Determination of acetylsalicylic acid by FIA-potentiometric system in drugs after on-line hydrolysis

A potentiometric flow injection (FI) system was developed for the acetylsal icylic acid (ASA) determination in drugs, without previous treatment. The t ubular potentiometric electrode for salicylate (SA) was based on tricapryly l-trimethyl-ammonium-salicylate (aliquat-salicylate) as the ion-exchanger, supported on poly(ethylene-co-vinyl-acetate) (EVA) matrix and applied direc tly onto a conducting support. The standards and samples were freshly prepa red in ethanol solution (0.10 mol l(-1) Tris-SO4 buffer, pH 8.0, containing 0.25 mol l(-1) Na2SO4 and 8.0% v/v ethanol) to facilitate the dissolution of ASA and were injected directly into the system. The SA formed due to the on-line alkaline hydrolysis of alcoholic ASA solution, with 0.50 mol l(-1) NaOH (coil, 50 cm length), was monitored by the tubular electrode after ne utralization with 0.25 mol l(-1) H2SO4. A solution of 0.10 mol l(-1) Tris-S O4 buffer (pH 8.0), containing 0.25 mol l(-1) Na2SO4 was employed as carrie r. In optimized conditions (flow rate of 2.1 ml min(-1) and volume of injec tion of 150 mu l), the tubular electrode showed a linear response to ASA in the concentration range between 4.0 x 10(-3) and 4.0 x 10(-2) mol l(-1). A conversion factor of ASA to SA of 85% occurs in these conditions with an i ncrease of about 130% in the signal to the system with on-line hydrolysis ( three-channel) in comparison to the system without (one-channel). The respo nse time of the electrode was about 5 s with an analytical frequency of 28 samples per h and a relative standard deviation (R.S.D.) of 2.1% for 30 suc cessive injections. Determinations of ASA in tablet samples by the proposed method exhibited relative differences of 1.0-3.5%, compared to the officia l method of the British Pharmacopoeia. The useful lifetime of the sensor wa s greater than I month, in continuous use. (C) 1999 Elsevier Science B.V, A il rights reserved.