Chemical Engineering

Estimation of Hardness.


Written by V R Sunday, 09 January 2011 18:44

The estimation of hardness of water is very essential for its use in boilers for steam generation as well as for industrial uses.


It is a complexometric method of determination of hardness of water by using EDTA(ethylene diamine tetra acetic acid) as the titrant and EBT(eriochrome Black-T) as indicator.At a PH 10,Ca2+ and Mg2+ ions in hard water forms a weak wine red colored complex with EBT,when EDTA is added to this,weak EBT is replaced by strong EDTA.At the end point the wine red color changes to blue.The disodium salt of EDTA available as versene(Na2H2V) is used for the titration.


Sodium Aluminate Conditioning.


Written by V R Sunday, 09 January 2011 18:37

Boiler water is treated with NaAlO2,it get hydrolyzed as

NaAlO2  + 2H2O=NaOH + Al(OH)3

The sodium hydroxide formed precipitates the magnesium salts present in water as magnesium hydroxide.

2NaOH + MgCl2=Mg(OH)2 + 2NaCl

Both Al(OH)3 and Mg(OH)2 flocculant precipitate formed entraps all the scale forming materials and colloidal impurities such as silica and oil.These loose precipitates can be removed by blow-down operation.NaAlO2 is a cheap by product obtained in bauxite refining.

Boiler Corrosion.


Written by V R Sunday, 09 January 2011 18:29

It is the decay of boiler material by a chemical or electrochemical attack by its environment.The main reasons for boiler corrosion are:

  • Dissolved oxygen

It is the main corrosion causing impurity in water,this oxygen at high temperature and pressure attacks the boiler material to form rust.

  • Dissolved carbondioxide.

It is carbonic acid which has a slow corrosive effect on the boiler material.Carbondioxide is also produced inside the boiler by the decomposition of bicarbonates.

  • Acid from dissolved magnesium salts.

Water containing dissolved magnesium salts liberate acids on hydrolysis.Liberated acids react with boiler material iron in a chain like reaction producing HCl again and again.Consequently presence of even small amount of MgCl2 will cause corrosion of iron to a large extent.It is avoided by adding alkalinity from outside.


Important relations in lime soda process and problems.


Written by V R Sunday, 09 January 2011 18:20

  • Hardness is always expressed as CaCO3 equivalents.
  • In calculating lime requirement one molecule of Ca(OH)2 is required for one molecule of Ca(HCO3)2,but two molecules of lime is required for one molecule of Mg(HCO3)2.
  • For every molecule of magnesium permanent hardness one molecule of lime is required for neutralization.
  • For every molecule of magnesium salt or calcium salt one molecule of soda is required.
  • There is no consideration for temporary or carbonate hardness during calculation on soda requirement.
  • CaCO3,MgCO3,whenever given as such as constituents of hard water it should be taken as actually present as their bicarbonates.
  • There is no effect on lime and soda on sodium salts such as NaCl,Na2SO4 etc.present in water.



Written by V R Sunday, 09 January 2011 16:36

If excess residual chlorine is present,water is subjected to dechlorination.For this water is filtered through activated carbon which removes the decomposition products and excess of chlorine.The other dechlorination methods are addition of SO2.Na2CO3,NaHSO3 etc.

SO2  +  Cl2  + 2H2O=H2SO4 + 2HCL

Na2SO3 + Cl2 + H2O=Na2SO4 + 2HCL


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