An asymmetrical FIA merging-zones manifold based on the dual injection of two sample microvolumes was developed for the simultaneous determination of salicylic acid and acetylsalicylic acid in pharmaceutical preparations at a sampling frequency of 30/h. The complex formed between the Fe(III) reagent continuously introduced in the system and salicylic acid was monitored photometrically at 520 nm. One of the sample plugs was prehydrolysed on injection into an NaOH stream and was circulated through a longer channel than the other plug. This yielded two FIA peaks corresponding to salicylic acid and the overall content, respectively. The proposed manifold was successfully used to control the dissolution test of a pharmaceutical preparation.
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