Alkalinity

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Alkalinity

• CE 370 Lab

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Definition

• Alkalinity is defined as the measure of the water
  capacity to neutralize acids. In other words, the
  ability to resist the change in pH.

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• Alkalinity of natural waters is primarily due to
  three species
• OH? (hydroxyl ions)
• CO3 (carbonate ions)
• HCO3? (bicarbonate ions)

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Importance of Alkalinity

• Alkalinity is involved (needed) in different
  water and wastewater treatment processes, such
  as
• Chemical coagulation
• Calculation of the dose of the chemical
  coagulants depends on the Alkalinity of the
  water.
• Al2(SO4)3 6H2O ? 2Al(OH)3 6H 3SO4
• Water softening Alkalinity is a major factor
  considered in calculating the lime and soda-ash
  requirements.
• Corrosion Control Alkalinity is an important
  parameter involving corrosion control
  calculations.

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Determination of the Different Species of
Alkalinity

• First, we have to make two Alkalinity
  measurements
• Phenolphthalein Alkalinity (P) Alkalinity
  measured to P end point
• Methyl Orange Alkalinity (M) Alkalinity measured
  to M end point
• Total Alkalinity (T) P M

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• Second, we determine the species as the
  following
• (we assume that OH? and HCO3? do not exist
  together)
• Possibilities
• OH? only
• CO3 only
• OH? and CO3
• CO3 and HCO3?
• HCO3? only

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• Then
• Case 1 M 0 ? P T OH? only
• Case 2 P M ? P ½ T CO3 only
• Case 3 P gt M ? P gt ½ T OH? and CO3
• OH? P M and M T P
• Then, OH? P (T-P) 2P T
• And, CO3 2M 2 (T - P)
• Case 4 P lt M ? P lt ½ T CO3 and HCO3?
• CO3 2 P
• HCO3? T - 2 P
• Case 5 P 0 ? T M HCO3? only
• HCO3? T

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Expressing the Concentration of Alkalinity

• Alkalinity concentration is expressed as CaCO3
  for convenience to make it possible to add and
  subtract.
• Conc. (mg/l) N ? eq. wt. ? 1000
• Conc. of Alkalinity NAlk ? eq. wt.Alk ? 1000
• Conc. of Alkalinity (as CaCO3) NAlk ? eq. wt.
  CaCO3 ? 1000
• eq. wt. CaCO3 50
• Nb ? Vb Na ? Va

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• The following equation can be used to express
  concentration as CaCO3

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Purpose of the Experiment

• To become familiar with the concept of alkalinity
  and its measurement in water

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Materials

• Burette, 25 ml
• Porcelain dish
• Magnetic stirrer and rod
• Beaker, 150 ml
• Pipetter
• Measuring cylinder, 100 ml
• pH meter
• 0.02N Sulphuric acid
• Methyl Orange indicator
• Phenolphthalein indicator
• Tap water sample
• Sweet water sample
• Synthetic samples

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Procedure 1 - Indicator Method

• Pipette exactly 100 ml of sample into a porcelain
  dish and drop in a magnetic rod.
• Mount a 25 ml burette and fill it to the mark
  with 0.02N sulphuric acid solution.
• Add 5 drops of Phenolphthalein indicator to the
  sample. If the solution turns pink, add acid
  slowly till pink color disappears. Record the
  volume of acid in milliliters as P.
• Add 5 drops of Methyl Orange indicator to the
  same sampling at the end of the first titration
  and add 0.02N sulphuric acid slowly till orange
  color turns to pink. Record this volume as M.
  Then, T PM.

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Procedure 2 - Potentiometric Method (pH meter)

• Pipette exactly 100 ml of sample into a 150 ml
  beaker and drop in a magnetic rod.
• Fill the burette with 0.02N sulfuric acid
  solution.
• If the pH of the sample is above 8.3 add 0.02N
  sulphuric acid slowly till pH 8.3. Record the
  volume of acid as P.
• Continue addition of acid till the pH of the
  sample reaches 4.5. Record volume of the acid as
  M. Then, T PM.

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Calculation of Alkalinity Species
Condition OH? CO3 HCO3?
1. P T T 0 0
2. P 1/2T 0 2P 0
3. P gt 1/2T (2P-T) 2(T-P) 0
4. P lt 1/2T 0 2P (T-2P)
5. P 0 0 0 T