,,.
,J.(}1
i:11
(xvi) ·
25.11
25.12
25 J 3
25.14
25 .15
25 .16
25.l 7
25.18
Integrated rate equations
Reactions involving more than three molecul~s
Methods for determination of order of a reactJ.on
Complex or simultaneous or composite reactions
Theories o( reaction rates
Effect of temperature on rates of reaction Arrhenius equation
Activation energy and catalysis
Examples
CATALYSIS
25 .19 Introduction
25.20 Action of a catalyst
25.21 Characteristics of catalytic reactions (or criteria of catalysis)
25.22 Types of catalysis
25.23 Catalytic promoters
25.24 Catalytic poisons
25.25 Negative catalysis and inhibition
25.26 Autocaialysis
25.27 Induced catalysis
25.28 Activation energy and catalysis
25.29 Theories.of catalysis
25.30 Acid-base catalysis
25.31 Enzyme catalysis
25.32 Some industrial processes using catalysts important
25.33 Criteria! for choosing a catalyst for industrial application
26. PHOTOCHEMISTRY
26. l Introduction
26.2 Photochemical reactions
26.3 Laws of Photochemistry
26.4 Quantum efficiency
26.5 High and low quantum yields
26.6 ·Mechanism of some photochemical reactions
26.7 Photosynthesis
26.8 Types of; photochemical reactions
26.9 Apparatus for photochemical studies
•26.10 ApplicatiC>ns of photochemistry in technology
26.11 Photochemistry of vision
26.12 Photosynthesis and Bioenergetics
QUFSTION BANK
APPENDiX-1 : El Nino Phenomenon and Its Effects
APPENDIX-2 : Basic Principles of Green Chemistry
BIBLIOGRAPHY
INDEX
862
874
874
877
883
887
88g
889
H
901
901
901
904
906
907
908
910
91!
91!
9!2
918
92[
922
923
925-947
925
925
926
927
931
931
935
936
937
938
938
940
948-962
963-964
965-965
966-96S
969-972
-~
Water Treatment
"Water is.one of the most abundant commodities in nature, but is al.so lhe most
. 1·
·
misused one."
. ·
. .
. .
.
.
.
1.1. INTRODUCTION
the
basic
necessities
of
life
1s
water.
L1vmg
things
exist
on
the
earth
because
this is.
0 ~of
.
the only planet that has_the p~esence of water. Water 1s necessary for the survival of all living
things be it plant or animal hfe.
.. .
.
Water is one of the most abundant commod1t1es m nature but 1s also the most misused one.
Although earth is a blue planet and 80% of its surface is covered by water, the hard fact of life
is that about 97% of it is locked in the oceans, and sea which is too saline to drink and for direct
use for agricultural or industrial purposes. 2.4 % is trapped in polar ice caps and giant glaciers,
from which icebergs-break off and slowly melt at sea. >1% wate( is used by man for various
development, industrial, agricultural, steam generation domestic.
1•2, SOURCES OF WATER
Water is required for agricultural, municipal and industrial purposes. For industrial purposes,
natural waters may be broadly divided into the following categories:
(l) Surface waters:
(a) Flowing waters e.g., streams and rivers (Moorland surface drainage)
(b) Still waters e.g.• ponds, lakes and reservoirs (Lowland surface drainage)
(2) Underground water: Water from shallow and deep springs and wells
(3) Rain water
(4) Estuarine and sea water
I From the poin_tof view_ o~ industrial applications, it is not usually f~asible to use rain w~ter
and sea water. Ram water 1s megular in supply and generally expensive to coJlect. Estuanne
and sea waters are too saline for most industrial uses except cooling. The three.inajor sources of
water for industrial use are
(a) Moorland surface drainage.
(b) Lowland surface drainage.
(c) Deep well water.
.
.
The important properties of these three types of waters ate give~-in Tobie - I.
1
.,
2
Sr..
A TEXTBOOK OF ENGINEERING CHEM
,
IS~
Table 1, Analyses or some types of water used in Industry
Typt of wattr
No.
Jonie constituents
pH
Si/lea, Dissolved Hardness in Jtrms ofc;:;:;;:-....
ppm
solids
(in ppm) •co, io
r.,.,
ppm
and their conctn
tration in ppm
- 1.
2.
3.
Moorland surface
drainage
Na·-8
c. ·-1
1
Mg"-6
HCO ; - 15
er- 10
so:- -31
NO,- - Traces
Low-land surface
drainage
Na' - 25
Ca' - 63
Mg'' - 18
HCO,_ - 160
cr-43
so,"- 80
NO,- - 15
Deep well water
Na' - 74
Ca' - 48
Mg'• - 20
HCOj - 350
cr-4s
so_'-- 10
NO ;, - Tra_ces
6.7
8
77
12
30
7.5
10
333
130
100
7.0
15
388
203
I
0
-------42
:230
203
MO(,rland surface drainage
.
..
.
Water from this source is fairly constant m compos1tJon. It 1s generally clear and cloured
brown. It is slightly acidic due to the presence of dissolved carbon dioxide and of weak
organic acids, which renders it corrosive. Although its hardness is _low, it c~n c_ause scaie
formation in boilers unless it is suitably treated before use. It contams some strams of iron
bacteria which must be removed by chlorination to prevent their deposition in the pipe-lines.
It possesses a tendency to dissolve lead and copper. This fact should be considered if the wate,
is used for drinking purposes.
( b) Lowland surface drainage
Water from this source vary widely in composition from place to place. It is not generan1
coloured but may contain fine mud in su.spension, which does not easily settle unless with !he
help of coagulants. Its hardness is usually high and hence can cause serious scale formation in
boilers, economizers and coolers, unless the water is properly treated before use. When the
water is heated in boilers, the CO 2 produced from the bicarbonate ions passes off with !he
steam and dissolves in the condensate, forming carbonic acid which is corrosive to the mild
steel. Riverand canal waters may get contaminated by sewage and industrial wastes, which
may require preliminary treatment prior to softening.
