| The pharmaceutical analysis is one of the most important component in the pharmacology. The quantity of the druggery affects people's health and safety directly, therefore, the whole control of druggery plays an important role in the clinic and medical treatment. Spectrophotometry, chromatography and spectrophotometry-chromatography, as the basic tools and essential methods in the druggery measurements, have been widely used in the pharmical products. Among them, UV-visible spectrophotometry is still the most used drug analysis method. There are many questions needed to be solved in the analysis of drudgery which include optimizing the analytical conditions, simplifying the procedure, improving the selectivity and sensitivity.This paper aims to improve selectivity and sensitivity. Series of studies are carried out in order to propose a simple spectrophotometric method and discuss reaction mechanism of association.In Britton-Robinson buffer solution at pH 4.1, pipemidic acid reacts with erythrosin B to form an 1:1 ion-association complex, which make the erythrosin B faded, the maximum fading wavelength is 525 nm. Accordingly, a decolor spectrophotometric method for determining pipemidic acid is established. Beer's law is obeyed in the range of 1.0×10-6 4.0×10-5 mol/L, the apparent molar absorptivityεis 3.2×104 L/(mol·cm), and the detection limit is 3.2×107 mol/L. The method is satisfactory for the determination of pipemidic in tablet.In weak acid medium, tetracycline (TC) reacts with Cu (II) to form cationic chelating, and further reacts with eosin Y (EY) to form a TC: Cu (II): EY = 3: 3: 2 ion-association complex, which results in the fading of eosin Y dramatically. The maximum fading wavelength is 516 nm. Based on this phenomenon, a fading spectrophotometric method for the determination of tetracycline is proposed. Its linear range is 3.0×10-6 3.0×10-5 mol/L, and the detection limit is 5.6×10(-7) mol/L. The mechanism of reaction and the composition of ion-association complex are also discussed.In the weak acid of Britton-Robinson buffer solution, erythrosin B reacts with Mo (VI) in molybdate to form a complex, which make the maximum absorption wavelength absorbance increased. This complex further reacts with fluoroquinolone derivatives including norfloxacin, ciprofloxacin, ofloxacin to form ion-association complex, leading to the decreasing of absorbance. The maximum fading wavelength is 525 nm. Based on this phenomenon, a simple, rapid and sensitive new method is developed. The linear ranges and detection limits are 1.0×10-6 3.1×10-5 mol/L and 3.2×10(-7) mol/L for norfloxacin, 1.0×10-6 3.4×10-5 mol/L and 5.6×10(-7) mol/L for ciprofloxacin and 1.0×10-6 3.2×10-5 mol/L and 6.3×10(-7) mol/L for ofloxacin, respectively.
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