Water Hardness

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Water Hardness
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Water Hardness

• Definition
• Concerns
• Scaling
• Cleaning
• Public Health

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Causes
Cations causing hardness Anions
Ca2 HCO3-
Mg2 SO42-
Sr2 Cl-
Fe2 NO-
Mn2 SiO32-
Table 19.1. Principle cations causing hardness
in water and the major anions associated with them
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Sources
From USGS
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Water Hardness Rating
mg/L as CaCO3 Degree of Hardness
0-60 Soft water
61-120 Moderately hard water
121-180 Hard water
gt180 Very hard water
From USGS
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Methods of Determination

• Complete Cation Analysis
• Most accurate
• Calculation of the hardness caused by each ion is
  performed by use of the general formula
• Hardness (mg/L) as CaCO3 M2 (mg/L) X 50
  
• EW of M2
• where M2 represents any divalent metallic ion
  and EW represents equivalent weight

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Example of Complete Cation Analysis

• Calculate the hardness of a water sample with the
  following analysis

Cation Concentration (mg/L) Anion Concentration (mg/L)
Na 20 Cl- 40
Ca2 15 SO42- 16
Mg2 10 NO3- 1
Sr2 2 Alkalinity 50
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Example of Complete Cation Analysis
Cation EW Hardness, mg/L as CaCO3
Ca2 20.0 (15)(50)/(20.0) 37.5
Mg2 12.2 (10)(50)/(12.2) 41.0
Sr2 43.8 (2)(50)/(43.8) 2.3
Total hardness 80.8
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Methods of Analysis

• Titrimetric Method
• Burette
• A piece of glassware designed to deliver known
  amounts of liquid into another container

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EDTA

• Ethylenediaminetetracetic acid
• Chelating agent
• Substance whose molecules can form several bonds
  to a single metal ion
• Multi-dentate ligands
• Many clawed holding onto the metal ion to form a
  very stable complex
• 4-6 bonds

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EBT

• Eriochrome Black
• Indicator

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A Word about Moles

• A mole used in chemistry is something like the
  term dozen we use every day.
• The atomic weight of any element, expressed in
  grams, contains 6.02 x 1023 atoms.
• There are 6.02 x 1023 atoms of oxygen in 16 g of
  oxygen
• There are 6.02 x 1023 atoms of hydrogen in 1 g
  of hydrogen
• A mole simply means that you have 6.02 x 1023 of
  whatever youre talking about
• Avogadros number

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Molarity

• Defined as the number of moles of solute divided
  by the number of liters of solution
• Molarity (mol/L M) moles of solute
• liters of solution
• The molar concentration of EDTA in this
  experiment is 0.005 mol/L.

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Example of EDTA Calculation

• 50.0 mL tap water sample
• 13.75 mL EDTA used in the titration
• Calculate the moles of EDTA used.
• Calculate the molarity of metal ion present in
  the tap water sample (Remember EDTA binds to
  metal ions on a 11 molar ratio).
• Find the ppm calcium ion concentration in the
  sample based on your results from question b).

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Example of EDTA Calculation

• Calculate the moles of EDTA used
• Molarity of EDTA 0.005 mol/L
• Moles of EDTA (Molarity)(Liters of EDTA used)

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Example of EDTA Calculation

• Calculate the molarity of metal ion present
  (Remember EDTA binds to metal ions on a 11
  ratio)
• Molarity of metal ion moles of EDTA
• liters of water sample

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Example of EDTA Calculation

• Calculate concentration of CaCO3 in ppm
• CaCO3 (ppm)
• (Molarity of metal ion)(FW of Ca2)(1000
  mg/g)
• CaCO3 (ppm)