Spectrophotometric flow injection determination of formetanate and m-aminophenol in water after reaction with p-aminophenol.

An automated procedure has been developed for the determination of formetanate and its metabolite m-aminophenol (MAP) in water samples. MAP can be selectively determined in the presence of formetanate by direct on-line reaction with p-aminophenol and spectrophotometric measurement of the absorbance at 576 nm in the presence of KIO(4), as oxidizing agent. The method has a limit of detection of 5 x 10(-7)M, it provides a recovery percentage from 95 to 104% and permits one to carry out 120 measurements/hr. The spectrophotometric determination of formetanate must be carried out after a previous hydrolysis to MAP. To determine formetanate in the presence of MAP, two steps are necessary. Firstly, the MAP content is selectively determined as has been mentioned above. After that, the sample is treated with 0.05M NaOH at 90' degrees C, to hydrolyze the formetanate to MAP, and then the sum of both is determined spectrophotometrically. The difference between the results obtained in each step gives the formetanate concentration. The developed procedure for the determination of formetanate provides a sensitivity of 1070 absorbance units mol(-1) l and a limit of detection of 1.9 x 10(-7)M, which corresponds to 50 mug/l of formetanate hydrochloride. The method has been applied to the analysis of natural water samples fortified with formetanate and MAP, and formetanate has also been quantitatively recovered in irrigation waters at a concentration level of 1.9 x 10(-6)M which corresponds to 500 mug/l. On the other hand, working in the stopped-flow mode, for a reaction time of 100 sec, the sensitivity of the formetanate determination can be increased to 4642 absorbance units mol(-1) l but the limit of detection remains of the order of 44 mug/l.