Determination of isothipendyl or dimethothiazine with their sulphoxides using third- and fourth-derivatives...

Third- and fourth-derivative spectrophotometry (3D + 4D), with a diode-array spectrophotometer, has been used for the assay of mixtures of each of isothipendyl hydrochloride (I) and its sulphoxide (II) or dimethothiazine mesylate (III) and its sulphoxide (IV). The parent compounds have been determined by measuring the 3D at 256 nm, 246-256 nm and 256-264 nm and 4D at 250 nm for (I) and 3D at 255 nm or 283 nm and 4D at 257 nm for (III). The sulphoxides have been assayed by measuring the 3D at 285-290 nm and 4D 285-290 nm for (II) and 3D at 305 nm and 4D at 303-310 nm for (IV). Beer's law is obeyed and the detection limits for the proposed 3D and 4D has been calculated. The recovery of these drugs and their sulphoxides in laboratory-made mixture and dosage forms has been reported.