الفهرس 
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Abstract
AS at 223 nm. The methods permit the determination of the three studied drugs in the range of 3.0-80 μg mL-1. The third method is based on the formation of yellow colored chloroform ion-pair complex between the basic nitrogen of the drugs (VARD New three simple and accurate spectrophotometric and atomic absorption spectrometric (AAS) methods have been developed for the determination of some phosphodiesterase inhibitors vardenafil HCl (VARD), sildenafil citrate (SLDN) and yohembin HCl (YOHM) in pure forms and pharmaceutical formulations. In the first method, we investigate the spectrophotometric methods for determination of VARD and YOHM based on the formation of charge-transfer complex in acetonitrile of VARD with p-CLA, DDQ and TCNQ at 535, 461 and 840 nm, respectively and YOHM with DDQ and TCNQ at 461 and 841 nm, respectively. Beer’s law is obeyed in the concentration range 5.0-200 g mL-1. The correlation coefficient was ≥ 0.9995 with a relative standard deviation (R.S.D.) of ≤ 0.70. In the second method, atomic absorption spectrometric (AAS) and spectrophotometric methods have been developed for the determination of VARD, SLDN, YOHM in pure forms and pharmaceutical formulations. The methods are based on the formation of ion-pair associates between the drugs and the inorganic complex, bismuth (III) tetraiodide. The reaction occurs in acidic medium to form orange-red ion-pair associates which are instantaneously precipitated. The formed precipitates are then filtered off and the residual unreacted metal complex in the filtrate is determined either spectrophotometrically at 455 nm or by AAS at 223 nm. Also, the precipitates may be dissolved in acetone and quantified spectrophotometrically at 497, 499 and 502 nm for VARD, SLDN and YOHM, respectively or decomposed by hydrochloric acid, and the bismuth