about 2.5 mL acetic anhydride should do it. That is about 1.5 molar equivalents. You will probably need 15 to 20 mL pyridine and you should be a bit careful adding the acetic anhydride as a lot of heat will likely be produced. It would be a good idea to cool the pyridine solution in an ice bath while adding the acetic anhydride slowly (a fast drip). Wear safety glasses. If the mixture became overheated and spattered in your eyes it would likely blind you. After the addition is complete, you can remove the ice bath.