Carbonate buffers are a combination of a weak acid and its conjugate salt, relying upon series of reverse reactions to buffer changes in pH.

To make 1 liter of carbonate buffer, add the following

• 1.6 g Na2CO3 (15 mM final)
• 2.9 g NaHCO3 (35 mM final)
• 0.2 g NaN3 (3.1 mM final). Use caution in handling sodium azide as it is extremely  poisonous; follow appropriate precautions for handling,
  storage, and disposal.
• H2O to 1 liter

Mix well, and then adjust to pH 9.5.