AcidBase Equilibrium: Outline

1
Acid-Base Equilibrium Outline

• Basics pH, pOH, Kw, Ka, Kb
• Strong acid vs. weak acid
• Strong base vs. weak base
• Acidity alkalinity
• Where do they come from?
• How to measure?

• CO2 The most important weak acid in natural
  water
• Why pH in natural water is not neutral?
• How to use Species Distribution Diagram?

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pH, pOH, Kw, Ka, Kb, pKa, pKb

• Water
• Kw HOH- ? 10-14
• Strong acid (e.g., HNO3, HCl)
• 100 dissociated HCl Cl- H
• Strong base (e.g., NaOH, KOH)
• 100 dissociated NaOH Na OH-
• Weak acid (e.g., H2CO3, CH3COOH)
• Monoprotonic weak acids (e.g., CH3COOH) Ka
• Polyprotonic acids (e.g., H2CO3) Ka1, Ka2..
• Weak base
• Equilibrium constant of basesKb, Kb1, Kb2

3
Examples

• Calculate pH, pOH, Kw, Ka, Kb, pKa, and/or pKb of
  the following
• A neutral water
• 0.01 M HNO3
• 10-4 M NaOH
• 0.05 M CH3COOH (Given Ka 1.8x10-5)
• Write the equilibrium expressions for the
  following weak acid/base
• H2CO3 (Ka1 4,3x10-7, Ka2 4.7 x10-11)
• H3PO4 (Ka1 7.5x10-3, Ka2 6.2x10-8, Ka3
  4.8x10-13)
• Ca(OH)2

4
Examples

• Which of the following is the strongest acid?
• (1) HAc, pKa4.74, (2) NH4, pKa9.26, (3) H3BO3,
  pKa 9.26, (4) HCN, pKa 9.32
• (1) H3PO4, Ka1 7.5x10-3, (2) H2PO4-,
  Ka26.2x10-8, (3) HPO42-, Ka3 4.8x10-13
• Which of the following is the strongest base?
• (1) CO32-, Kb1 2.13x10-4, (2) HCO3-, Kb2
  2.33x10-2, (3) CaOH, Kb2 3.5x10-2, (4) MgOH,
  Kb2 2.6x10-3

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Examples of Environmentally Important Acids and
Bases

• Strong acids and bases
• H2SO4, HNO3, HCl, HF
• NaOH, KOH
• Weak acids, bases and salts
• Acids HAC, NH4, H2CO3, HCN, H2S, phenol, H3PO4,
  protein, fatty acid
• Bases AC-, NH3, Ca(OH)2 (lime), Mg(OH)2
• Salts Na2CO3 (soda ash), Al2(SO4)3 (alum)

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Acidity Alkalinity

• Definition
• Acidity The capacity of water to neutralize OH-
• Alkalinity The capacity of water to neutralize
  H
• Source of acidity
• Natural water CO2 (air, bacteria, etc), H2PO4-,
  H2S, protein, fatty acids, Fe3, hydrated Al3,
  e.g.
• Fe(H2O)63 ltgt Fe(H2O)5OH2 H
• Al(H2O)63 ltgt Al(H2O)5OH2 H
• Polluted water free mineral acid (H2SO4, HCl)
  from metallurgical industry (steel pickling
  liquor), acid mine drainage, acid rain, organic
  acid waste
• Source of alkalinity
• Major HCO3-, CO32-, OH-
• Minor NH3, conjugate bases of H3PO4, HBO3, and
  organic acids

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Effects of Acidity and Alkalinity

• Acidity
• Increase corrosion
• Affect aquatic life
• Increase soil leaching and therefore water
  quality
• Affect dosage of chemicals used in water
  treatment
• Alkalinity
• Corrosive if high in alkalinity, hence a
  parameter used for corrosion control
• A parameter in deciding whether treated waters
  meet drinking water standards, and whether
  industrial water can discharged into municipal
  wastewater treatment plant for biological
  treatment
• Moderate alkalinity is needed in swimming pool,
  for complete coagulation in water treatment
  plant, or for natural water to resist acid rain /
  pH change

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Acidity Measurement
10
9
Phenolphthalein end point (pH 8.2 Colorless ?
Red)
8
7
Range of CO2 acidity
6
5
Titrate with NaOH
4
Methyl orange end point (pH 4.3 Red ? Yellow
Orange)
3
2
Range of mineral acidity
1
9
Alkalinity MeasurementTitration Curve for a
Hydroxide-Carbonate Mixture
12
11
10
Titrate with H2SO4
Point of inflection (Phenolphthalein)
9
pH
8
7
6
Point of inflection (Methyl Orange)
5
Carbonate
Hydroxide
4
OH- H ? H2O CO32- H ? HCO3- HCO3-
H ? H2CO3
3
mL Acid
2
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Calculation of Acidity

• Two different types of acidity
• Methyl orange acidity free mineral acids
• Total acidity Phenolphthalein acidity
• Acidity by measurement of titration
• Titration with NaOH and is reported as methyl
  orange acidity and total acidity in CaCO3 in
  mg/L
• Acidity V x N x 5 x 104/Vs
• V volume of base used in titration (mL)
• V1 mL of NaOH needed to pH 4.3
• V2 ml of NaOH needed to pH 8.3
• N normality of base used in titration (N)
• Vs volume of water sample (ml)

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Calculation of alkalinity

• Two different types of alkalinity
• Phenolphthalein alkalinity
• Total alkalinity methyl orange alkalinity
• Alkalinity by measurement of titration
• Titration with H2SO4, and is reported as
  Phenolphthalein alkalinity and Total
  alkalinity in mg/L as CaCO3
• Alkalinity V x N x 5 x 104/Vs
• V volume of acid used in titration (mL)
• V1 mL of H2SO4 needed to pH 8.3
• V2 ml of H2SO4 needed to pH 4.3
• N normality of acid used in titration (N)
• Vs volume of water sample (ml)

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ExampleAlkalinity Calculation

• A sample of water from the overflow of the
  recarbonation basin that follows a
  precipitation/softening process has a pH of 9.0
  200 mL of the water require 1.1 mL of 0.02 N
  H2SO4 to titrate it to the phenolphthalein
  endpoint and additional 22.9 mL of 0.02 N H2SO4
  to titrate it further to the orange endpoint.
  Assuming the sample contains no calcite
  particles, what are phenolphthalein alkalinity
  and the total alkalinity in mg/L as CaCO3?

