(c) Dr. Payal Joshi

Hardness of Water

yWater containing dissolved salts of Ca and Mg is called hard water

yHard water prevents lathering of soap yCa2+ & Mg2+ ions react with soaps which are salts of

fatty acids to give insoluble scums or precipitates yThis property or tendency ---------- hardness of

water

(c) Dr. Payal Joshi

y 2C17H35COO-Na+ + Ca2+ → (C17H35COO)2 Ca +2Na+

(scum)

y 2C17H35COO- Na++ Mg2+→(C17H35COO)2 Mg (scum) +2Na+

Types of Hardness

Temporary/ Alkaline/Carbonate

Permanent/ Non-alkaline/Non-Carbonate

(c) Dr. Payal Joshi

y Other metal ions like Fe2+, Mn2+, Al3+ also react with soap

in a similar manner, thus contributing to water hardness

y However, for practical purposes only Ca2+ and Mg2+ content

is considered for deciding water hardness

(c) Dr. Payal Joshi

Temporary Hardness y Due to dissolved bicarbonates of Ca & Mg in water y It is so called since it can be easily removed simply by

boiling y Heat decomposes bicarbonates of Ca & Mg as follows

(c) Dr. Payal Joshi

Permanent Hardness y Due to dissolved sulfates, chlorides, nitrates of Ca &

Mg in water y It can be eliminated by special chemical methods y Temporary & Permanent hardness added together gives

Total hardness Expression of Hardness y Dissolved Ca & Mg salts calculated as Calcium

carbonate equivalent (mg CaCO3 equivalent)

(c) Dr. Payal Joshi

y Classify the following impurities into temporary, permanent & non-hardness causing impurities

Impurities Type of hardness

Ca(HCO3)2 Temporary MgSO4 Permanent CaCl2 Permanent CO2 Non-hardness HCl Non-hardness

Mg(HCO3)2 Temporary CaSO4 Permanent NaCl Non-hardness

(c) Dr. Payal Joshi

yHardness of water in terms of Degree Clark (0Cl) 0.07 0Cl ≡ 1 ppm yHardness of water in terms of Degree French (0Fr) 0.1 0Fr ≡ 1 ppm yHardness of water in terms of mg/lit 1 mg/lit ≡ 1 ppm

(c) Dr. Payal Joshi

y A water sample has a hardness of 807 ppm. Find

hardness in terms of 0Cl (Degree Clark) and 0Fr (Degree French)

a) Hardness in terms of 0Cl Since, 0.070Cl = 1 ppm Thus, 807 ppm ≡ 807 x 0.07 = 56.49 0Cl 1 b) Hardness in terms of 0Fr Since, 0.10Fr ≡ 1 ppm Thus, 807 ppm ≡ 807 x 0.1 = 80.7 0Fr 1

(c) Dr. Payal Joshi

y Conversion to mg CaCO3 equivalent = Mass of hardness producing substance x Eq.wt of CaCO3

Eq.wt of hardness producing substance

(c) Dr. Payal Joshi

Disadvantages of using hard water

Domestic use:

y Washing : no lather formation, wastage of soap

y Bathing: no lather formation. Also the resulting ppt sticks on body

y Cooking: due to dissolved salts, boiling point of water is elevated, causing wastage of time & fuel.

y Drinking: bad effect on metabolic system. Calcium oxalate stones may develop in urinary tracts, if used regularly.

(c) Dr. Payal Joshi

Industrial use: y Textiles: loss of soap during washing of yarn, & fabrics.

Ppt of Ca & Mg sticks on fabric. Fe, Mn salts leave colored spots on fabrics

y Sugar: crystallization of sugar is affected due to presence of sulfate ions.

y Dyeing: Dissolved Ca, Mg, Fe salts react with dye forming undesirable ppts giving poor shades of color on the fabric.

y Pharmaceuticals: If used in drug, injections, ointments– results in undesirable products in them causing ill health

(c) Dr. Payal Joshi

Determination of hardness by EDTA Method

y Determining total amount of Ca2+ & Mg2+ ions in water y Titrating sample with standard EDTA solution using an

organic dye indicator EBT (Eriochrome Black T) y EDTA is a weak acid with the structure given below

(a) Structure of ethylene diamine tetraacetic acid, EDTA (b) Structure of tetracarboxylate ion, [EDTA]4–, formed by dissociation of EDTA

(c) Dr. Payal Joshi

y Mg2+ + EBT (blue) ------------> [Mg2+ ---EBT] [Mg2+ ---EBT] (wine red unstable complex) ---> [Mg2+ --- EDTA] (stable)+ EBT (blue)

Structure of [Mg–EDTA]2– chelate (1:1 complex), known as chelates Complexation Reaction

