I am running an experiment on the degradation/removal of oxytetracycline and for the analysis, I am using UV-detector HPLC system with C18 column, 355nm wavelength. My mobile phase is 60/25/15 v/v of oxalic acid, methanol, and acetonitrile. For the methanol and acetonitrile, I suppose that I just need to measure it into desired volume, but for the oxalic acid, I do not know how to prepare it into liquid solution as my lab only have the solid only. Your help is much appreciated.