(c) Deep well waters
This type of water is fairly constant in composition, unless contaminated by other waters
percolating through faults in the surrounding strata. When freshly drawn, this is usually
colourless, clear and devoid of finely divided sq.spended matter and hence sparkling. This type
of water may develop a brown opalescencr-on exposure to air due to the presence of small
amounts of ferrous iron, which gets converted into hydrated ferric oxide. Traces of manganese
as well as H1S may al~o be present.
In many deep well waters, the concentration of the bicarbonate is more than equivalent to
the combined concentrations of Ca2• and Mi• ions so that N&iC03 may be considered to be
(a)
3
WATER TREATMENT
present (vide Table-I). The hardness is then entirely alkaline hardness. The sulphate content is
often very low•
Water from shallow wells possess a composition similar to that of low-land surface drainage
waters. The concentration of bicarbonate ions is less than the combined concentrations of Ca''
and Mg" ions. Hence, the water contains non-alkaline hardness.
Some deep well waters contain considerable amounts of free CO 2• Because of the hardness
present and because of the carbonate ions produced due to thermal decomposition of bicarbonate
ions, deep well waters, like low-land waters, give rise to severe scale formation in &iilers. The
high silica content also contributes to the formation of hard scales in boilers. CO, formed by
the decomposition of bicarbonate ions, similar to lowlantl waters, also results in the production
of acid condensate leading to corrosion.
1•3. EFFECT OF WATER ON ROCKS AND MINERALS
J. Dissolution. Some mineral constituents of rocks such as NaCl and CaSO,. 2H 20 readily
dissolve in water.
j 2. Hydration. Some minerals are easily hydrated with the consequent increase in volume
leading to disintegration of the rocks in which these minerals are present. Example are:
(a)
CaS0 4
Hydration
an hydrate
CaS0 4 .2H 20
(accompanied by an expansion of 33%)
gypsum
Mg 2Si04 Hydration
Mg,SiO,. XH,0
Olivine
Serpentine
3. Effect of dissolved oxygen. This leads to oxidation and hydration
Fe,O,
oxidation
Fe,O,
hydration
3 Fe,O,. 2H,O
(b)
Magnatite
haematite
2FeS 2 + 702 + 2H20 -
Marccs1te
Limonitc
2FeSO, + 2H2S04
4. Effect of dissolved CO,
.
(a) Water contmning dissolved CO 2 convert the insoluble carbonates of Ca, Mg and Fe
into their relatively soluble bicarbonates
CaCO,<,i + H,C03 Ca (HC03) 2
Mgco,"l + H,co, Mg (Hco,J,
(b) Rock forming minerals like silicates and alumino-silicates of Na, K, Ca and Fe are
attacked by CO,, producing soluble carbonates, bicarbonates and silica
K,O. Al 20 3.6Si01 + 2H 20 + CO, orthoclase
Al,03.2Si01.2H 20 + K,C03 + 4Si01
kaolin
(c) Rocks containing felspar disintegrate an\! charge n·earby river water with dissolved
salts, fine clay and silica in suspension. ·
Thus, water collects impurities from the ground, rocks or soil with which it comes into
contact. Contamination of water may also result from sewage or industrial wastes, either by
actual contact when these are allowed to flow into 'the running water, or by percolation through
the fround.
1-4. TYPES OF IMPURITIES PRESENT IN WATER
The impurities present in natural waters may be broadly classifad as follows:
(I) Di~olved impurities
(a) Inorganic salts e.g.,
(i) Cations : Ca'', Mg'', Na', K', Fe'', AI" and sometimes traces of Zn" and Cu''
(ii) Anions: Cr, SO/, No,·, HCo,·. and sometimes F. and No,·
,,
4
A TEXTBOOK OF ENGINEERING CHEMISTRi
(b) Gases , .g., CO,, O,. N,. oxides ofN, and sometimes NH,. H,S
(<) Organic salts
(2) Suspended impurities
(a) inorganic ,.g., clay and sand
(b) Organic ,.g., oil globules, vegetable and animal mailer.
Finely divided clay and silica, aluminium hydroxide, ferric hydroxide, organic wa
products. humic aci~s. colou~ng matter, complex protein, aminoacids, which are genera;::
classified as albuno1d ammorua.
.
(3) Colloidal impurities
Clay and finly divided silica colloidal pertical of 10' - IO~ mm size.
(4) Micro-organisms
Bacterias, fungi, algae other micro-organisms and other fonns of animal and vegetable Iir,
1•5, EFFECTS OF IMPURmES IN NATURAL WATERS
The various ty~s of i_mpuri~es pre~nt in natural wate~s .impart some properties 00 th,
waters. From the potnl of view of mdustrtal use, the charactenst1cs and effects of the impurir
on the water quality arc discussed under the following headings:
"
(I) Colour
(2) Tastes and odours
(3) Turbidity and sediment
(4) Micro-organisms
(5) Dissolved mineral mailer:
(a) hardness,
(b) alkalinity,
(c) total solids, and
(ti) corrosion
(6) Dissolved gases
(7) Silica content and
(8) Oxidability.
1•5, (I) Colour. Colour is found mostly in surface waters, although water from so,n,
shallow wells, springs and deep wells may also be occasionally coloured. The colours 0
natural waters range from yellowish-brown to dark brown. The colours and the materials whicl
cause it are often objectionable in which the water and the manufactured product come int,
contact, e.g., dyeing, scouring and laundering.
The colour of natural waters is mainly due to the presence of dissolv~d or colloidall,
dispersed organic mailer. The measurement of colour is usually made with a tlntometer and th;
result is expressed in "Hazen units" or "Standard units of colour". In determining the colour o
water, it is the true colour as expressed in the standard units, that is of interest and not th,
apparent colour.
It is known that solutions of potassium chloroplatinate tinted with small amounts of cobal
chloride give colours similar to the colours of natural waters. The colour produced by I mg
litre (I ppm) of platinum (used as K, Pt CI,) is taken as the standard unit of colour. The colo,
determination is done only after the removal of suspended mailer by centrifugation.
The colour standards are usually prepared by dissolving 1.2545 g of K2PtCl 6 (con1ainin1
0.5 g of Pt) and I g of crystalliz~d cobaltious chloride (CoCl 2 • 6H,0) containing about 0.241
g of cobalt in water with 100 ml of concentrated HCI and diluting to I liter with distilled wa~r
This solution is deemed to possess 500 units of colour, as per the American Public Healtl
Association's books of standard method~ of water analysis.
