Buffer Example and Titration Calculations

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Buffer Example and Titration Calculations

• With your Chemistry 102 Host
• Dr. Mike Daniel

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pH Change to 1M Acetic Acid/1M Acetate Ion Soln.
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Moles OH- Added
Moles H Added
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Buffers

• Buffer Solutions resist a change in pH
• Buffers contain relatively large concentrations
  of either
• An acid, HA and its conjugate base A-
• A base, B, and its conjugate acid (BH)

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Buffers
NH4 to react with OH-
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Buffers
NH3 to react with H
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Buffers

• When a strong base, H is added, it reacts
  essentially to completion with the weak base
  present
• H A- ? HA or
• H B ? BH

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Buffers

• When OH- is added, it reacts essentially to
  completion with the weak acid present
• OH- HA ? H2O A-
• OH- BH ? H2O B

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Weak Acid Ionization

• HA H A-
• Ka H A- / HA
• Rearrange
• H Ka HA / A-

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Weak Acid Ionization

• H Ka HA / A-
• H and pH depend mainly on Ka, and to lesser
  extent ratio HA / A-

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Weak Acid Ionization

• H Ka HA / A-
• Taking the LOG of both sides
• -Log H -Log Ka Log( HA / A- )
• pH pKa Log( HA / A- )
• pH pKa Log ( HA / A- )

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Buffers

• pH pKa log (base/acid)
• Want pH ? pKa ? 1
• pH determined by
• Mainly by Ka of acid or and ratio of conjugate
  base/acid
• or
• Kb of base and ratio base/conjugate acid

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Buffer Choice

• Want pH ? pKa ? 1
• How do I make a pH 4.0 buffer?
• Choose a pKa near the desired pH

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Buffer Table
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Buffer Choice

• Choose a pKa near the desired pH
• pH pKa log (base/acid)
• 4.0 3.74 log (base/acid)
• 0.26 log (base/acid)
• 10.26 1.8 (Na formate / formic acid)

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Basic Buffer Choice

• pKa pKb pKw 14.00
• Ammonia pKb -Log (1.8 X 10-5)
• pKa NH4 14.00 4.74 9.26
• NH3 / NH4Cl used to buffer around pH 9.26

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Buffer Capacity

• As long as ratio remains virtually constant, the
  pH will be virtually constant
• This is true as long as concentrations of
  buffering materials (HA/A-) or (B/BH) are large
  compared with H or OH- added.

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Acid / Base Titrations

• To measure concentration of unknown
• Titrant (solution added) must be a strong acid or
  strong base so reaction goes to completion.

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Acid / Base Titrations

• Calculations to equivalence point
• 1. Stoichiometry Calc to determine how much
  product made
• 2. Equilibrium Calc using conjugate pair
  concentrations.

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Acid / Base Titrations Strong A B

• Follow Text page 672
• 50.0 mL of .100 M HCl titrated with .100 M NaOH
• Equivalence Point Where Stoichiometric amounts
  of acid and base have been added
• End Point Where indicator color change occurs
• Reaction HCl NaOH -gt NaCl H2O

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Acid / Base Titrations Strong A B

• VMHCl 5 X 10-3 mol
• -VMNaOH
• moles of H leftover till equivalent point
  reached
• At equivalence point,
• initial mol HCl mol NaOH added
• 5 X 10-3 mol NaCl/(50 mL 50 mL solution)
• pH 7

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Acid / Base Titrations Strong A B
Equivalence Point
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Acid / Base Titrations Strong A B
HCl/NaCl buffer
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Acid / Base Titrations Strong A B
Phenolphthalein
Bromcresol Green
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Acid / Base Titrations Weak Acid with Strong
Base

• 50. mL of .100 M Acetic Acid (HAc) titrated with
  .100 M NaOH
• Initial pH calculated as before

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Acid / Base Titrations Weak Acid with Strong
Base

• During titration up to equivalence point
• VMHAc 5 X 10-3 mol
• -VMNaOH
• moles of HAc leftover
• VMNaOH moles OH- added moles Ac-
• made
• Say 5. mL of .1 M NaOH added to 10. mL of .1 M
  HAc

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Acid / Base Titrations Weak Acid with Strong
Base

• HC2H3O2 ? H C2H3O-

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Acid / Base Titrations Weak Acid with Strong
Base
HAc/Ac- buffer
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Acid / Base Titrations Weak Acid with Strong
Base

• At Equivalence Point all acetic acid converted
  to Acetate ion
• At Equivalence Point you have a Sodium Acetate
  Solution

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Acid / Base Titrations Weak Acid with Strong
Base

• At Equivalence Point you have a Sodium Acetate
  Solution
• To determine pH
• Use Kb and
• C2H3O2- H2O ? HC2H3O2 OH-
• To determine OH- and H and pH

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Acid / Base Titrations Weak Acid with Strong
Base

• At Equivalence Point you have a Sodium Acetate
  Solution
• pH lt or gt 7 ?
• pH 8.88

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Acid / Base Titrations Weak Acid with Strong
Base
Equivalence Pt pH 8.88
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Acid / Base Titrations Weak Acid with Strong
Base

• After Equivalence Point
• VMNaOH VMHAc(initial) moles OH- in total
  volume.
• From OH- determine H and pH

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Acid / Base Titrations Weak Acid with Strong
Base
Phenolphthalein
Bromocresol Green
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Acid / Base Titrations Weak Base with Strong
Acid
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Acid / Base Titrations Weak Base with Strong
Acid
NH3/NH4 buffer
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Acid / Base Titrations Weak Base with Strong
Acid
Phenolphthalein
Methyl Red