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Another way of calculating total alkalinity

• Alkalinity can be calculated if the
  concentrations of CO32-, HCO3-, and OH- (pH) are
  known
• Total alkalinity 2CO32- HCO3- OH-
• Total alkalinity 2CO32- HCO3- OH-
  H (textbook, p. 70)
• Reported as Total alkalinity in mole/liter (M)

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Examples of calculating total alkalinity

• Two water samples have pH values of 7 and 10,
  respectively. The concentrations (M) of dissolved
  CO2, HCO3- and CO32- are given below
• pH 7 pH 10
• CO2 2.25x10-4 0
• HCO3- 1x10-3 4.64x10-4
• CO32- 0 2.18x10-4
• Calculate the alkalinity in M at pH 7
• Calculate the alkalinity in M at pH 10
• Calculate the total carbon in M at pH 7 and pH 10

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Carbon Dioxide (CO2)The Most Important Weak Acid

• Abundant in air (CO2 (g) 0.037 v/v in dry air)
• Dissolved CO2 is present in virtually all natural
  waters and wastewater Even rainfall in
  unpolluted air is slightly acidic due to CO2
• CO2 (g) ? CO2 (aq)
• CO2 (aq) H2O ? H HCO3-
• Algae need CO2 for photosynthesis
• 2HCO3- hv ? (CH2O) O2 CO3- (pH ?)
• Bacteria produce CO2 during decay of organic
  compounds
• (CH2O) O2 ? CO2 H2O (pH ?)

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pH of Unpolluted Natural Water Why pure water in
equilibrium with CO2 is NOT neutral?

• In air, CO2 (g) 0.035, vapor pressure of
  water PH2O 0.031 atm
• Partial pressure of CO2 (g) in air PCO2
• PCO2 (1 atm - 0.031 atm) x 0.035 0.000339
  atm
• Equilibrium between air and water CO2 (g) ltgt
  CO2 (aq)
• Use of Henrys law to calculate CO2 concentration
  in water
• CO2 (aq) H ? PCO2 (The value of H is given in
  Table 5.1 p. 126)
• CO2 (aq) 3.38x10-2 (mol/Latm-1) 0.00339 atm
• 1.14610-5 (M)
• Dissociation of CO2 (aq) in water
• CO2 (aq) H2O ltgt HCO3- H
• Ka1 HHCO3-/CO2 Ka1 4.45x10-7
• H HCO3- (Ka1xCO2 (aq))1/2 (4.45x10-7)
  x (1.14610-5) 1/2
• 2.25 x 10-6 (M)
• pH -lg H -lg (2.25 x 10-6) 5.65
• Therefore, the pH of natural water 5.65 ? 7.00
  (neutral)

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ExampleThe effects of algae growth global
warming algae on pH

• Describe whether the pH will be increased or
  decreased as a result of algae growth?
• If the CO2 (g) concentration is increased 1 ppm
  per year for the next 50 years due to the global
  warming, the CO2 (g) concentration in year 2050
  will be approximately 400 ppm (i.e., 0.040).
  Estimate the pH of natural water in 2050?

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Species Distribution Diagram
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To understand the species distribution
diagramCO2-HCO3--CO32- in Water (p. 67-70)

• CO2 H2O ltgt HCO3- H pKa1 6.35
• HCO3- ltgt CO32- H pKa2 10.33
• Species fraction
• ?CO2 CO2 / (CO2 HCO3- CO32-)
• ?HCO2- HCO3- / (CO2 HCO3- CO32-)
• ?CO32- CO32- / (CO2 HCO3- CO32-)
• Species as a function of pH
• ?CO2 H2 / (H2 Ka1H Ka1Ka2)
• ?HCO3- Ka1H / (H2 Ka1H Ka1Ka2)
• ?CO32- Ka1Ka2 / (H2 Ka1H Ka1Ka2)

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ExampleUse of Species Distribution Diagram
1.      The predominant species in the water at
pH 8 is (are) (a) CO2 (b) HCO3- (c)
CO32- (d) HCO3- and CO32- 2.      When pH pKa1
(6.35), which of the following two species have
the equal mole fraction (a) CO2 (g) and CO2
(l) (b) CO2 and HCO3- (c) HCO3- and CO32- (d)
CO2 and CO32- 3.      At pH 11, the species
distribution is most likely (a) 82 CO32- (b)
82 HCO3- (c) 82 CO2 (d) none of above 4.     
The alkalinity of a water sample in equilibrium
with atmospheric CO2 at pH 12 is most likely to
be due to (a) HCO3- and CO32- (b) CO32- and
OH- (c) HCO3- and OH- (d) None of above
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Closure Acid-Base in Water

• Other application of acid-base equilibrium
• Formation of acid-rain
• Wastewater neutralization
• Buffer in environmental analysis
• Titration in environmental analysis
• Useful websites
• Groundwater http//www.groundwater.org
• Acid precipitation http//www.epa.gov/docs/acidra
  in/ardhome.html
• Water quality http//www.epa.gov/watrhome.html