Molarity of multicomponent buffer solutions - (Apr/04/2008 )
How can I make a 0.2 Molar phosphate (or any other) buffer solution? By adding 0.2 molar solutions of each component or by adding 0.1 mole of each component?
It depends on the buffer: In the case of phosphate buffers the 0.2 refers to the phosphate concentration, not the other components (e.g. Na, K etc) of the chemical, so you need to mix appropriate proportions of, for example, 0.4 M NaH2PO4 and 0.4 M Na2HPO4 buffers to give the pH you want (it is hard to go from less concentrated to more concentrated, so you can't make get to 0.2 M from 2 0.1 M solutions). Acetate buffers work the same way I think.
In the case of Tris buffers, just add 0.2 Mol of Tris to an appropriate volume of water, pH, and make up to your final volume.
to make 0.2M phosphate just mix 0.2M acidic component (monobasic or acid) with 0.2M basic component (dibasic or tribasic). this will give you a 0.2M buffer with no specific final volume after you've made your final pH adjustment.
bob1 was correct if you want to then dilute to 0.2M and get a specific volume. if final volume doesn't matter, then just mix 0.2M with 0.2M and make final adjustments to pH with 0.2M in the direction that you need to adjust.
Many thanks tp both of you for your consideration.