13 Buffer Systems

Question 1

What makes a buffer system?

Answer 1

• Weak electrolyte + strong electrolyte = buffer system
• 1. Weak acid + salt of this acid => CH3COOH/CH3COONa
• 2. Weak base + salt of this base => NH4OH/NH4Cl
• 3. Weak multiprotonic acid + salt of this acid => H2CO3/NaHCO3
• 4. 2 salts of a weak multiprotonic acid => NaH2PO4/Na2HPO4; Na2HPO4/Na3PO4

Question 2

Why do we need buffer systems?

Answer 2

=> Many body fluids have a certain pH

• To maintain a certain pH level
  => buffer systems resist gradual pH changes
• To maintain homeostasis
• To ensure enzyme functioning
  => enzymes work at specific pH (—> highest activity)

Question 3

Explain, what a buffer system is.

Answer 3

• Mixture of 2 components => Acid + conjugated base; Base + conjugated acid
• Resists a change in pH when a strong acid or base is added

Question 4

Describe, how the buffer system works, when a strong base is added.

Answer 4

=> ACIDS react with BASES

• acid/salt => CH3COOH/CH3COONa
  
  • CH3COOH = weak acid = weak electrolyte
  • CH3COONa = soluble salt = strong electrolyte

=> CH3COOH CH3COO- + H+

- Strong base (OH-) is added (H+ + OH- —> H2O)
- =>  CH3COOH  CH3COO- + H+ => The H+ - ion concentration decreases, equilibrium is shifted to the right side, more ions and less unsplit molecules

• ACID neutralizes the added BASE
  
  • => OH- + CH3COOH —> CH3COO- + H2O

Question 5

Describe, how the buffer system works, when a strong acid is added.

Answer 5

=> ACIDS react with BASES

• acid/salt => CH3COOH/CH3COONa
  
  • CH3COOH = weak acid = weak electrolyte
  • CH3COONa = soluble salt = strong electrolyte

=> CH3COOH CH3COO- + H+

- Strong acid (H+) is added
- =>  CH3COOH  CH3COO- + H+ => The H+ - ion concentration increases, equilibrium is shifted to the left side, less ions and more unsplit molecules

• CONJUGATED BASE neutralizes the added ACID
  => H+ + CH3COONa —> CH3COOH + Na+
  => less ions and more unsplit molecules

Question 6

How does a buffer system work in general?

Answer 6

• Strongly based on its weak electrolyte properties => weak electrolyte dissociation equilibrium
• ACIDS react with BASES —> bases neutralizes added acid; acid neutralizes added base
• Buffer systems change the ratio of ions to unsplit molecules by the shift of equilibrium
• Example: acid/salt => CH3COOH/CH3COONa
  
  • CH3COOH = weak acid = weak electrolyte
  • CH3COONa = soluble salt = strong electrolyte
  • CH3COOH CH3COO- + H+

Question 7

In which form is the weak electrolyte present in a buffer solution and why?

Answer 7

=> mostly present as the undissociated molecular form

1. Only CH3COOH
   - => CH3COOH CH3COO- + H+
   - => unsplit molecules ions
2. CH3COONa is added => buffer system is formed!

=> CH3COOH CH3COO- + H+
- common ion CH3COO- of the salt/conjugated base CH3COONa increases the CH3COO- concentration and forces the equilibrium of the weak acid CH3COOH to shift to the left side, towards the undissociated molecular form.

=> After the formation of a buffer system, the weak acid CH3COOH is mostly in molecular form
=> The salt splits fully!
=> CH3COONa —> CH3COO- + Na+
—> provides all the required acetate ions CH3COO- to force the equilibrium to towards the unsplit molecules CH3COOH

Question 8

Are buffer systems limited or not? When yes, what limitation factors are there?

Answer 8

Buffer systems are limited by two factors:

1. Buffer capacity beta = The amount of strong acid (H+) or strong base (OH-) in moles that 1 L of a buffer system can take in order to change the pH by 1 unit
2. Effective buffer pH region => pH = pKa +/- 1