Potentiometric determination of urea by sequential injection using Jack bean meal crude extract as a source of urease

A sequential injection system was proposed to accomplish the potentiometric determination of urea. This procedure used an ammonium tubular selective e lectrode to assess ammonium concentration produced by enzymatic hydrolysis of urea from Jack bean meal (Canavalia ensiformis DC) crude extract. A gase ous diffusion device was coupled to the flow set-up allowing on-line sampli ng and suitable selectivity for determinations. A detection limit of 6.0 x 10(-4) mol urea l(-1), a relative standard deviation of 1.9% (n = 10) and a sampling rate of 20 samples h(-1) were observed when 172 Sumner units (SU) of urease and 900 mul of sample were used. Results agreeing with a compara tive method were obtained by the proposed procedure and the use of the crud e extract solution combined with the sequential injection approach improved the performance, producing reproducible results and low costs in compariso n with procedures using commercial enzymes. (C) 2000 Elsevier Science B.V. All rights reserved.