You can buffer YPD at almost any pH using various phosphate or citrate buffers.
However, if you plan to pour plates, add the buffer after autoclaving, since at
extremely acid pH, the agar will be damaged under autoclave conditions and the
plates will never solidify (and they are really messy to discard!).
You will need to test how much buffer to add; I have never done the whole range
of pH values.
UMDNJ-RW Johnson Medical School