Continuous-flow determination of low concentrations of urea in natural waters using an immobilized urease enzyme reactor and an ammonia gas-sensing membrane electrode detector system

Abstract

The continuous-flow determination of low concentrations of urea in river and lake water is described. Coexisting ammonium ions up to a concentration of 2 × 10^–4 mol dm^–3 were effectively removed by passing through a gas-permeable poly(tetrafluoroethylene)(PTFE) tube after the addition of an alkaline reagent solution. The solution was then brought back to neutral pH by passing through another gas-permeable PTFE tube immersed in hydrochloric acid, to ensure the optimum pH for the urease enzyme. Urea in the sample solution was enzymically decomposed into ammonium ions and then detected by an ammonia gas-sensing membrane electrode, as ammonia, after the addition of sodium hydroxide solution. The detection limit of urea was about 1 × 10^–7 mol dm^–3. This system was applied to the analysis of natural waters containing (0.2–4)× 10^–6 mol dm^–3 urea and the results were compared with those obtained by spectrophotometry using the reagent diacetylmonoxime.