Alkalinity of water. Def : 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
Def: 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
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
OH- + H+ H2O
CO32- + H+ HCO3-
HCO3- + H+ H2CO3 CO2 + H2O
Conclusion: Result of titration i.e. P and M end point
EXPRESSION OF ALKALINITY IN TERMS OF CaCO3 EQUIVALENTS
be present together to make the water alkaline.
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
Priming and Foaming
Scale: This is hard and adherent, coating formed on inner wall of boilers, during steam generation
lime soda process, zeolite or ion exchange process.
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
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 conditioning
In 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 NaCl
The 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.
In 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 Conditioning
Addition 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 conditioning
Boiler 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
1. 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
The 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
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
Iron at plane surface
(i) Boiler corrosion
(ii) Caustic embrittlement
(iii) Internal conditioning of boiler feed water.