Alkalinity of waterDef: It is due to the presence of those types of substances in water which have tendency to increase the concentration of OH- ions either by hydrolysis or by dissociation of water

Factors

1. The presence of salts of weak organic acids which undergo hydrolysis and consume H+ ions of water. As a result concentration of OH- ions increases in water and water becomes alkaline

2. The presence of HCO3-, HSiO3

- and SiO3-2 ions in water

which makes the water alkaline because they have tendency to take up H+ ions from water. Hence the concentration of OH- ions increases in water and water becomes alkaline

Classification

1. Caustic Alkalinity (Due to OH- and CO3-2)

2. Temperory hardness (Due to HCO3-)

Alkalinity is a measure of ability of water to neutralize the acids

Determination of alkalinity by titrimetric method

A Known volume of hard water sample is titrated against standard acid using methyl orange and phenolphthalein as indicator

Color Change at end point

Methyl orange: Pale yellow to red

Phenolphthalein: Pink to colourless

Determination is based on following reactions

OH- + H+ H2O

CO32- + H+ HCO3

-

HCO3- + H+ H2CO3 CO2 + H2O

Causes of alkalinity1. OH- only

2. CO32- only

3. HCO3- only

4. OH- and CO3-2

5. CO3-2 and HCO3

-

Combination of OH- and HCO3- is not possible because they combine instantaneously to form carbonate ions

OH- + HCO3- CO3

-2 + H2O

Procedure

Calculation

Conclusion: Result of titration i.e. P and M end point OH- CO3

-2 HCO3-

(i) P=0 0 0 M

(ii) P=M P or M 0 0

(iii) P=1/2M 0 2P 0

(iv) P>1/2M M-2(M-P)=2P-M

2(M-P) 0

(v) P<1/2M 0 2P M-2P

EXPRESSION OF ALKALINITY IN TERMS OF CaCO3 EQUIVALENTS

Assignment

• Define alkalinity. Can OH- and CO3–

be present together to make the water alkaline.

• What are the ions responsible for causing alkalinity? Explain the process of determination of alkalinity.

Water for industrial use

In most of the industries, water is used for production of steam. This water should be free from dissolved Ca and Mg salts. Such water is called BOILER FEED WATER

If boiler water contains impurities beyond prescribed limit, they lead to following problems

Scale and sludge formation in boiler

Boiler Corrosion

Priming and Foaming Caustic Embrittlement

Scale and Sludge formation in boiler

Sludge: It is soft, loose and slimy precipitate formed in the boiler water, during the generation of steam

It is formed by MgCO3, MgCl2, MgSO4 and CaCl2

Disadvantages:

(i) Sludge is poor conductor of heat, so they tend to waste the portion of heat generated

(ii) Its excess, decrease the efficiency of boiler

Prevention

(iii)Boiler feed water should be well softened

(iv)Blow-down operation should be carried out

Scale: This is hard and adherent, coating formed on inner wall of boilers, during steam generation

Low pressure boilers Ca(HCO3)2, MgCl2

High pressure boilers CaSO4, Silica

Disadvantages

(i) It is poor conductor of heat so reduce the rate of heat transfer.

(ii) Due to scale formation on heating surfaces, overheating occurs, so metal becomes soft and weak, and this make boiler unsafe

(iii)Boiler tubes may clog due to scale formation

(iv) Scale formation leads to uneven expansion of boiler metal, thick scale cracks and when water comes suddenly in contact with overheated iron plate. This result in the formation of large amount of heat suddenly, which in turn develops a sudden high pressure that may lead to explosion of boiler

Prevention of scales

a.Internal treatment b. External treatment(Sequestration)

lime soda process, zeolite or ion exchange process.

Conditioning methods

Sequestration:

The process where suitable chemicals are added to the boiler containing hard water with which scales are either precipitated or converted in to soluble complexes

The chemical used is called Sequestrating agent

Internal Treatments or Internal Conditioning

In this method appropriate chemicals are added to water in the boiler when the hardness producing ions are converted in to either more soluble salts or insoluble ppts. The following are the important internal treatment methods done inside the boiler.

1. Colloidal Conditioning Certain organic substance like tannin is added to the boiler water they

get absorbed over scale forming substances thus preventing scale deposition.

2. Phosphate conditioningIn high pressure boilers, scale formation is prevented by adding sodium

phosphate to boiler feed water. This reacts with Ca2+ and Mg2+ ions in water forming non adherent and easily removable soft sludge of Ca and Mg phosphate. The commonly used phosphate are NaH2PO4, Na2HPO4 and Na3PO4.

2Na3PO4 + 3CaCl2 → Ca3(PO4)2 + 6 NaClThe choice of phosphate depends up on alkalinity of boiler feed water.

For eg. Ca cannot be precipitated below pH 9.5. Hence Na3PO4 is selected to remove Ca2+ ions.