WATER TREATMENT
Removal or rcduc1ion of colour and organic matter is generally accomplished by
coagulation, settling, adsorption. filtration and sometimes super chlorination . .
1,5, (2) Tastes and odours. Most of the odours in natural waters, with the exception of
H s and iron are organic in nature. The odours and tastes observed in chlorinated waters arc
d~e '10 compounds formed by the reaction of chlorine on traces of organic matter present in the
water. These organic tastes and odours are usually confined to surface wa1ers and arc either
very low or totally absent in deep well waters.
.
Disagreeable odours and tastes are objectionable for various industnal processes such as
beverages, food products, paper, pulp and textiles. Organic tastes and odours may be removed
by means of activated carbon, aeration, or aeration followed by activated carbon treatment.
The removal of inorganic odours and tastes due to H2S or iron will have to be removed by
chemical methods like oxidation, chlorination or precipitation.
1•5, (3) Turbldlly and sediment. "Turbidity" is imparted to natural waters due to the
presence of finely divided, insoluble impurities which remain suspended in water and reduce
its clarity. These suspended impurities may be inorganic in nature (e.g., clay, silt, silica, ferric
hydroxide, calcium carbonate, sulphur, etc.) or organic (e.g., finely divided vegetable or animal
matter, oils, fats, greases, micro-organisms etc.).
Since the different materials that causes turbidity in natural water, an arbitary standard is
used I mg SiO/L = I unit of turbidity. Standard suspensions of pure silica are used for
measuring turbidity. However, the actual standards of turbidity are determined from the depths
of liquid in a standard Jac~son candle turbiditimeter, in which the flame of a candle disappears
when viewed lengthwise through the tube. For instance, the depths of 72.9 cm, 21 .5 cm, 4.5 cm
and 2.3 cm ccitrespond to the turbidities of 25 ppm, I 00 ppm, 500 ppm and 1000 ppm
respectively.
Nowadays, 1urbidity is conveniently measured with the help of more reliable, sensitive
instrument photometers. A beam of light from a source produced by a standtlrdized electric
bulb is passed through a sample. The light emerging from the sample is then directed through
a photometer which measures the light absorbed. The reading on the meter is calibrated in
terms of turbidity. Formazin polymer suspension has now mostly replaced silica suspension as
the standard because it provides more reproducible results. Therefore, the turbidity is now also
expressed as "forma,in turbidity units" (FTU).
Another method used for measurement of turbidity is by light scattering. The light falling
on the sample is scattered because of t~e turbidity present. The scattered light is then measured
hy putting a photometer at right angles to the path of the incident light generated by the light
source. This technique of measurement of light scattered at an angle of 90° is called
"neph,lomrtry", Accordingly, the unit of turbidity is "n,ph,lometric turbidity unit" (NTU).
Tolerances of turbidity for different industries depend upon the type of industry, the grade
of the product being manufactured, the nature of the turbidity present and the type of wet
processing being practised. Suspended silt and mud which cause turbidity may be objectionable
in boilers and in cooling-water systems. Colloidal or dissolved organic matter causing turbidity
may i_nterfere with v.:ater-softening processes. For example, the Zeolitcs and cation~exchangers
used m watcr-softemng processes may be coated with coagulated organic or-&uspended matter
which leads to reduction in their efficiency.
. T_urbidity of wate'. may be removed by sedimentation, followed by (a) coagulation and
flltratton, (b) coagulatton and s,ettling, or (c) coagulation, settling and filtration.
1•5, (4) Micro-organisms. Micro-oganisms are more abundant in surface waters (since
these co":1e into contact with air, soil and vegetation in which the organisms originally existed).
whereas m ~eep _well waters, the bacterial count is often low or even absent. The growth of
these oreamsms m water used for industrial purposes may cause serious problems and hence
,,
ATEXTBOOK OF ENGINEERING CHEMISTRy
6
effeclive measures have 10 be taken 10 prevent the growth of these organisms. Organic grow11i
generally take place most_ readHy in water at te~peraMes ranging from_ Io•c -: 35°C. Many 0;
them form coatings m ptpe Imes, thus reducmg their carrymg capactty considerably. Thes
coatings frequently break loose in large masses which may completely block lhe flow throug~
valves, pumps. nozzles and other parts of the waler distribution syslems. In filters and waie1
softeners employing granular media, the granules inay become malled together by such organ;
growths. 1hus impairing_ their operation by lowering their flow rates and also resulting i~
channeling and overturmng of the beds.
The commonest 1ypes of living organisms that are impor1ant from lhe point of view 0 f
1reatmenl are algae, fungi and bacteria, all of which often form slime wilh consequen1 fouli
and corrosion. The slime surrounding the organisms causes them to adhere to metal surfac;g
This leads to_ difficulti~s such as reduced heat transfers and tube bldckage~. The growth ;;
marine organtSms. part1cularly mussels, m sea-water sys1ems may lead lo senous reduclion i
the carrying capacity of the pipe lines.
n
Conlrol of algal, fungal and bacterial growlhs is usually achieved by chlorinalion. Wai
soluble solid sterilizing agents such as CuSO,, sodium penlachlorophenale and organ;r
mercurials are used in special circumslances. The growth of mussels !Day be prevenled bc
chlorinalion.
Y
Algae and other chlorophyl-containing plant need sunlighl for !heir growth. Hence, lhe
growth of these organisms can be prevented by stQring the water in covered reservoirs. In many
industrial plants, 1he organic growths are removed in the setlling basin of the water-1rea1meni
plant This is usually done with chlorine and coagulation, sellling an\! filtralion, lo remove the
remains. The use of chlorine i~ this way is called pre-chlorination, which is hel_pful in reducing
the dosages of coagulanl required. In many other cases, lhe bulk of 1he orgamc mauer is first
removed by coagulation, sellling and filtration followed by chlorination. This method is called
pos1-chlorination. Somelimes, bolh pre-chlorinalion and post-chlorination are used.
