f. Stock nitrate standard, 1000 mg N/L: In a 1-L volumetric flask dissolve 7.221 g potassium nitrate, KNO3 (dried at 60 C for 1 h), or 4.93 g sodium nitrite, NaNO2 , in about 800 mL water. Dilute to mark and invert to mix. When refrigerated the standard may be stored for up to 3 months.