Spectrophotometric analysis of some guanidino drugs by acid-dye and charge-transfer complexation methods

Two spectrophotometric methods are described for the determination of guanethidine sulphate (I), guanfacine hydrochloride (II), guanoclor sulphate (III), guanoxan sulphate (IV) and debrisoquine sulphate (V). The first method involves ion-pair formation of the selected compounds (I-V) with bromocresol purple at pH 3.8. The yellow ion pair is extracted with chloroform and the absorbance is measured at about 415 nm. The second method is based on the reaction of the basic guanidino compounds (I, III-V) with iodine in chloroform to give molecular charge-transfer complexes with maximum absorbance at 292 and 345 nm. Beer's law was obeyed for both methods and the relative standard deviations were found to be less than 2%. The apparent molar absorptivities were found to be 2.1 x 10(4) to 6.9 x 10(4) l mol-1 cm-1 using bromocresol purple and 0.7 x 10(4) to 2.4 x 10(4) l mol-1 cm-1 using iodine. The investigated drugs were assayed in tablets. The mean percentage recoveries were found to be 99.8-100.8% by the acid-dye method and around 100.4% by the charge-transfer complexation method.