AN INTEGRATED REDUCTION METHOD FOR THE DETERMINATION OF UREA AS AMMONIA IN FRESH-WATER SAMPLES

A flow-injection system for the on-line microwave preparation of natural water samples with determination of urea as ammonia by colorimetric detection is described. Urea present in the samples is reduced on line to ammonium ions which on reaction with NaOH release gaseous ammonia which passes through a gas permeable membrane causing a pH change in a Bromothymol Blue (BTB) indicator stream which is colorimetrically determined at 635 nm. Signals from the flow through detector were recorded as peak height measurements on a chart recorder. Calibration proved to be linear up to 45 mu mol 1(-1) urea with an LOD of 2.40 mu mol 1(-1) 1 urea (R.S.D. < 4%, n = 10). Samples were analysed at a rate of 14 min per sample and proved to show superior results when compared to a conventional batch preparation methodology.