Titrations

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Titrations
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Strong Acid with Strong Base

• Starting pH
• pH -logFAcid
• Just before the Equivalence Point
• H (VacidFacid-VbaseFbase)/(VsolVbase)
• Very Sharp Equivalence Point
• pH 7.00
• Excess base
• pH 14 (-logOH-)
• OH- Fbase(Vbase-V)/(VbaseVsol)

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25.00 mL of 0.10 M HCl titrated with 0.10 M NaOH
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25.00 mL of ? M HCl titrated with 0.10 M NaOH
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• V 12.50 mL of NaOH
• HCl (12.50 mL)(0.1000 M)/(25.00 mL)
• 0.0500 M

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Strong Base with Strong Acid

• Starting pH
• pH 14 -logBase
• Very Sharp Equivalence Point
• pH 7.00
• Excess acid
• pH -logH)
• H Facid(Vacid-V)/(VacidVsol)

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25.00 mL of 0.10 M NaOH titrated with 0.10 M HCl
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25.00 mL of ? NaOH titrated with 0.20 M HCl
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• V 12.50 mL of HCl
• NaOH (12.50 mL)(0.2000 M)/(25.00 mL)
• 0.1000 M

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Titrating Monoprotic Weak Acids with Strong Base

• Starting pH Use weak acid dissociation
• Ka H2/(FAcid H)
• pH -logH
• HA OH- ? A- H2O
• Before the Equivalence Point use H-H eq
• pH pKa
• log (FNaOHVNaOH)/(FHAVHA- FNaOHVNaOH)
• ½ way to Equivalence point, pH pKa

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• At Equivalence point use weak base dissociation
• Kb OH-2/(Fbase OH-),
• where Fbase FHAVHA/(VsolVeq)
• Excess base
• OH- Fbase(Vbase-V)/(VbaseVsol)
• pH 14 (-logOH-)

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Ka 1.010-5
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Example for Ka 10-5

• Starting pH
• H ((110-50.1)1/2 0.0010 M
• pH 3.00
• ½ way pH pKa 5.00
• At Equiv. pt
• OH- ((110-90.05)1/2 7.110-6 M
• pH 14.00 -log(7.110-6) 8.85

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Titrating Monoprotic Weak Bases with Strong Acid

• Starting pH Use weak base dissociation
• Kb OH-2/(Fbase OH-)
• pH 14.00 (-logOH-)
• A- H ? HA
• Before the Equivalence Point use H-H eq
• pH pKa
• log (FA-VA-- FHClVHCl)/ (FHClVHCl)
• ½ way to Equivalence point, pH pKa

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• At Equivalence point use weak acid dissociation
• Kb H2/(Facid H),
• where FACID FA-VA-/(VsolVeq)
• Excess HCl
• H Facid(Vacid-V)/(VacidVsol)
• pH logH)

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Titrate with 0.10 M HCl
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Example for Ka 10-7

• Starting pH
• OH- ((110-70.1)1/2 0.00010 M
• pH 10.00
• ½ way pH pKa 7.00
• At Equiv. pt
• H- ((110-70.05)1/2 7.110-5 M
• pH 4.15

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Titration of H2A with strong base

• Starting pH weak acid equilibrium
• ½ way to first eq. pH pKa1
• At first equivalence pt
• pH (pKa1 pKa2)/2
• ½ way to 2nd eq. pH pKa2
• At 2nd equiv. pt. weak base equilibrium

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Example

• Starting pH H (10-30.1)1/2 0.0100 M
• ½ way to first eq. pH 3.00
• At first equivalence pt
• pH (pKa1 pKa2)/2 6.00
• ½ way to 2nd eq. pH 9.00
• At 2nd equiv. pt. OH- (10-50.05)1/2
  7.0710-4 M, pH 10.85

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Amino acids

• Amphoteric
• Can act as an acid and a base
• Glycine (GH2, GH, G-)
• pKa1 2.350, pKa2 9.778

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Alpha Fraction Plot for Glycine
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Triprotic system

• Example H3PO4

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Titration of a triprotic acid with strong base

• Starting pH weak acid equilibrium
• ½ way to first eq. pH pKa1
• At first equivalence pt
• pH (pKa1 pKa2)/2
• ½ way to 2nd eq. pH pKa2

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• At 2nd equiv. pt.
• pH (pKa2 pKa3)/2
• ½ way to third equiv. pt. pH pKa3
• At third equiv pt weak base equilibrium

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pKas are 4, 7, and 10
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H3PO4 (pKas 2.2, 7.2, 12.2)