Iron and manganese baclerial growllts, known as "Crenolhrix" are best prevented by removal
of these melals, followed by chlorinalion. In the case of sulphur waters, lhe H,S should be first
removed followed by chlorination to remove lhe last traces of H,S and to kill any sulphalereducing bacteria which may be presenl.
l•S, (5) Dissolved mineral matter. For most of the induslrial uses, only the following
mineral conslituents are usually delermined: Ca, Mg, Na, K, bicarbonale, carbonate, hydroxide
chloride, sulphate, nitrale, fluoride, silica, Fe, Mn and mineral acid. The mosl imporlant qualit;
of lite dissolved mineral maller from the poinl of industrial application include hardness and
alkalinity.
·
Hardness. Hardness was originally defined as lhe soap consuming capacily of a waler
sample. Soaps generally consists of the sodium sails oflong-chain falty acids such as oleic
acid, palmelic acid and stearic acid;, The soap consuming capacily of waler is mainly due to lhe
-l!D'sence of calcium and magnesium ions. These ions react with the sodium sails of long-chain
fatty acids present in the soap to form insoluble scums of calcium and magnesium soaps which
do not possess any detergent value (cleaning tendency).
2 C1,H,,COONa + CaCI, --+ (C 17H,,COO)2 Ca ,j,
+ 2 NaCl
Soap (,olublc)
Calcium soap (insoluble ppt.)
Other melal ions like Fe'', Mn'' and Al" also react with the soap in lhe same fashion, lhus
contributing to hardness but generally, these are present in nalural walers only in !races.
Further, acids such as carbonic acid can also cause free fatly acid to separate from soap solution
and lhus contribute to hardness. However, in praclice, the hardness of a waler sample is usuall)
taken as a measure of its Ca'' and Mg'' conteµt.
·
7
WATER mEATMENT
Temporary and permanent hardne11
When nalural waler is boiled, lhe bicarbonale ions present are decomposed lo fonn carbonale
ions and carbon dioxide is sel free. The hardness so precipitated was referred 10 as "lemporary
hardness", bul lhis tenn is now referred to all the hardness associated with the bicarbonate
content of the water (i.e., that detenninable by tilralion with acid). This is due to lhe fact that
caCO and more particularly MgCO, have appreciable, !hough slight, solubility ip water.
'
,I
Ca(HC03)2
i+HzO+COz
i
Mg(HC03)2 ~Mg(OH)2 i+HzO+COz
i
(Insoluble)
The difference between the temporary and tolal hardness is referred to as "pennanent
hardness", since this is not removed by boiling the water. The pennanent hardness is regarded
as comprisin•g of the dissolved chlorides, sulphales and nitrates of calcium and magnesium.
Alkaline and non-alkaline hardnesa
1
The tenns, temporary and pennanent hardness, are gradually being replaced by the preferred
terms alkaline and non-alkaline hardness.
"Alkaline hardness" is defined as the hardness due 10 lhe bicarbonates, carbonates and
hydroxides of 1he hardness-producing melals. II is also called "carbonate hardness".
1
In a raw water, the alkaline hardness is almosl always lhe hardness associated with the
bicarbonates. However, a ireated or boiler water may also contain hardness due to small quantities
., of CaCO, and Mg(OH)2 in solution. The alkalinily as measured by titration with mineral acid
using methyl orange as indicalor is equal lo lhe sum of the concentrations of the bicarbonates,
carbonate and hydroxide expressed in equivalents. If this alkalinily is less lhan the 1otal
hardness also expressed in equivalents, lhen the alkaline hardness is equal to lhe alkalinily.
Conversely, when the alkalini1y to methyl orange is equal to or greater lhan the tolal hardness,
the alkaline hardness is equal 10 the tolal hardness".
The "non-alkaline hardness" is obtained by subtracting the "alkaline hardness" from the
"101al hardness". This is also known as "non-carbonale hardness".
Estimation of hardness
Hardness is usually detennined by the following two methods:
Soap solution method
Soluble soaps con~ist of_sodium or polassium salts of higher fatty acids, ~uch as oleic acid,
sleanc acid and palrru11c acid. These soaps give lather with hard water only after ,sufficient
quantity of lhe soap is added to precipitale all the hardness causing metal ions presenl in
water.
(I)
Ca or Mg (HCO,), + 2C 17 H,,COONa -
Calcium or maginesium
bicarbonate
Sodium stearatc
(solllble soap with
detergent value)
CaCI, or MgCI, + 2C 17 H,,COONa CaSO, or MgSO, + 2C 1,H,,COONa -
(C 17H,,C00)2 Ca or Mg + 2 Na H CO,
Ca or Mg slearate
(insoluble soap with
no detergent value)
(C 17 H35C00)2 Ca or Mg+ 2NaCI
(C 17 H,,C00)2 Ca or Mg+ Na,SO,.
Thus, after pricipilation of all the hardness causing metal ions present in the hard water
,sample further addition of soap gives lather.
The Iola! hardness of a water sample can be detennined by Iitrating an aljquol of the ·
sample ~gainst a standard soap solution in alcohol. The appearance of a stable lather persistin!!
A TEXTBOOK OF ENGINEERING CH
8
!:Mis
even after shaking for about 2 minutes marks end-point. If the water saniple i~ I
minutes to remove the temporary hardness and then is titrated with the standard so Cd fot l
as described above, the titre value corresponds to the permanent hardness of the s:: so1~1; 0
difference between the two measurements correspond to the temporary hardness.
Pie. \
(2) EDTAmethod
>
This method gives more accurate results than the soap solution method.
HOOC H,C
N-CH, -CH, -N
<
CH,COOH
HOOC H,C
CH,COOH
Fig. 1.1. Structure of EDTA
EDTA can be represented by HY. HY from is not used because of its )imited
again NaAY form also not used beca~se of\ts extensive hydrolysis in solution wbichSOlubiliit
solution highly alkaline Na.,H Y form is mostly used in analytical work since it can b "'•••
in high state of purity.
'
e oblain.;
NaOOCH,C
<
CH,COONa
N-CH, -CH, -N
HOOCH,Cl
•
.
