Add 30 g of dry potassium nitrate and 40 mL of conc. sulfuric acid in a distillation flask, with stirring, collect the fraction which comes off at 75-85℃ in a vacuum. The collecting flask should be cooled with cold water. Store in freezer.
KNO3 + H2SO4 → KHSO4 + HNO3
Distillation under vacuum