Abstract: Effects of potassium persulfate from Guoyao group, Younaide and Fluka, and storage time of runoff-water on total nitrogen determination were studied. The results showed that the 3 drugs without purification were unable to reach the requirements of total nitrogen determination but purified potassium persulfate of Fluka could. And the total nitrogen concentrations of runoff water keeping 4 month in -20℃ had no significantly changed.