Flow-injection analysis of Kjeldahl nitrogen in milk and dairy products bypotentiometric detection

A flow-injection analysis (FIA) system with a potentiometric tubular defect or and a gas diffusion unit was developed for the determination of Kjeldahl nitrogen in milk and dairy products. The samples were digested by the Kjel dahl method and afterwards injected into the RA system without prior treatm ent. NaOH solution was introduced into the system to promote the conversion of ammonium ions to ammonia gas. The gas produced diffused through a gas-p ermeable membrane towards an acceptor channel (Tris-HCl, pH=7.5). When in c ontact with this solution, ammonia gas was reconverted to amonium cation an d carried to a tubular electrode sensitive to this species. Therefore, a PV C tubular ammonium ion-selective electrode was prepared without inner refer ence solution and with the sensor system based on nonactine and monactine d issoved in Tris-(2- ethylhexyl) phosphate. The results obtained with the developed FIA system were in good agreement w ith those of the reference method the relative deviations being less than 4 %. Reproducibility of the method was assessed by the relative standard devi ation, which was always less than 0.5% for 12 consecutive injections of sam ple digests. A sampling rate of 100 samples h(-1) was obtained. (C) 1999 El sevier Science B.V. All rights reserved.