I need to prepare a 0.2 M phosphate buffer. In my research, I've seen people prepare it with both monobasic and dibasic sodium phosphate... is this necessary? Can I just use one or the other?
You can do that, but the problem is maintaining the right molarity of phosphate. If you begin with 0.2M of either one and then adjust the pH with acid or base, say you need 10-20 ml of acid or base to adjust the pH correctly for 100ml solution, now your phosphate concentration is no longer 0.2M but more like 0.16-0.18. Whether or not that makes a difference will be dependent upon the experiment you are doing. But by making 0.2 M stocks of both mono and dibasic and just mixing them to the proper pH, you will always have 0.2M regardless of how much you need to add. You will be surprised at how much acid or base you need to add to adjust a 0.2M stock.
It is better to use both phosphate salts (Mono and Dibasic) for the preparation of phosphate buffer. For the rapid calculation you can use online calculator or check the protocols.
e.g. http://cshprotocols.cshlp.org/content/2010/1/pdb.rec12105.full
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