Use of solid boric acid as an ammonia absorbent in the determination of nitrogen

Abstract

An inexpensive trap-de-mister assembly utilising solid crystalline boric acid as an ammonia absorbent was developed to replace the specialised trap-condenser-impinger apparatus normally used in Kjeldahl-type distillations. The boric acid is subsequently dissolved in water and a final determination of the ammonium ion is made either by acidimetric titration, a conductance measurement or spectrophotometric measurement after the addition of Nessler's reagent. The apparatus fits into the neck of a calibrated flask during the distillation and is itself constructed from disposable plastic pipette tips, glass-wool and rubber tubing. Both the high surface area and the tortuosity of the paths through the randomly packed bed of crystals makes solid boric acid an efficient ammonia absorbent. The system is useful for small-scale separations only because, when the volume of solution from which the ammonia is distilled exceeds 25 ml, the acid crystals become saturated with water and begin to dissolve before all of the ammonia is trapped.