrated solvent flows to higher one. This is known
as osmosis.
REVERSE OSMOSIS: When water flows from a region of lower concentration to higher
concentration, when pressure (15-40kg) applied on concentrated side to reverse the flow of water
get reverse direction and the semipermeable membrane only allows pure water to pass through it.
Thus dessolved ionic and non ionic solvents are left behind also purified from salt. The
membrane is made up of cellulose acetate,cellulose butyrate,polymethacrylate
.

Advantages of Reverse Osmosis:

1.High life time
2.Removes ionic, non-ionic and colloidal silica impurities , which can not be removed by
demineralization method.
3. Low capital cost. 4. Simple operational procedure.
5. The membrane can be replaced within a few minutes, thereby providing uninterrupted water
supply
 Electrodialysis
Efficient technique is used for desalination of water done by membrane process.
Principle:
Under the influence of an electric potential across salt solution, charged ions are moved towards
respective electrodes through ions and selective membranes. If selective membranes is either
cation selective, it allows only cations and reject anions. If selective membranes is either anion
selective, it allows only anions and reject cations.

Process:
Multiple membranes alternatively allows anions or cations to flow through it to get fresh water
from saline water. when emf(electromotive force applied across two electrodes, Na+ present in salt
water move towards cathode through cation selective membrane and cl- present in salt water move
towards anion through anion selective membrane. As s result, pure water obtained.

Advantages:
1. Hardness can be completely removed.
2. Process can be made automatically
3. Easy operation. No experts required.
4. Less time and small area requires.
 DOMESTIC DRINKING WATER TREATMENT
Requirements of drinking ( potable) water: i. Free from colour, odour, bacteria, dissolved
gases ii. Should have pleasant taste iii. Dissolved oxygen should be below 10 ppm iv.
Chloride content should be below 250 ppm. . Flouride content should be below 1.5 ppm.
vi. Hardness salt content should be below 500 ppm. vii. P H should be in the range of 6.5
– 8.0 To get these properties, the water is treated properly.
Water treatment process
1. Screening : shutters with variable sized holes, Removes floating material like wood,
plastic, papers
2. Sedimentation :Allowing the water to stand for 2 – 6 hrs in a tank Removes 75% of
suspended impurities
3. Coagulation: Adding alum, that Removes 100% suspended and colloidal
impurities, clay, silica Filtration Filter bed Removes bacteria, colour, odour, small
dust particles.
Al (OH)3 Al2(SO4)3

4. Filtration: Gravity Sand filter Filter bed Removes bacteria, colour, odour, small dust
particles. Filtration: Filter bed consists of four layers i.e., Fine sand, coarse sand, fine
gravel and coarse gravel. When water is passed through this, all the colour, odour,
part of bacteria are removed. The bed needs periodic regeneration.

5. Sterilisation: Removal of bacteria is sterilization or disinfection.

There are four important sterilization methods.
1. Boiling
By boiling for 10 – 15 minutes, water can be sterilized . But it affects the taste and
can not be used for higher volume water.
2. By using Ozone,(with the usual dosage of 2-3ppm for 10 – 15 miuntes in disinfection
tank) we produce nascent oxygen which is powerful disinfectant.Its excess is not harmful, since
it is unstable and decomposes into oxygen. But, it is costly. It cannot be used and stored for long
time as it is unstable. O2 + [ O] , this nascent oxygen kills bacteria
 3. By passing UV radiations from mercury vapour lamp, the swimming pool water can
be sterilized. But it is costly and turbid water cannot be treated.

4. By chlorination: a) Chlroine gas or solution bubbling will kill bacteria. b) Adding

chlorine and ammonia (2:1) will form chloramine. It decomposes slowly to release chlorine
which kills bacteria. Its excess does not produce any irritating odour. It imparts a good taste to
treated water.

Bleaching powder (CaOCl2) reacts with water and forms hypochlorous acid which kills bacteria.
Genereally, 1kg powder is used for 1000 kilolitres of water. It should be noted that chlorine
should be always added a little higher than the break point chlorination for perfect disinfection.

Breakpoint chlorination

The point at which the residual chlorine begins to appear is called Breakpoint chlorination.
Stage 1: The added chlorine is completely consumed, ie. No residual chlorine is formed.
Stage 2: Further addition of chlorine is used to combine with ammonia to form chloramines
compounds. So, the residual chlorine content is increased.
Stage 3: As saturation level is attained, the chloramines begin to decompose to release
chlorine which is utilized to remove all the types organic and inorganic impurities. So,
residual chlorine level decreases. At point, the added chlorine removes all the types of
impurities. So, the point is known as “ Break point chlorination”.
Stage 4: After this point, further addition of chlorine is not at all utilized and simply
increases the residual chlorine content.