A simple and accrued spectrophotometric method was developed to estimate Doxycycline as pure and in its formulation. The method included using a novel reagent neutral red dye the present method. The method was based on the oxidation of Doxycycline with an excess amount of N-bromosucinimide in an acid medium, then the residual amount of N-bromosucinimide oxidized the neutral red dye and caused bleaching of the color of the dye. The absorbance of the dye's color did not bleach measured at wavelength 525 nm. The absorbance is proportional to the concentration of Doxycycline in the solution. The linearity was from 1 to 15 µg/ ml, and the higher concentration above 15 µg/ ml gave deviation from Beer's law. The important analytical parameter included molar absorptivity and Sandell's sensitivity index, which were calculated and equal to 3.67 x104 l/mol.cm, 0.01261µg/cm2, respectively. An application part included estimating Doxycycline in its dosage form (capsule) from different manufacturers with satisfactory results. Keywords: Doxycycline, Neutral red, Bleaching Color, Oxidation,

A simple and accurate spectrophotometric method has been developed based on diazotization and coupling reactions. The method relied on the diazotization of sulphamerazine (SMEZ) via reaction with HNO2 obtained from the reaction of hydrochloric acid with sodium nitrite. The un-reacted nitrous acid destroyed via a reaction with sulphamic acid. The formed diazotized sulphamerazine (D-SMEZ) was coupled with m-hydroxylaniline to produce yellow azo dye, which gives the highest absorption at 450 nm. The formed azo dye is freely soluble in water and has good stability. The linearity is observed by drawing absorbance of various amounts of sulphamerazine from 0. 5 to 25 μg.mL-1, which exhibits molar absorptivity of 1.4592x105 l.mol.-1cm-1 and Sandell's sensitivity factor 0.001811 μg.cm-2. The low detection limit, low quantitation limit, relative error, and percent relative standard deviation have been estimated, and their values are 0.0022 and 0.0076 μg.mL-1 as well as 0.02 to 0.2%, respectively. The present method has successfully been applied to determine sulphamerazine in trisulphoprim injection (veterinary drug).