2. Using the buffer pKa

2. Using the buffer pKa

  • In this method, calculate the amount (in moles) of acid/salt or base/salt present in the buffer at the desired pH.
  • If both forms (i.e., the acid and the salt) are available, convert the amount required from moles to grams, using the molecular weight of that component, and then weigh out the correct amounts of both forms.
  • If only one form is available, prepare the buffer by adding the entire buffer as one form, and then adding acid or base to convert some of the added buffer to the other form.
  • Decide what the total concentration of buffer will be in the solution, and convert the concentration to amount (in moles) using the volume of solution, and then to grams, using the molecular weight of the buffer form available.
  • Then calculate the amount (in moles) of each form that will be present in the final solution, using the buffer pKa and the desired pH.
  • Then calculate how much strong acid or base must be added to convert enough of the buffer form added to the other form, to give the correct amount of each form at the pH of the final solution.
  • Dissolve the buffer and strong acid or base in slightly less water than is required for the final solution volume.
  • Check the pH and correct if necessary. Add water to the final volume.
Last modified: Monday, 19 March 2012, 6:02 AM