Na-K phosphate buffer recipe? - (Jan/28/2008 )

I have an enzyme assay proctol that calls for 0.5M Na-K phosphate buffer. Does anyone know how to make this or have a protocol? I have made Sodium phosphate buffer and Potassium phosphate buffer, but never Sodium Potassium phosphate buffer. Any help would be greatly appreciated.

prepare it the same way you would with sodium phosphate, but switch one of the solutions with the potassium salt (i would use the monobasic potassium phosphate to keep a higher sodium concentration).