Buffer Capacity (College Board AP Chemistry)

• Buffer capacity tells you how much acid or base can be added to the buffer without the pH changing
• The greater the buffer capacity, the more acid or base that can be added before the pH starts to change
• The buffer capacity depends on the concentrations of both the weak acid and its conjugate base (or weak base and its conjugate acid

• A buffer is formed from ethanoic acid and sodium ethanoate
  • If acid, H⁺, is added the following reaction will occur

CH₃COO^– (aq) + H⁺ (aq) CH₃COOH (aq) 

the equilibrium will shift to the right-hand side as H+ ions are 'mopped' up

• • If a base, OH^–, is added the following reaction will occur

CH₃COOH (aq) + OH^– (aq)  CH₃COO^– (aq) + H₂O (l)

the equilibrium will shift to the right-hand side as OH^– ions are 'mopped up'

• Using the different concentrations of ethanoic acid and sodium ethanoate we can demonstrate the difference in buffer capacity (the pK_a of ethanoic acid is 4.74)

	Buffer 1 - higher concentration	Buffer 2 - lower concentration
Initial pH of buffer solution	[CH3COOH] = 0.80 M  [CH3COO–] = 0.40 M  pH = pKa + log10   pH = pKa + log10 = 5.04	[CH3COOH] = 0.080 M  [CH3COO–] = 0.040 M pH = pKa + log10 = 5.04  pH = pKa + log10 = 5.04
pH on addition of 0.05 M NaOH	[CH3COOH] = 0.80 M - 0.05 M = 0.75 M  [CH3COO–] = 0.40 M + 0.05 M = 0.45 M  pH = pKa + log10   pH = 4.74 + log10 = 5.56	[CH3COOH] = 0.080 M - 0.05 M = 0.075 M  [CH3COO–] = 0.040 M + 0.05 M = 0.045 M  pH = pKa + log10   pH = 4.74 + log10 = 6.18

 

• Buffer 1 has a higher capacity than buffer 2