Buffer Calculations (HL) (DP IB Chemistry)

Buffer Calculations

• The pH of a buffer solution can be calculated using:

  • The K_a of the weak acid

  • The equilibrium concentration of the weak acid and its conjugate base (salt)

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

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• To simplify the calculations, logarithms are used such that the expression becomes:

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• Since -log₁₀ [H⁺] = pH, the expression can also be rewritten as:

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• This is known as the Hendersen-Hasselbalch equation

Basic buffers

• [OH^–] = K_b  and pOH = pK_b + log₁₀ 

The pH of a buffer can be determined from:

• 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

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

• Dilution does not change the ratio of acid or base to the salt concentration as both components will be decreased by the same amount

• The overall pH change of the buffer does not change

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