Total nitrogen determining for plant material containing nitrate

A nonfoaming method for semimicro Kieldahl determination of total nitrogen in plant samples containing appreciable amounts of nitrate was developed for use with a digestion block utilizing test tubes for digestion flasks. The sample (30 to 200 mg) is treated with 5 ml of a sulfuric acid:salicylie acid (30:1 $\text{v}\text{w}$) mixture at room temperature for 1 hr. Catalyst [K₂SO₄:CuSeO₃·2H₂O:pumice 970:19:11 w/w/w)] is added and the mixture is digested at 360 to 380°C for 1 hr after the mixture clears. Ammonium in the digest is determined by a suitable method. This semimicro Kjeldahl procedure results in a 95% or better recovery of nitrate, either from KNO₃ or from KNO₃ added to plant material.