Prepare the Phosphate Buffer. Decide on the concentration of the buffer. Most buffers are used at a concentration between 0.1 M and 10 M. If you make up a concentrated buffer solution, you can dilute it as needed. Decide on the pH for your buffer. This pH should be within one pH unit from the pKa of the acid/conjugate base. So, you can prepare a buffer at pH 2 or pH 7, for example, but pH 9 would be pushing it. Use the Henderson-Hasselbach equation to calculate how much acid and base you need. You can simplify the calculation if you make 1 liter of buffer. Select the pKa value that is closest to the pH of your buffer.