Buffer Calculations (HL) (DP IB Chemistry): Revision Note

Buffer calculations

• The pH of a buffer depends on:

  • The pK_a (or pK_b) of the weak acid (or base)

  • The ratio of the concentration of acid to conjugate base, or base to conjugate acid

• To determine the pH, the concentration of hydrogen ions is needed which can be found using the equilibrium expression:

  • 

• For acidic buffers (e.g. CH₃COOH + CH₃COO^-)

• For basic buffers (e.g. NH₃ + NH₄⁺)

pH = 14 - pOH

• This is known as the Hendersen-Hasselbalch equation

Calculating the pH of a buffer

• The pK_a or pK_b values of its component acid or base

• The ratio of initial concentrations of acid and salt used to prepare the buffer

Factors that can influence buffers

Dilution

• Diluting a buffer reduces the concentration of both the weak acid/base and its conjugate:

• The ratio of stays almost the same

• K_a and K_b are equilibrium constants so are not changed by dilution

• Therefore, the pH changes only very slightly so the buffer still works

• However, buffer capacity decreases, it can now resist less added acid or base before the pH changes significantly

  • This is because the concentrations are lower so there are fewer particles of buffer components available to neutralise added H⁺ or OH^-

Temperature

• A constant temperature must be maintained when using buffers as temperature will influence the pH of the solution

• Temperature affects the values of K_a and K_b 

• In medical procedures, temperature fluctuations should be avoided due to the effect on the buffers in the blood