27 feb 2013 lab no 3 hardness
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Lab # 3Determine the
Hardness of Water
Lokhaze AliVisiting Faculty
NICE, SCEE, NUST
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Lab # 3 Water Hardness 2
Water Hardness
Water's hardness is determined by the concentration of multivalent cations in water. There are many different divalent salts, however, calcium and magnesium are the most common sources of water hardness.
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Lab # 3 Water Hardness 3
Water Hardness
• Water is said to be “hard” when it contains Ca+2 and Mg+2 ions. These ions react with soap to form an insoluble substance called “soap scum”
2C17H35COONa + Ca+2 (C17H35COO)2Ca + 2Na+
Sodium Stearate Calcium Stearate
• Hardness of water is caused by the presence of multivalent cations such as Ca+2, Mg+2, Fe+2, Al+3, Mn+2, Ba+2, Sr+2 in water.
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Lab # 3 Water Hardness 4
Total Hardness
Calcium and Magnesium Hardness
Total Hardness = Ca+2 Hardness + Mg+2 Hardness
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Lab # 3 Water Hardness 5
Temporary/Permanent Hardness
a) Carbonate Hardness/Temporary HardnessThe part of total hardness that is caused by Calcium bicarbonate and Magnesium bicarbonateCa(HCO3)2 Ca+2 + 2HCO3
Mg(HCO3)2 Mg+2 + 2HCO3
b) Non-Carbonate Hardness/Permanent HardnessDue to the presence of chlorides or sulphates of calcium and magnesium such as MgCl2, MgSO4 and CaCl22CaCl2 2Ca+2 + 2Cl-
MgSO4 Mg+2 + SO4-2
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Lab # 3 Water Hardness 6
Temporary Hardness
Temporary hardness can be removed by boiling
Ca(HCO3) aq + Heat CO2 + H2O + CaCO3
Mg(HCO3) aq + Heat CO2 + H2O + MgCO3
No health-based guideline value is proposed for hardness. However, the degree of hardness in water may affect its acceptability to the consumer in terms of taste and scale deposition
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Lab # 3 Water Hardness 7
Water Hardness
mg/L as CaCO3 Degree of Hardness
0 - 50 Soft
50 - 100 Moderately Soft
100 - 150 Slightly Hard
150 - 200 Moderately Hard
> 200 Hard
>300 Very Hard
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Lab # 3 Water Hardness 8
Concerns
• Scaling• Cleaning• Public Health
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Lab # 3 Water Hardness 9
Methods of Analysis
Titrimetric MethodBurette
• A piece of glassware designed to deliver known amounts of liquid into another container
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Lab # 3 Water Hardness 10
EDTA
• Ethylenediaminetetracetic acid• Chelating agent
Substance whose molecules can form several bonds to a single metal ionMulti-dentate ligands
• Many clawed; holding onto the metal ion to form a very stable complex
• 4-6 bonds
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Lab # 3 Water Hardness 11
Water HardnessDetermination with EDTA
• H2EDTA2- binds to a Ca2+ ion by forming four special covalent bonds called coordinate covalent bonds.
H2EDTA2-
Ca2++ + 2 H+
Ca2+ ion chelated by
EDTA
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Lab # 3 Water Hardness 12
EBT
Eriochrome BlackIndicator
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Lab # 3 Water Hardness 13
Procedure (Titration of Sample)
• Use a volumetric pipet to dispense 50.00 mL of hard water sample into a 250 mL flask.
• Add 5 mL of pH 10 buffer, 2 drops of Eriochrome Black T indicator, and 15 drops of 0.03 M MgCl2.
• Titrate the solution with EDTA from your burette to the blue endpoint you observed when titrating the blank.
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Thank You