The development of a modified electrode for the selective determination of p-nitrophenol (PNP) to be applied in the determination of the enzymatic activity (EA) of acid phosphatase using p-nitrophenylphosphate (PNPP) as substrate is described. Elimination of the interference of PNPP was made by covering the electrode surface using 25 mul of a 5% (m/v) Nafion (R) solution. In comparison to the glassy carbon (GC) electrode without modification, the presence of the membrane promoted a reduction in the sensitivity and an increase in the detection limit for the determination of PNP, in addition to a decrease of the electron transfer constant. The GC electrode covered with a Nafion (R) membrane showed a linear response range between 20 and 230 mu mol l(-1) for PNP, adjusted by the equation I-p = 0.157 + 0.0089 [PNP], for N = 22 with a correlation coefficient of 0.9984. This electrode was used for the determination of the EA of acid phosphatase from castor bean seeds and an activity of 38.40 U mg(-1) was found. (C) 2001 Elsevier Science B.V. All rights reserved.4412207214