CH,COOH
>
Ethylene diamine tetra acetic acid (EDTA) (Fig. 1.1) forms complexes with Ca'•
as well as with many other metal cations, in aqueous solution. These complexes ~d M:gi,,
general formula given in Fig. 1.2 below:
av, ~,
CH,COO-M-OOCH, C
1./ ~.1
N-'--CH, -CH, - - N
I
I
·ooctt,c
CH,COO"
Fla. 1.2. EDTA complex wilh a divalent mc1al calion, Mi.. such as Cal•/ Mg1o , etc.
Thus in a hard water sample, the total hardness can be determined by titrating the C 2,
Mg'• present in an aliquot of the sample with Na,EDTA solution, using NH,Cl-NH 6H 11and
solution of pH 10 and Eriochrome Black-T as the metal indicator. The colour ch~nge " •
end-point is from wine red to blue.
at the
Na.,H,Y
---+
Mg2+ + HD2-
----->
2Na• + H 1 Y- -
disodium
EDTA
- solution
(Mg or Ca)
Metal indicator
complex (wine
red colour)
o·
+ H,Y- From
disodium
EDTA
solution
MgD· + H'
(Metal-indicator complex)
wine red
(lndicator)
blu e
----->
(Mg or Ca) y ·- + HD-- + H'
metal-EDT A
complex
(colourle ss)
Free
indicator
(blue colour)
•Note. For details regarding the various methods of determining different types of hardness, the students llllJ
refer to lhc Textbook on "Experiments and Calculations in Enoinceriog Chemistry.. by S.S. DARA
9
WATER TREATMENT
pennanent hardness can be detennined by precipitating the temporary hardness by prolonged
boiling for about 30 minutes followed by titration with the Na,EDTA solution as above. The
difference in the titre values corresponds to the temporary hardness of the water sample.•
IFILifllli!thiiN
Parts per million (ppm). One part per million (ppm) is a unit weight of solute per
million unit weights of solution. In dilute solutions of density = 1, I ppm = I mgniter. It is
customary to express hardness in tenns of equivalents of CaCO,. Hence, all the hardness causing
impurities are first convened in terms of their respective weights equivalent to CaC03 and the
sum total of the same is expressed in parts per million.
Equivalent of
}
weight of the substance causing hardness x 50
CaCO
a hardness = Equivalent
.
. of the substance causmg
. hardness
causin'gfor
substance
weight
( I)
(Since chemical equivalent weight of CaCO, = 50)
For instance, 136 parts by weight of CaSO, would react with the same amount of soap as
JOO parts by weight of CaCO, (i.e. , 2 equivalents of CaCO,). Hence, in order to conven the
. weight of CaSO, present as its CaCO, equivalent, the weight of CaSO, should be multiplied by
afactorof IOO or ~ . Similarly,if 'a' gmsofCaCl 2,'b'gmsofMgSO 'c'gms ofMgCI 'ti'
136
68
••
2
gms of Ca (HCO,) and 'e' gms of Mg (HCO,)2 are present in a hard water sample, each of them
2
can be converted in terms of their weight equivalent of O, by multiplying with
.
JOO 100 , ~•
100 .100
1
and 100 respecu.vely. The sum total of the values so obtained expressed
146
62 represents
120per milhon
as parts
the hardness of the water sample under consideration.
111
.
IO0g CaCO, " 111 g CaCI,
aa 120 g MgSO,
= 95 g MgCl2
= 162 g Ca (HCO,) 2
= 146 g Mg (HCO,)2
" 148 g Mg (NO,),
=
44 g
co,
a 136 g CaSO
Equivalents per mlllio_n _(epm): One e'quivalent per million is a unit chemical
eqm~alen_t weight of solute per mllhon weight units of solution. In dilute solutions of density
not d1ffenng very_ much from unity, I epm = I milligram equivalent per litre; and in titrimet
I epm IS convenuonally taken as equal to I ml of t N solution per litre.
ry,
Thus,
I epm of Mg a 12 ppm of Mg
" 50 ppm of CaCO,
= 42 ppm of MgCO,
"' 73 ppm Mg (HCO,),
'" 81 ppm Ca (HCO,),
= 68 ppm CaSO,
" 47.5 ppm MgCI,
" 55 .5 ppm CaCI,
" 60 ppm MgSO, and so on
It should
be noted that for any dissolved sub Slance,_a concentrat,on
.
. of I epm is equal to
"50 ppm
as CaCO,".
. (2)
'?"
ATEXTBOOKOF ENGINEERINGCH!::
10
-
-~
(3) Gnlns per Imperial gallon (gpg). In English system, hardness 1s exp;essed .
in te¾I
of grains
(1 grain = _I_
lb) per gallon ( 10 lbs); i.e. parts per 70,000 parts 1 grain p
7,000
. .
er &al!ll\
is also called as degree Clark. Thus, 9 degrees Clark means that 9 grams rn terms of Cac
present per gallon of water; or 9 parts arc present per 70,000 parts of water. On PPm scO>lit
~ 1-
means that 9 x - - = 128.57 ppm of hardness (as CaCO,) is present in the water
'
70,000
samp1,
lnter-relatlon1hlp between various units of hardneBB
·
I p.p.m. = I mg/I = 0.1°French= 0.07° Clark
= 0.07 grains per imperial gallon
= 0.0583 grains per U.S . gallon
= 0.02 epm as CaCO,
1• Clark = 14.3 ppm= 1.43° French
= 1 grain per imperial gallon
= 0.833 grains per U.S. gallon
1 grain per U.S. gallon = 17.1 ppm : 17.1 mg/I
= 1.2 grains per imperial gallon
l'Fr '= I0ppm= 10mg/1=0.7'Clark
I O Russian = 1 part Ca/ I06 parts of water
I' German= I part Ca/10' parts water.
10 ppm as Cao _or 17 .9 ppm as CaCO,.
Water containing less than 150 ppm of hardness are classified generally as "good" th
containing 150 to 300 ppm as fair and those exceeding 300 ppm as "bad" as per Bu' o~
Indian standard BIS.
reau 01
Example l . Calculate the temporary and permanent hardness of a water sample h .
the following analysis:
· a"ing
Mg (HC0,) 1 73 mg/1
Ca(HC0,) 1 - 162 mg/1
CaSO, - 136 mg/1
MgC/ 2 95 mg/1
(:aCl1 - 1Jl mg/1
NaCl - 100 mg/II
Solution.
