Improved colorimetric determination of salicylic acid and its metabolites in urine.

We describe an improvement in the Levy and Procknal method [J. Pharm. Sci. 57, 1330 (1968)] for determination of salicylic acid and its metabolites in urine. Salicylic acid and salicyluric acid are successively extracted from 1 or 2 ml of urine (acidified with HCl) by two 10-ml portions each of carbon tetrachloride and ethylene dichloride. The extracts of each solvent are shaken with 5 ml of ferric nitrate solution (a 10-fold dilution of 17 g of Fe(NO-3)-9H-2O in 1 liter of 70 mmol/liter HNO-3). The aqueous phases are centrifuged and their absorbances measured at 530 nm. For total salicylate, 3 ml of urine and 3 ml of HCl are heated in a partially evacuated serum vial at 100 degrees C for 16 h and then salicylic acid is assayed in the hydrolyzed sample. Recovery of a weighed oral dose of sodium salicylate in urine was 105.4%; it was 127.9% by the Levy and Procknal method for the same sample. The improved method is faster and more accurate.