Strength of AcidsStrength of an acid is

measured by the extent it reacts with water to form hydronium ions (H3O+).

Strong acids ionize ~100% so pH = -log[acid].eg. HCl, HBr, HNO3, H2SO4

Strength of AcidsWeak acids (or bases) only

partially ionize; typically only 1-5%.eg. CH3COCOOH, pyruvic acid

CH3CHOHCOOH, lactic acid

CH3COOH, acetic acid

Strength of AcidsStrength of an acid is indicated by

its Ka or pKa (= -log(Ka)).

HA + H2O H3O+ + A-

Larger Ka (smaller pKa) valuesindicate more dissociation and stronger acids.

Ka = [H3O+][A-]

[HA]

Strength of Acids Ka pKa

CH3COCOOH 3.2x10-3 2.5

CH3CHOHCOOH 1.4x10-4 3.9

CH3COOH 1.8x10-5 4.8

Larger Ka and smaller pKa values

indicate stronger acids.

Monitoring Acidity

The Henderson-Hasselbalch (HH) equation is derived from the equilibrium expression for dissociation of a weak acid.

HA A- + H+

Ka =

[A-][H+][HA]

pH = pKa + log [A-][HA]

Henderson-Hasselbalch Equation

The HH equation enables us to calculate the pH of a weak acid solution during a titration and to make predictions regarding buffer solutions.

What is a titration?It is a process in which carefully

measured volumes of a base are added to a solution of an acid while monitoring the change in pH.

When chemically equivalent amounts of acid and base are present during a titration, the equivalence point is reached.

At the equivalence point, only the salt (eg. Na+ CH3COO-) is present in solution.

Titration Curve (CH3COOH with NaOH)

moles OH- per mole HA

pH

Titration Curve

At the inflection point, equal moles of A- and HA are present in solution.[A-] = [HA]

pH = pKa + log (1) = pKa + 0

[A-][HA]

pH = pKa + log

Sample calculation

The pKa for acetic acid is 4.76.

1. Calculate the relative amounts of acetic acid (HA) and acetate ion (A-) when 0.7 equivalents of NaOH have been added to an acetic acid solution.

2. Use the Henderson-Hasselbalch equation to calculate the pH at this point. (Click for answer.)

Titration Curve (HOAc with NaOH)-4

0.7 equivalents of NaOH neutralizes 0.7 eq of acid (HA) producing 0.7 eq of acetate (A-) and leaving 0.3 eq of HA.

pKa of HOAc is 4.76

pH = 4.76 + log [0.7] [0.3]

30% acid and 70% salt. pH=5.13

Buffer SolutionsBuffer: a solution that resists

change in pH when small amounts of strong acid or base are added.

A buffer consists of:

a weak acid and its conjugate base or

a weak base and its conjugate acid

Buffer SolutionsMaximum buffer effect occurs at

the pKa for an acid.

Effective buffer range is +/- 1 pH unit of the pKa for the acid or base.

eg. H2PO4-/HPO4

2-, pKa=7.20

buffer range 6.20-8.20 pH

Buffer SolutionsWithin cells the primary buffer

is phosphate : H2PO4-/HPO4

2-

The primary buffer in blood is bicarbonate : HCO3

-/H2CO3.Proteins also provide buffer

capacity. Some side chains can accept or donate protons.

Buffer SolutionsBuffers work by chemically neutralizing

added acid or base. Eg.:

HCO3- + H3O

+ H2CO3 + H2O

H2CO3 + OH- HCO3- + H2O

As long as amount of added acid or base is small, log [A-]/[HA] will not change significantly and pH will not change.

Buffer SolutionsCalculate the ratio of lactic acid to

lactate in a buffer at pH 5.00. The pKa for lactic acid is 3.86

5.00 = 3.86 + log [lactate] [lactic acid]

5.00-3.86 = log [lactate] [lactic acid]

antilog 1.14 = [lactate] [lactic acid]

= 13.8