Buffering Systems and Water as a Reactant

CHEM 7784

Biochemistry

Professor Bensley

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

Buffers are mixtures of weak acids and their anions

Phosphate has three

ionizable H+ and three

pKas

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)

Case where 10% acetate ion/ 90% acetic acid

• pH = pKa + log10 [0.1 ]

¯¯¯¯¯¯¯¯¯¯

[0.9]

• pH = 4.76 + (-0.95)

• pH = 3.81

• pH = pKa + log10 [0.5 ]

¯¯¯¯¯¯¯¯¯¯

[0.5]

• pH = 4.76 + 0

• pH = 4.76 = pKa

Case where 50% acetate ion/50% acetic acid

• pH = pKa + log10 [0.9 ]

¯¯¯¯¯¯¯¯¯¯

[0.1]

• pH = 4.76 + 0.95

• pH = 5.71

Case where 90% acetate ion/ 10% acetic acid

• 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

Water as a reactant in biochemistry

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