Improved colorimetric determination of aspirin and salicylic acid concentrations in human plasma

An improvement in a previously described method for the determination of plasma salicylic acid and aspirin levels in humans is described. The procedure was simplified by employing only one plasma sample for both salicylates. More accurate estimation of salicylates, particularly aspirin, was achieved by using two different calibration curves. Salicylic acid was estimated by reaction with an aqueous solution of the Folin-Ciocalteu phenol reagent. Absorbance of the blue-colored complex, which formed on addition of sodium hydroxide, was measured at 670 nm. The influence of alkalinity in the formation of the colored complex is discussed. The average recovery of aspirin added to plasma was 94.61%; it was 214.72% by the previous method.