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Buffering Systems and Water as a Reactant

Buffering Systems and Water as a Reactant. CHEM 7784 Biochemistry Professor Bensley. Chapter 2.3 through 2.5 Buffering 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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Buffering Systems and Water as a Reactant

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  1. Buffering Systems and Water as a Reactant CHEM 7784 Biochemistry Professor Bensley

  2. 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

  3. Buffers are mixtures of weak acids and their anions

  4. Phosphate has three ionizable H+ and three pKas

  5. Henderson–Hasselbalch Equation:Derivation  HA H+ + A-  • Solve for [H+] • Take negative logarithm of both sides • Substitute pH for –log[H+] and pKa for –log Ka • Invert –log [HA]/[A-] (changes sign)

  6. Case where 10% acetate ion/ 90% acetic acid • pH = pKa + log10[0.1 ] ¯¯¯¯¯¯¯¯¯¯ [0.9] • pH = 4.76 + (-0.95) • pH = 3.81

  7. Case where 50% acetate ion/50% acetic acid • pH = pKa + log10[0.5 ] ¯¯¯¯¯¯¯¯¯¯ [0.5] • pH = 4.76 + 0 • pH = 4.76 = pKa

  8. Case where 90% acetate ion/ 10% acetic acid • pH = pKa + log10[0.9 ] ¯¯¯¯¯¯¯¯¯¯ [0.1] • pH = 4.76 + 0.95 • pH = 5.71

  9. Cases when buffering fails • 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 So why do we care so much about buffers in Biochemistry?

  10. Water as a reactant in biochemistry

  11. 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

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