3. Carbonate ConditioningIn low pressure boilers, scale formation can be prevented by adding Na2CO3 to boiler feed water. Ca salts are precipitated as loose sludge of CaCO3 which can be removed by blow down operation.

4. Calgon ConditioningAddition of sodium hexameta phosphate called calgon prevents scale formation by converting Ca2+ and Mg2+ ions in to soluble complexes.Na2[Na4(PO3)6] Na+ + [Na4(PO3)6]2-

2Ca2+ + [Na4(PO3)6]2- 4Na+ + [Ca2(PO3)6]2-

5. Aluminium conditioningBoiler water is treated with sodium meta aluminate when NaOH and Al(OH)3 are formed. NaOH precipitates Mg salt as Mg(OH)2. Both Al(OH)3 and Mg(OH)2 ppts formed entraps all the scale forming materials. These loose ppt formed can be removed by blow down operation.NaAlO2 + 2H2O → NaOH + Al(OH)3

2NaOH + MgCl2 → Mg(OH)2 + 2NaCl

CaCl2 + Na2CO3 CaCO3 + 2NaCl

Boiler corrosionDecay of boiler material with environment is called boiler

corrosion

Factors causing:

a) Presence of dissolved oxygen: Liberated dissolved oxygen attack the boiler material

4Fe + 3O2 +2xH2O 2Fe2O3.xH2O

b) Presence of dissolved CO2

CO2 + H2O H2CO3 (pitting corrosion)

c) Presence of acid forming salts

MgCl2 + H2O Mg(OH)2 + 2HCl

2HCL + Fe FeCl2 + H2

FeCl2 + 2 H2O Fe(OH)2 + 2 HCl

Here HCl is formed again and again in a chain like process and silicic acid catalyse the reaction, if silica is present in water rate of the reaction increases

Prevention1. By mechanical deaerator

Dissolved gases like CO2 and O2 can

be removed by this method

Water undergo deaeration at high

temperature and low pressure because

Solubility of gases in water α P /T

2. By addition of O2 removing substances

N2H4 + O2 N2 + 2H2O

Na2S + 2O2 Na2SO4

2Na2SO3 + O2 2 Na2SO4

3. By addition of alkali

Priming and FoamingPrimingWhen steam produced rapidly in boilers some droplets of water are carried along with the steam. This process of wet steam formation is called priming.

Causes of Priming• Presence of considerable quantities of dissolved salts.• Sudden boiling• High steam velocity• Sudden increase in steam production rate

Priming can be avoided by : Fitting mechanical steam purifiers Controlling rapid change in steam velocities Maintaining low water level Efficient solftening of boiler water.

Foaming is the production of froth or bubbles which do not break easily on the surface of boiler water. It may occur when the pH of the solution is between 7 and 9.

Causes1. Presence of dissolved impurities and suspended oil content which will

reduce the surface tension of water resulting in bubble formation. 2. Presence of finely divided sludge particlesPrevention1) By removing oil or sludge by addition of coagulants .e.g FeSO4, NaAlO2

etc2) By addition of antifoaming chemicals e.g. Caster oil Disadvantages of Priming and Foaming1. Water hammer which disrupts operation of the equipment. Uneven heating of water2. Actual height of water column in the boiler can not be judged properly,

thereby maintenance of boiler pressure becomes difficult.3. Dissolved solids in boiler water are carried by the wet steam to super

heater and turbine blades where they get deposited as water evaporates. This deposit reduces the efficiency of the machine.

4. Erosion of turbine blades

Foaming

Caustic EmbrittlementThe formation of irregular intergranular cracks on boiler metal

particularly at the point of high local stress by the use of highly alkaline water in the boiler is called Caustic Embrittlement

Causes:

Na2CO3 + H2O 2NaOH + O2

This caustic water flows in to the minute hair cracks usually present on boiler material through capillary action

Here water evaporates and the concentration of NaOH increases progressively which attacks the surrounding area, thereby dissolving iron of the boiler as sodium ferroate

3Na2FeO2 + 4 H2O 6NaOH + Fe3O4 + H2

Mechanistically embrittlement arises due to setting up of concentration cell

Iron at points of local stress

Conc NaOH

dil NaOH(aq)

Iron at plane surface

Prevention: 1. By phosphate conditioning of water

2. By addition of certain chemicals like lignin, tannin etc. These prevent the infilteration of NaOH through air cracks and thus help in preventing it

3. By addition of Na2SO4 to boiler water Ratio of Na2SO4 : NaOH(present in boiler water should be 1:1 For up to 10 atm boiler pressure 2:1 For up to 20 atm boiler pressure 3:1 For more than 20 atm boiler pressure

4. Neutralization of excess alkali by suitable technique

Assignment

• Write short note on the following

(i) Boiler corrosion

(ii) Caustic embrittlement

(iii) Internal conditioning of boiler feed water.