My lab has a couple containers of Potassium Phosphate and Sodium Phosphate that are past the retest date printed on the bottle. I contacted the supplier of the chemicals, but they do not supply new Certificates of analysis for sold products. Can anyone recommend any testing procedures to check the efficacy of these chemicals?

Also, on a side note, does anyone know if you can use Sodium Phosphate instead of Disodium Phosphate to make PBS?