Buffering Systems and Water as a Reactant
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Buffering Systems and Water as a Reactant
CHEM 7784
Biochemistry
Professor Bensley
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Chapter 2.3 through 2.5Buffering Systems and Water as
a Reactant
How buffers work and why we need them How water participates in biochemical reactions
Today’s Objectives: to understand
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Buffers are mixtures of weak acids and their anions
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Phosphate has three
ionizable H+ and three
pKas
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Henderson–Hasselbalch Equation:Derivation
HA][
]A][H[ -
aKHA H+ + A-
HA][
]A[logppH
-
aK
1. Solve for [H+]2. Take negative logarithm of both sides3. Substitute pH for –log[H+] and pKa for –log Ka
4. Invert –log [HA]/[A-] (changes sign)
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Case where 10% acetate ion/ 90% acetic acid
• pH = pKa + log10 [0.1 ]
¯¯¯¯¯¯¯¯¯¯
[0.9]
• pH = 4.76 + (-0.95)
• pH = 3.81
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• pH = pKa + log10 [0.5 ]
¯¯¯¯¯¯¯¯¯¯
[0.5]
• pH = 4.76 + 0
• pH = 4.76 = pKa
Case where 50% acetate ion/50% acetic acid
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• pH = pKa + log10 [0.9 ]
¯¯¯¯¯¯¯¯¯¯
[0.1]
• pH = 4.76 + 0.95
• pH = 5.71
Case where 90% acetate ion/ 10% acetic acid
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• pH = pKa + log10 [0.99 ]
¯¯¯¯¯¯¯¯¯¯
[0.01]
• pH = 4.76 + 2.00• pH = 6.76
• pH = pKa + log10 [0.01 ]
¯¯¯¯¯¯¯¯¯
[0.99]
• pH = 4.76 - 2.00• pH = 2.76
Cases when buffering fails
So why do we care so much about buffers in Biochemistry?
N NOH
SO3Na
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Water as a reactant in biochemistry
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Chapter 2: Summary
The goal of this chapter was to help you to better understand:
• The nature of intermolecular forces • The properties and structure of liquid water• The behavior of weak acids and bases in
water• The way water can participate in biochemical
reactions