Buffers and Titration of Acids and Bases | General Chemistry 3

Buffer solutions and titrations are studied in this chapter: Henderson-Hasselbach equation, buffer solutions and indicators, strong acid - strong base and weak acid - strong base titrations

Henderson-Hasselbalch Equation

pH ≈ pKa + log base0acid0

 

 

General conditions to apply the Henderson-Hasselbalch equation:

- Ka should be in the range of about 10-4 M to 10-11 M
- The ratio base0acid0 must be between 0.1 and 10
- The values of [base]0 and [acid]0 should be in the range of 10-3 M to 1 M

Buffers

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base. Its pH changes very little when a small amount of strong acid or base is added to it and does not change when the solution is diluted
 

CH3COOH is a weak acid and CH3COO- is its conjugate base
⇒ a mixture of a solution of CH3COOH and a solution of CH3COO- results in a buffer solution
Adding few mL of a strong acid (for example HCl) to this mixture will change the pH of the solution by around 0.1 pH unit

Indicators

pH indicators: substances which change color with pH ⇒ they are used to signal the end point of titrations
The pH region in which an indicator changes color is equal to the pKai for this indicator
 

Bromthymol blue (pKai = 7.3) changes from yellow to blue around a pH = 7.3

If the solution is yellow ⇒ acids are in excess
If the solution is blue ⇒ bases are in excess
When the solution changes color ⇒ equivalence point

 

At the equivalence point: stoichiometrically equivalent amounts of acid and base have reacted

nacida = nbaseb

nacid and nbase = number of moles of acid and base
a = stoichiometric coefficient of acid
b = stoichiometric coefficient of base

 

Strong Acid-Strong Base Titration

Titration of a strong acid with a strong base (or vice versa): pH changes abruptly at the equivalence point
Titration curve for the titration of a 0.100M HCl (aq) solution with a 0.100M NaOH (aq) solution:
 

Weak Acid – Strong Base Titration

Titration of a weak acid with a strong base (or vice versa): pH = pKa at the Midpoint
Titration curve for the titration of a 0.100M CH3COOH (aq) solution with a 0.100M NaOH (aq) solution: