Optical sensor for salicylic acid and aspirin based on a new lipophilic carrier for aromatic carboxylic acids

Abstract

The optode with fair selectivity for salicylate and aspirin is based on the use of a new lipophilic carrier for aromatic carboxylic acids, namely (±) n-butyl O-(1-naphthylaminocarbonyl) lactate (BNAL). The sensing scheme involves co-extraction of both salicylate anion and a proton from the aqueous sample medium into a pvc layer using BNAL as a salicylate carrier, and a pH-sensitive dye as a lipoid proton carrier, both contained in the pvc membrane. BNAL transports the salicylate anion into the pvc lipid membrane. In order to maintain electro-neutrality, the proton carrier dye simultaneously extracts a proton into the pvc membrane, thereby becoming protonated. As a result, the dye undergoes a colour change which is detected via measurement of fluorescence intensity. The sensor is fully reversible and has a dynamic range from 0.1 to 30 mmol/l salicylate at pH 5.00, but the response function is highly pH-dependent. The effect of 25 interfering anions was investigated, and optical selectivity coefficients were determined. It is also shown that by proper variation of the membrane composition, selectivities for other carboxy anions can be obtained. The method has been applied to determine aspirin in tablets using flow injection analysis, and the results were found to be satisfactory.