Department of Biochemistry and Molecular Biology

1
Acid-Base Concepts and Buffer Action
Lecture 3

• Acids
• Strong Tendency to give up a proton
  readily---HCl
• HCl H2O? Cl-- H3O
• Weak Weak tendency to give up a proton---Acetic
  Acid
• CH3COOH H2O CH3COO-- H3O
• Bases
• Stronggreat tendency to accept a proton--- OH
  ion
• Weak--- Poor tendency to accept a proton---
• CH3-COO- anion

2
Dissociation of a Weak Acid

• HA H
  A
• (Weak Acid) (proton) (conjugate
  Base)
• Ka (acid dissociation constant) H A--
• 
  HA
• log Ka log H log A--/HA
• --log H --log Ka log A--/HA
• pH pKa log conjugate base/acid
• Henderson Hasselbach equation
• pH -logH and pKa --log Ka
• When A- HA
• pH pKa log HA/HA or pH pKa log 1
• So pH pKa

3
Significance of pH and pKa

• pH -log H
• If H 1 molar (1M) then pH - log 1 0
• If H 10-7 M then pH -log 10-7
  -(-7)log10
• 
  7
• When H concentration is high pH is a low
  number
• When H concentration is low pH is a higher
  number
• pH scale is logarithmic and is usually given from
  0-14

4
pH Scale

• pH H Molar
• 0 10 1
• 10 -1
  0.1
• 10-2
  0.01
• 10-3
  0.001
• 10-4
  0.0001
• 10-5
  0.00001
• 6.7 10-6.7
  (2x10-7) 0.0000002
• 10-7
• 10-8
• 10-9
• 10-10

5
Dissociation of water

• Water is a weak acid and a weak base
• H2O H OH-
• Kw HOH-
• H2O
• Kw(H2O) 10-14 HOH- and pH pOH14
• Since H OH-
• H2 10-14 and H 10-7
• So for water, pH 7
• and pOH 7

6
Titration of a weak acid
7
Buffering Capacity of Acid-Conjugate base

• A buffer is a mixture of weak acid and its
  conjugate base and its pH should not change
  appreciably when small amount of acid or base is
  added
• CH3-CHOH-COOH CH3-CHOH-COO-
  H
• (Lactic acid pKa4.5)
• At pH4.5 which is equal to pKa,
  lactatelactic acid
• What will be the change in pH if we add 0.01 M
  base (OH) to a buffer which contains 0.1 M each
  of acid and conjugate base?
• pH 4.5 log 0.11/0.099 4.5 log 1.11
  4.5 .0414 4.5414
• What will be the change in pH if 0.01 M H were
  added to the system?
• pH 4.5 log 0.099/0.11 4.5 log 0.9
  4.5-0.045 4.455
• So the buffering capacity of a weak
  acid-conjugate base system is very good near the
  pKa of the acid

8
Salivary Buffering-Bicarbonate and pH

• CO2 H2O H2CO3 HCO-3 H
• In the oral cavity conc. Of H2CO3 remains
  essentially constant at 1.3 mM but pH and
  bicarbonate concentrations do change
• 1.When acid is produced within the dental plaque,
  the reaction is driven to the left and mouth
  being an open system, CO2 escapes and acid is
  neutralized and thus saliva protects teeth from
  decay assuming there is enough bicarbonate
  present in the plaque. Conversion of H2CO3 to CO2
  and water requires carbonic anhydrase VI which is
  produced by acinar cells of parotid gland and
  appears to form part the tooth pellicle.

9
Importance of salivary pH

• The normal salivary pH is 6.3 because of
  salivary bicarbonate.
• H2CO3 HCO3-1 H
• pKa6.1
• pH pKa log HCO3/H2CO3
• After meals salivary flow is increased from 2mM
  to 30 mM. Using these values pH at 2 mM will be
  6.29 and at 30 mM, it should be 7.46.

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11
Problem Set

• Please do these calculations
• A pH of 2.5 means that the H
  is----------------
• For an acetate buffer at pH 4.9, what will be the
  concentration of acetate ion if the concentration
  of acetic acid is 0.06 M (pKa of acetic acid is
  4.8).
• Calculate the pKa of lactic acid given that the
  pH is 5.28 and the concentrations of lactic acid
  and lactate are 0.01 and 0.06 M respectively.
• Ans
• 3.16 x10 3 M
• 0.076 M
• 4.5

12
Learning Objectives Acids and Bases

• Know the definitions of strong and weak acids
  and bases?
• Know how to calculate pH and pKa.
• Know the use of Henderson-Hasselbach equation.
• Know the conditions for good buffering action
  which are good buffers?
• Do the problems at the end of this lecture.