How To Make a Phosphate Buffer

Here's How:

1. Decide on the Buffer Properties
   

   Before making a buffer you must know what molarity you want it to be, what volume to make and what the desired pH is. Most buffers work best at concentrations between 0.1 M and 10 M. The pH should be within 1 pH unit of the acid/ conjugate base pKa. For simplicity, this sample calculation will be for 1 L of buffer.

2. Determine the Ratio of Acid to Base
   

   Use the Henderson-Hasselbalch equation (below) to determine what ratio of acid to base is required to make a buffer of the desired pH. Use the pKa value nearest your desired pH and the ratio will refer to the acid-base conjugate pair that correspond to that pKa.

   
   HH Equation: pH = pKa + log ([Base]/[Acid])
   

   For a buffer of pH 6.9, [Base]/[Acid] = 1.096

3. Substitute for [Acid] and Solve for [Base]

   The desired molarity of the buffer is the sum of [Acid] + [Base].

   For a 1 M buffer, [Base] + [Acid] = 1 and [Base] = 1 - [Acid]

   By substituting this into the ratio equation, from step 2, you get:

   [Base] = 0.523 moles/L

4. Solve for [Acid]
   

   Using the equation: [Base] = 1 - [Acid], you can calculate that:

   [Acid] = 0.477 moles/L

5. Mix the Acid and Conjugate Base
   

   Prepare just under 1 L of solution using the correct amounts of monosodium phosphate and disodium phosphate.

6. Check the pH
   

   Use a pH probe to confirm that the correct pH for the buffer is reached. Adjust slightly as necessary, using phosphoric acid or sodium hydroxide.

7. Correct the Volume
   

   Once the desired pH is reached, bring the volume of buffer to 1 L.

8. Dilute as Desired
   

   This same buffer can be diluted to create buffers of 0.5 M, 0.1 M, 0.05 M or anything in-between.

What You Need:

• Monosodium phosphate.
• Disodium phosphate.
• pH meter and probe.
• Phosphoric acid or sodium hydroxide to adjust pH.
• Appropriate labware - volumetric flask, graduated cylinders, beakers, stir bars.
• Stirring hotplate.