Ca CO, equivalent
Salt
Ca(HCO,) 2
IOO = 50 mg/I
146
100
162 X
= 50 mg/I
162
Caso,
136 X
MgCl 2
95
cac1,
100
lll x Ill= lOOmg/1
NaCl
Does not contribute to hardness -and hence ignored. •
Mg(HCO,),
73
X
X
ioo
136
100
9S
= 100 mg/I
= (00 mg/I
11
IATER TREATMENT
Temporary hardness " [Mg(HCO,) 2) + [Ca (HCO,)2)
= 50 mg/I + 100 mg/I
150 mg/I or 150 ppm
= 150 x 0.07° Clark= 10.5° Clark
Permanent hardness e [CaSO,) + [MgC1 2] + [CaC1 2)
100 mg/I + 100 mg/I + 100 mg/I
300 mg/ I or 300 ppm
300 x 0.07° Clark = 21 ° Clark
: . Total hardness = 150 + 300 = 450 ppm
= 450 x 0.07° Clark
31.5° Clark
Alternative method of calculation
Equivalent weights of the different salts involved are as follows :
Salt
Equivalent weight
146
Mg (HCO,) 2 = 73
2
Ca(HCO,)2
162 = 81
2
CaSO,
136 = 68
2
MgCl 2
2
CaCI,
!!!
2
CaCO,
100 = 50
2
Also,
= 47.5
= 55.5
Equivalents per million, e.p.m. = Weight in ppm or mg/I
Equivalent weight
Now,
Salt
Mg(HCO,)2
Ca(HCO,)2
CaSO,
MgCI ,
CaCI,
NaCl
Temporary hardness
Equivalents per million
73
= I epm
73
162
81
= 2epm
68
= 2epm
136
95
47.5 = 2 epm
111
55.5 = 2 epm
Does not contribute to hardness and hence ignored.
50 x I[Mg(HCO,) 2] + [Ca (HCO,),11
50 x 11 + 2} = 150 ppm as CaCO,
A TEXTBOOK OF ENGINEERING CHEMis,-
12
w.
150 x 0.07° Clark
= 10.5° Clark
(since I epm of each salt 50 ppm of CaCO,)
Permanent hardness
hy
= 50 x t[CaSO,l
+ [Mg Cl,] + [CaCI,]}
50 x (2 + 2 + 2) = 300 ppm as CaCO,
300 x O.o7° Clark
21° Clark
Temporary hardness + permanent hardness
Total hardness
=
150 ppm + 300 ppm = 450 ppm
450 x 0.07° Clark 31.5° Clark.
=
Alkalinity. By alkalinity of water we mean the total .content
.
· · of those
d substances •nw
that ~a~se an increased concentration of OJ-r ions upon d1ssociat1on or ue t~ hydrolysis.
atei
2
alkahmty of natural waters is generally due 10 the presence ID them of HCO, , S10 - Hs· lne
and sometimes CO 2 - ions and also due 10 the presence of dsaltsI of. some
•O,-,
I
h weak· or'ga'n.IC ac·d
3
known as _humates, that bind H' ions as a result of hy ro Y_S•~• I ereb_y mcreasin s,
concentrallon of OJ-r ions. In addition to the above, the alkahmty of b01ler water . g the
condition~d by the presence of
and Off ions. ~!so, the presenc~ of salts of wea~\al1o I
such as stl1cates and borates induces buffer capacity ID water and resists the lowerin
Cids
Surface waters containing algae and also water treated by lime-soda process ma/ of
considerable quantities of alkalinity due 10 co,' · and Off.
contain
Depending on the anion that is present in water (HCO, -. co,' - or Off), alkar . 1
' fi d
.
lk 1· .
h
IDity .
c asst 1e respectively as bicarbonate alkalinity, carbonate a a 1mty or ydroxide alk . . 1
I
The maximum contraminant level for alkalinity is 200 ppm for domastic purpose as ahnuy_
Highly alkaline waters may lead to caustic embrittlement and also may cause d~er BIS.
ro;-
N/
P".
arr
I hru
as
of precipitates and sludges in boiler tubes and pipes.
?Osition
With respect to the constituents causing alkalinity in natural waters, the following situ .
may arise :
ations
I , Hydroxides only
2. Carbonates only
3. Bicarbonates only
grc
4. Hydroxides and carbonates
5. Carbonates and bicarbonates
(Notes. The possibility of h~droxides and bicarbonates existing together is ruled
because of the fact that they comblDe with each other as follows forming the carbonat .. ou, b
Off+ HCO - = CO - H O
es.
Y
!
3
l
+
2
The types and extent of alkalinity present in a water sample may be co
•
determined
b
f
·
·
nvenien1ly
.
Y 1tratmg an ahquot of the sam11le with a standard acid to phenolphthal · nd
pomt, P, and contmuing the titration to methyl orange end-point M. The reacf em e .
place may be represented by the following equations:
10ns takrng
Off + H'
-
H,O
•..(I )
co,· · + H'
-
HCO,-
... (2)
HCo,- + H'
H,co, H,O + co,
...(JI o/1
The volume of acid run-down upto phenolphthalein end-point P corresponds to the De,
completion of equations (I) and (2) given above, while the volume of acid run-down afterP
corresponds to the completion of equation (3). The total amount of acid used from beginning
of the experiment, i.e., the methyl orange end-point M, corresponds to the total alkalinicy
present which represents the completion of reactions (I) to (3).
13
/\TER TREATMENT
The results may be summarized in the following Table 2, from which the amounts of
•droxides, carbonates and bicarbonates present in the water sample may be computed:
Table 2
R11ulls of dtrations to phenolphthaltln tnd point, P, and
,ntthyl orange end-point M.
Hydroxide
OF
p = 0
P=M
P>
P<
I
Hco;
Nil
Nil
M
Nil
Nil
2 p
Nil
[2 P - Ml
2 [M -Pl
Nil
Nil
2P
IM - 2 Pl
I
2M
$.
