The easiest method sequentially in one still then in the second. However, if the duoPUR is used to purify more than one acid type, using the same still twice is better option, because it will limit the draining and rinsing time. Also, the impurities leached from the quartz are not the same from acid to acid. This might result in elevated concentrations for some elements or recontamination of the quartz. Repeated distillations using the same acid results in a “cleaning effect” on the quartz, effectively making it cleaner each time the system is run.