NH4Cl+NH4OH

pH = 10

Titration EDTA

(c) Dr. Payal Joshi

yPreparation of Reagents y Standard Hard Water: 1 gm pure dry CaCO3 dissolved

in min.quantity of dil.HCl. y Solution evaporated to dryness on water bath &

residue left is dissolved in distilled water & solution is diluted to 1 lit=> Hardness of solution= 1 mg of CaCO3 equivalent

y EDTA solution, EBT indicator => prepared y Buffer solution : NH4Cl + conc. NH3

(c) Dr. Payal Joshi

y Standardization of EDTA solution y Pipette 50 ml of std hard water (SHW) in conical flask. y Add 10-15 ml buffer soln (pH=10), 4-5 drops of EBT indicator. y Solution is titrated against EDTA till colour changes from wine red

to deep blue (V1 ml)

y Water Sample Analysis (Estimation) y Total hardness : 50 ml hard water sample titrated against

EDTA (V2 ml) y Permanent hardness : 50 ml hard water sample is boiled

for 10-15 min, filtered, diluted with D/W to make up to 50 ml & titrated against EDTA (V3 ml)

y Temporary Hardness = (Total– Permanent) hardness

(c) Dr. Payal Joshi

Softening of Water y Process of removing hardness/ reducing concentration of

hardness causing salts from water y Clark’s Process

y Calculated amount of lime [Ca(OH)2] added to hard water y Bicarbonates are converted into insoluble carbonates &

removed by settling & filtration y Lime Soda Process

y Lime treatment followed by addition of soda ash (Na2CO3) y Calcium ions in hard water are removed as CaCO3 and Mg as

Mg(OH)2

(c) Dr. Payal Joshi

Lime soda method

(c) Dr. Payal Joshi

Reactions with Lime y Lime removes free acids

y 2HCl + Ca(OH)2 Æ CaCl2 + 2H2O y H2SO4 + Ca(OH)2 Æ CaSO4 + 2H2O

y Lime removes temporary hardness y Ca(HCO3)2 + Ca(OH)2 Æ 2CaCO3 + 2H2O y Mg(HCO3)2 + 2Ca(OH)2 Æ 2CaCO3 + Mg(OH)2 + 2H2O

y Lime removes permanent hardness y MgCl2/MgSO4 + Ca(OH)2 Æ Mg(OH)2 + CaCl2/CaSO4

y Lime removes dissolved iron and aluminium salts y Al2(SO4)3 + 3Ca(OH)2 Æ 2Al(OH)3 + 3CaSO4 y FeSO4 + Ca(OH)2 Æ Fe(OH)2 + CaSO4

(c) Dr. Payal Joshi

Reactions with soda y Removes permanent hardness due to Ca salts y CaCl2 + Na2CO3 Æ CaCO3 + 2NaCl y CaSO4 + Na2CO3 Æ CaCO3 + Na2SO4 y NaAlO2 + 2H2O → NaOH + Al(OH)3 (hydrolysis) y Al(OH)3 gelatinous ppt that traps fine ppt/particles y Residual Hardness = 50-60 ppm

The method discussed is called Cold Lime Soda Process

(c) Dr. Payal Joshi

Hot lime soda softener

(c) Dr. Payal Joshi

Hot process is operated at temperature close to boiling point of the solution, (80-1500C)

y Reaction proceeds faster y Softening capacity of hot process increases manifold y Precipitate & sludge formed settle down rapidly, hence no

coagulants needed y Much of the dissolved gases (like CO2 & air) driven out of

water y Viscosity of softened water is lower, so filtration becomes

easier y Hot-Lime soda process produces water of comparatively

lower residual hardness of 15-30 ppm

(c) Dr. Payal Joshi

Amount of Lime (in kg) = 74/100 [Temp Ca2+ + (2 × Temp Mg2+ ) + perm Mg2+ + Fe2+ , Al3+

+ H+ (HCl/H2SO4) + HCO3- - NaAlO2] ×Vol of water x 100/%pure

106

Amount of Soda (in kg) 106/100 [Perm (Ca2++Mg2++Fe2++Al3++ H+ - HCO3

-] × Vwater x 106 100/%pure

(c) Dr. Payal Joshi

Ion-Exchange Process

y Reversible exchange of ions of same charge between mobile liquid phase & an insoluble solid (stationary phase)

y Ion-exchanger is an insoluble material liberating counter ions by electrolytic dissociation

y Cation exchanger: High mol.wt, cross-linked polymer containing sulfonic (-SO3H), carboxylic (-COOH), or phenolic (-OH) as a part of resin and equivalent amount of cations

y H-R (resin) + Na+ Ù Na-R (resin) + H+

y 2NaR (resin) + Ca2+ Ù CaR2 (resin) + 2Na+

(c) Dr. Payal Joshi

y Anion exchanger : Is a polymer containing quaternary ammonium groups (-N+R2 ) containing equivalent amount of anions, Cl-, OH-, etc

y 2RCl + SO42- Ù R2(SO4) + 2Cl-

(resin) (solution) (resin) (solution) Water from which all the cations & anions are removed is called

demineralized or deionized water. Water is first passed to cation exchanger in acid form. All cations

will be exchanged for H+ ions Next, water coming from cation exchanger is passed into anion

exchanger in basic form and anions are exchanged for OH--

(c) Dr. Payal Joshi

Major Impurity=> CaSO4 2RCOOH + Ca2+ Ù (RCOO)2Ca + 2H+

2R’OH + SO42- Ù R’2SO4 + 2OH-

H+ + OH- Ù H2O (c) Dr. Payal Joshi

Regeneration

yWith continued operations, exchange capacity of the column decreases

yAdding a strong acid regenerates cation resin and the anion resin is regenerated by adding strong base.