Bicorbonale
Nil
M
2M
p =
Carbon.alt
I
2M
Alkalinity is generally expressed as parts per million (ppm) in terms of Ca co,. \000 ml of
50 acid solution= IOOO mg of CaCO,. Hence
Vol.of
50acidxl000
r· -----NI --------mg/lor m
Alka mity - Vol.ofsarnpletakenfortitration
pp
on the basis of the analysis of water with respect of alkalinity and total hardness, the
,ounts of carbonate hardness (temporary hardness or alkaline hardness) and non-carbonate
rdness (non-alkaline hardness or permanent hardness) present in the water can be determined
follows:
I. If the methyl orange alkalinity of the water equals or exceeds the total hardness, all
the hardness is present as carbonate hardness.
2. If the methyl organe alkalinity of water is less than the total hardness, the carbonate
hardness equals the alkalinity.
3. The non-carbonate hardness, under conditions in (2) above, is equal to the total hardness
minus the methyl organe alkalinity.
Further, on the basis of hardness and alkalinity, natural waters can be subdivided into iwo
,ups : Non-alkaline and alkaline. If the hardness is greater than alkalinity, the water is called
,-alkaline. If the hardness of the water is lesser than alkalinity, the water is characterised as
aline. Non-alkaline waters are more frequently encountereg in nature, which are characterized
different kinds of hardness as follows:
where
H, =
H, =
H0 =
HM =
He. =
H.,1 =
(H + HM) = (He,+ H.,,)
total hardness,
carbonate hardness,
non-carbonate hardness,
calcium hardness -and
magnesium hardness.
0
Example 1. 100 ml of a raw warer sample on titration with N/50 HfO, required 12.4 ml
!he acid to phenolphthalei11 end-point and J5.2 ml of the acid 10 methyl orange end-poinl.
!ermine the type and extent of alkalinity present in the water sample. __
_ _j
Solution.
P = 12.4 ml of N/50 H2SO,
M = 15.2 ml of N/50 H1S0,
,,
r--v-reOOK OF ENl:111'"'-"" ''-' vn
·
~s1
A 1:1' •
14
.
I
.
off and CO -- alkalinities
1
Smee P > -2 M, the water sample roust contain on Y
'
and "'I
••
cannot be any HCO,- alkalinity (vide Table 2).
th
Further,
I. The volume of N/50 H SO equivalent to Off present in !OO ml of e water sarn
[ P-M] = (2xl2.4)IDl-15.2ml
Pie
2
nd
2
= 24.8 ml - 15.2 ml= 9.6 ml. a 1
th
8
2. The volume of N/50 H,SO, equivalent 10 CO,-- present in IOO ml of e water al!J
= 2 [M- Pl = 2[15.2- 12.4) ro
Pl
= 2 x 2.8 ml= 5.6 ml
3. Since the equivalent weight of CaC03 = 50
I ml of IN H SO, = 50 mg of CaCO,
2
Alkallnlty due to OH.
Since I ml of IN H2SO,= 50 mg CaCO,
•
I
9.6 ml of N/50 H SO• = 50 x 9•6 x -50 mg CaCO,
2
= 9.6 mg CaCO/100 ml of water sample
Amount of mr present in I litre of the water sample
= 9.6 x 1000 = 96 mgn as CaCO,
100
Alkalinity of the water sample due to 0~
= 96 ppm
Alkallnlty due to co,-Similarly,
I
5.6 ml of N/50 H2SO,= 50 x 5.6 x - mg CaCO,
50
= 5.6 mg CaCO/100 ml of water sample
Amount of co,-- present in I litre of the water sample
= 5.6 x 1000 = 56 mg/I as CaCO,
100
Alkalinity of the water sample due to co,-= 56 ppm
Report
The given water sample contains:
Off alkalinity = 96 ppm
co,- - alkalinity 7' 56 ppm
Total alkalinity = 152 ppm
Example 2. A water sample is not alkaline to phenolphthalein. However, 100 ml o/ 11,
sample, 0 11 titration with N/50 HCI, required 16.9 ml to _obtain the end-point, using mtl</
orange as indicator. What are the types and amount of alkalinity present in the sample ? ·
Solution.
P = O; M= 16.9ml.
Hence, the alkalinity of the water sample is only due to HCO,- ions.
: . Volume of N/50 HO equivalent to the HCO,-present in I00 ml of the water sample = 16.9
Since I ml of IN HCI = ~O mg CaCO,
15
WATER TREATMENT
~rt
16.9 ml of N/50 HCI " 50 x 16.9 x
mg
50
= 16.9 mg CaCO/100 ml of water sample
Amount of HCo,- present in I litre of the water sample
s
16.9 x
1000
= 169 mg as caco,
100
a 169 ppm
:. Alkalinity
Report
The given water sample consists of only HCo,- alkalinity and ii is 169 p.p.m.
f Example 3. A water sample is allcaline to both phenolphthalein as well as methyl orange.
JOO ml of the water sample on titration with N/50 HCI uquired 4.7 ml of the acid /o
11_henolphthalein end-point. When a few drops of methyl orange are added to the same solution
and the litration further continued, the yellow colour of tbe solution just turned red afier
addition of another 10.5 ml of the acid solution. Elucidate on the type an extent of alkalinity
:'present in the water sample.
' Solution.
P = 4.7 ml of N/50 HCI
M = (4.7 + 10.5) = 15.2 ml of N/50 HCI
Since P <
i
M, the waler.sample must contain co,-- alkalinities and HCO,- alkalinity
only and no! Off alkalinity.
Further,
(,) The volume ofN/50 HCI equivalent to co,-- present in 100 ml of the waler sample,.
2[PJ = 2x4.7=9.4ml,and
(ii) The volume of N/50 HCI equivalent lo HCO,- present in 100 ml of the water sample "'
M-2P = (l5.2-9.4)ml=5 .8ml
Alkillnlty due to Co,-Since I ml of l N HCI
= 50 mg of CaCO,
9.4 ml of N/50 HCI a 50 x 9.4 x
I
50
= 9.4 mg CaCO/100 ml of water sample
Amount of Co,- - present in I litre of the water sample
9.4 x
1000
100
= 94 mg/I = 94 ppm as CaCO,
Alkalinity due to co,-- = 94 ppm
Ukallnlty due to HCO, 1
5.6 ml of N/50 HCl a 50 x·5.8 x - = 5.8 mg ofCaCO3/100 ml of water sample
50
Amount of HCO, - present in 1 litre of the water sample
5.8 x
Alkalinity due to HCO,- = 58 ppm
!port
The given water sample contains:
CO, alkalinity = 94 ppm
IOOO
lOO
= 58 mg/I as CaCO,
r
A TEXTBOOK OF ENGINEERING CHEMIST~
16
HCO,- alkalinity = 58 ppm
Total alkalinity = 152 ppm
Example 4. 10!) ml of a water sample, on titration with N/50 H,SO,. ga~e a titre value op
5.8 ml to phtnolphthalein end-poi'nt and J 1.6 ml ro methyl orange end-point, Calculate the
alkaUni'ty of the water sample ;11 terms of CaCO, and comment on the type of alkalin;I)
pr,sent.
