In this section:

Calculation of the Buffer pH Using Concentrations
Calculation of the Buffer pH Using Activities

Calculation of the Buffer pH Using Concentrations

Recall that a buffer solution is a solution containing comparable amounts of a weak acid and its conjugate base. The equilibrium calculations for a buffer are generally set up in a ledger of intital and equilibrium conditions for theweak acid dissociation reaction. The buffer equilibrium terms are readily determined if 2 conditions hold: (1) the K_a of the weak acid is less than 10^-3 and (2) the concentrations of the weak acid and conjugate base are both greater than 10^-3 M. If both conditions hold, then the equilibrium concetrations of the weak acid and conjugate base can be approximated by the intital conditions, and the equilibrium hydrogen ion concentration is represented by x. The next step is to set the mass action expression (based on concentrations) equal to K_a , substitute the equilibrium values and solve for x. Once the hydrogen ion concentration is known, the pH is determined from pH = -log[H⁺].

Calculation of the Buffer pH Using Activities

Use the following steps to incorporate activities into the buffer calculation.

• Determine the intial and equilibrium concentrations as described above.
• Set the mass action ratio based on activities equal to K_a.
• Substitute the weak acid and conjugate base activities with the product of the concentration times activity coefficient.
• Determine the ionic strength of the solution.
• Look up the activity coefficients for the weak acid and conjuagte base.
• Substitute the values for the equilibrium concentrations and activity coefficients and solve for the hydrogen ion activity.
• Calculate the pH using pH = -log a_H+.

Page author: David Miller
URL: http://www.csun.edu/~hcchm003/cnnect.html
Last updated: July 19, 1997