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

A potentiometric flow injection (FI) system was developed for the acetylsalicylic acid (ASA) determination in drugs, without previous treatment. The tubular potentiometric electrode for salicylate (SA) was based on tricaprylyl-trimethyl-ammonium-salicylate (aliquat-salicylate) as the ion-exchanger, supported on poly(ethylene-co-vinyl-acetate) (EVA) matrix and applied directly onto a conducting support. The standards and samples were freshly prepared 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 neutralization with 0.25 mol l−1 H2SO4. A solution of 0.10 mol l−1 Tris–SO4 buffer (pH 8.0), containing 0.25 mol l−1 Na2SO4 was employed as carrier. In optimized conditions (flow rate of 2.1 ml min−1 and volume of injection of 150 μl), the tubular electrode showed a linear response to ASA in the concentration range between 4.0×10−3 and 4.0×10−2 mol l−1. A conversion factor of ASA to SA of 85% occurs in these conditions with an increase of about 130% in the signal to the system with on-line hydrolysis (three-channel) in comparison to the system without (one-channel). The response 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 successive injections. Determinations of ASA in tablet samples by the proposed method exhibited relative differences of 1.0–3.5%, compared to the official method of the British Pharmacopoeia. The useful lifetime of the sensor was greater than 1 month, in continuous use.