Solution.
P = 5.8 ml; M = 11.6 ml.
·
. due to CO Smee
P = -I M, it means that all the alkalinity present in the water samp1e ts
2
J
- only: while OH- and HCo,- are absent.
Further, the volume of N/50 H,SO, equivalent to co,- - present in 100 ml of .the Water
sample
2P =2 x 5.8 =11.6 ml
Since I ml of IN HCI
a 50 mg of CaCO,
x .2_
I
50
= 11.6 mg of CaCO/100 ml of water sample
Strength of col-- in terms of Caco,
'
1000
11.6 X lOO = 116 mg/I
11.6 ml of N/50 HCI
s
50 x
= 116 ppm
Report
The alkalinity of the water sample is 116 ppm, which is only due to co,- -.
Example S. JOO ml of a water sample, on titration with N/50 HfO,. using phenolphthalein
as indicator, gave the end-point when 5.0 ml of acid were run down. Another aliquot of JOO ml
of the sample also required 5.0 ml of the acid to obtain methylorange end-point. What is the
,type of alkalinity present in the sample and what is its magnitude? _ _
,._.
Solution.
P = 5.0ml; M=5.0ml.
Since P = M, it is obvious that the water sample contains only hydroxide alkalinity aud
it is not a natural water sample. Further, since I ml of IN H,SO, "' 50 mg ofCaCO 3
I
50
= 5 mg of 9co,t 100 ml of water sample
The amount of Off" present in I litre of the water sample
5mlofN/50H2SO4
= 5x
5
= 50x 1 x
1000
lOO
= 50 mg ofCaCO3
Alkalinity of the water sample ;: 50 mg/I or 50 ppm
Chlorides
Chlorides are present in water generally as NaCl, MgCI, and CaC1 2• Although chlorides are
not considered as harmful as such, their concentrations over 250 mg/L impart peculiar taste to
the water which is objectionable or unacceptable fQr drinking purposes for most people from
aesthetic point of view. Hence the secondary standard for chlorides is 250 mg/L. Further,
presence of unusually high concentrations,?f chloride in water generally indicates pollution
from domestic sewage or from industrial wastewaters, presence of chlorides is also undesirable
WATER TREATMENT
17
in boiler feed water. Salls like MgCI , may undergo hydrolysis under the hi gh pressure and
temperature prevailing in the ho1ler, ge nerating hydrochl oric ac id which causes corrosion of
boiler parts.
Sulphates
Sulphates are among the major anions present in natural water. When sulphate s are present
in excessive amounts in drinking water, they may produce a laxative or cathartic effect on the
people consuming such waler. The secondary maximum contaminant level (SMCL) for sulphates
is 250 mg/L.
.
Nitrates
Excessive concentrations of nitrates are objectionable particularly for infants. The
maximum contaminant level (MCL) for nitrates is 10 mg/L.
In agricultural regions, ground water can have significant concentrations of nitrates from
unused fertilizer leaching into the underlying aquifers.
Surface waters can be polluted by nitrates both from discharge of municipal wastewater
and from drainage from agricultural lands.
Ingestion of excessive nitrates in drinking water by infants causes a disease known as
·'methemoglobinemia" (infant cyanosis or blue baby syndrome). In the intestines of infants
(particularly those below 6 months of age), nitrates can be reduced to nitrites, which are
absorbed into the blood, oxidizing the iron present in the blood. thereby resulting in cyanosis
which causes a blue colour to the baby. That is why, thi s condition is known as " Blue-1,aby
syndrome". Infant methemoglobinemia can be readily diagnosed by medical docwrs and is
treated readily by injecting methylene blue into the infant's blood.
Nitrates can be effectively removed from water with the help of strongly basic anion
exchange resins. However, high operating cost and the disposal of huge quantity of waste brine
from regeneration of the resin pose major limitations for this treatment process.
Fluorides
Flouride is found in groundwater as a result of dissolution from geologic formations. It is
particularly found in ground waters that come into contact with fluoride containing minerals
such as fluorspar (CaF,), fluorapatite [Ca 10 F,(PO,J.l, cryolite (Na,AIF,) and igneous rocks
containing fluosilicate s.
Surface waters generally contain much smaller concentrations of fluoride, unless they are
contaminated otherwise. Water pollution by fluorides may be caused by the contaminated
domestic sewage and the run-off from agricultural lands where phosphatic fertilizers have been
used. Phosphatic fertilizers may contain 0.5 to 4% of fluorine by weight as an impurity.
The fluoride concentration found in some water samples is 1.5 to 6 mg/L, and in extreme
cases, it may be as high as 16 to 36 mg/L. Unsightly fluoro sis may be caused when the fluoride
level in drinking water exceeds 4 mg/L.
Optimum tluoride concentrations prescribed in public water supplies generally are in the
Lange of 0. 7 to 1.2 mg/L. depending _on the annual average of maximum daily air temperature
of the place, on which the consumption of the water by the people depends. Beneficial health
effects have been observed where the fluoride levels are optimum. However, too low or too
:high concentrations of fluoride in drinking water are problematic , and such si tuations may
have to be tackled by fluoridation (addition of fluoride) or dcfluoridation (removal of fluoride)
respectively.
·
Abse~ce or low concentration of flu oride in drinking water causes a high incidence of
dental c~nes, particularly in children. On the other hand, excessive concentration of fluoride
10
drinking water causes "fluorosis" which is manifested in mottling of teeth. discoloration
and at times, chipping of teeth.
.