Acids/BasesLesson 14

Buffers

Buffer Solutions

Buffer solutions are equilibrium systems, which maintain a relatively constant pH when small amounts of acids or bases are added.

How can you prepare a buffer?

You need a high concentration of weak acid or a weak base, and add to it its conjugate acid or its conjugate base also in high concentrations. This combination is known as a BUFFER.

Buffer Solutions

Acid Buffer Recipe

Weak Acid + Salt (conjugate base) HF NaF

*Na+ is a spectator

Buffer Equation Same as weak acid

HF ⇋ H+ + F-

HF ⇋ H+ + F-

[High] [low] [High]

[HF] is high and [H+] is low because HF is a weak acid.

[F-] is high due to adding the salt NaF.

This buffer solution can shift left or right and the [H+] and the pH remains fairly constant.

Note on buffersA solution of CH3COOH by itself cannot act as a buffer because the concentration of CH3COO- would be too small and will not satisfy the conditions of a buffer system.

A buffer requires large amounts of both the acid and its conjugate.

This is why we mix the weak acid or base with its conjugate salt.

Acidic buffer Mix 1.0 mol of acetic acid (CH3COOH ) and 1.0 mol of sodium acetate (NaCH3COO) and dilute to 1.0 L of solution

Base Buffer Recipe

Weak Base + Salt (conjugate acid)

NH3 NH4Cl *Cl- spectator

Buffer Equation Same as weak base

NH3 + H2O ⇋ NH4+ + OH-

[High] [High] [Low]

Fill in the blanks to get buffer solutions.Acid conjugate Base from a salt 

 

 

 

 

 

 

HCN NaCN

H2CO3 NaHCO3

H3PO4 NaH2PO4

CH3COOH KCH3COO

NaHCO3Na2CO3

NaH2PO4Na2HPO4

How buffers work…

The ‘buffering effect’ works due to the fact that the value of the ratio:[conjugate acid] remains fairly constant.

[conjugate base]

* This ratio does not change much when a small amount of either an acid or a base is added to the buffer system. Why ???

The conjugate acid present can react with the added OH to neutralize it, and the conjugate base present can react with the added H3O to neutralize it.

Buffer Animation 1

 Buffer Application

 1. Pick the two buffer solutions 

A. NH4Cl and HCl

 B. NaCl and NaOH

 C. HCN and NaCN

 D. NH4Cl and NH3

 

 

 2. Write the equations for the above two buffer solutions. Label the concentrations as high or low.

   

3. A few drops of 0.10 M NaOH is added to a buffer solution at pH = 5.00. The new pH could be:

 A 4.98

 B. 5.02

 C. 8.05

 D. 2.11

Buffers in biological systems

HomeworkRead page 182 and 183 for “Buffers in biological systems”, must know these systems and how they work.

Page 181: #132,133,138,139

Page 183: #143 on buffers…interesting story/question.