y (RCOO)2Ca + HCl Æ 2RCOOH + Ca2+ yR’2SO4 + NaOH Æ R’OH + SO4

2-

(c) Dr. Payal Joshi

y Advantages: (i) Process can be used to soften highly acidic or alkaline water (ii) Water of low hardness(2 ppm) can be obtained y Disadvantages: (i) Equipment is costly (ii) Turbidity of water to be treated should be below 10 ppm

(c) Dr. Payal Joshi

Zeolite or Permutit Process yHydrated sodium alumino silicate

Na2O.Al2O3.xSiO2.yH2O

yZeolite is capable of exchanging reversibly their sodium ions for hardness producing ions in water.

(c) Dr. Payal Joshi

• Hard water enters from top at a specified rate & passes over a bed of sodium zeolite kept in a cylinder.

• Softened water is collected at the bottom of cylinder & is taken out from time to time.

(c) Dr. Payal Joshi

y Theory: When hard water is passed over a bed of sodium zeolite , Ca2+, Mg2+ ions are taken up by the zeolite simultaneously releasing equivalent Na+ ions in exchange for them.

y CaCl2 + Na2Ze → CaZe + 2NaCl

y MgSO4 + Na2Ze → MgZe + Na2SO4

(c) Dr. Payal Joshi

y Regeneration: When Zeolite is completely converted into calcium & magnesium Zeolites, it ceases to soften water i.e. it gets exhausted. It is generated by treating with 10% brine solution.

y CaZe + 2NaCl → Na2Ze + CaCl2

y MgZe + 2NaCl → Na2Ze + MgCl2

(c) Dr. Payal Joshi

Advantages:

y Water of about 10 ppm hardness is produced.

y Process automatically adjusts itself for different hardness of incoming water.

y Requires less skill in maintenance as well as operation.

(c) Dr. Payal Joshi

Disadvantages:

y Treated water contains more sodium salts

y Method only replaces Ca2+ & Mg2+ ions by Na+ ions, but leaves all ions (HCO3

- & CO3 2-) in soft water

y Such soft water containing NaHCO3 , Na2CO3 etc decomposes to give dissolved CO2 which cause corrosion in steel container walls

(c) Dr. Payal Joshi

Drinking Water Purification

(c) Dr. Payal Joshi

Removal of microorganisms

y Process of destroying or killing disease-producing microorganisms from water and making it safer for use is called disinfection

y Chemicals or substances added to water for killing bacteria are known as disinfectants

y Chlorination y Bleaching powder treatment y Ozonization

(c) Dr. Payal Joshi

Chlorination y Chlorine (gas or in concentrated solution form) produces

hypochlorous acid Cl2 + H2O Æ HOCl + HCl y Hypochlorous acid, so produced kills the bacteria.

Bacteria + HOCl Æ Bacteria killed

y HOCl Ù H+ + OCl-

y Hypochlorite ion inactivates the enzymes present in microorganisms—leading to death

(c) Dr. Payal Joshi

Action of bleaching powder

y Only calculated quantity of bleaching powder (calcium hypochlorite) should be used, since an excess of it gives bad taste and smell to treated water

Ca(OCl)2 + H2O Æ Ca(OH)2 + Cl2

Cl2 + H2O Æ HCl + HOCl (hypochlorous acid)

Germs + HOCl Æ Germs killed

(c) Dr. Payal Joshi

Action of Ozone y Ozone is an excellent disinfectant produced by passing electric

discharge through oxygen 3O2 Æ 2O3 y Ozone is highly unstable and breaks down liberating nascent

oxygen

O3 Æ O2 + [O]

y Nascent oxygen kills all bacteria and oxidizes organic matter present in water

(c) Dr. Payal Joshi

Reverse Osmosis Semipermeable membrane

Pressure

Initial condition Equilibrium Reverse Osmosis

(c) Dr. Payal Joshi

Ultrafiltration

y Ultrafiltration is a cross flow separation process- similar to reverse osmosis

y Pore size of the membrane used in ultrafiltration is in the range of 0.1 – 0.001 microns

y Low MW compounds pass through membrane & high MW compounds are retained in the membrane.

y It is suitable for retaining biomolecules, bacteria, viruses, polymers, colloidal particles and sugar molecules.

(c) Dr. Payal Joshi

• The stream of liquid that comes

out through membrane is

permeate and the other side of

the liquid stream (which contains

the macromolecules) is called the

concentrate (retained by the

membrane)

(c) Dr. Payal Joshi

Applications

yPretreatment in sea water desalination plants in combination with reverse osmosis

y Sterile filtration of drinking and beverage water yRemoval of metal hydroxides in wastewater

treatment

(c) Dr. Payal Joshi