Chem. Pharm. Bull. 53(10) 1251—1254 (2005)

soluble vitamin and needed for the growth and repair of tissues in all parts of the body. AA has vital importance in processes of oxidation and reduction in human organism participating of several metabolic reactions. AA has also been used for the prevention and treatment of the common cold, mental illness, infertility, cancer and AIDS. Due to the great importance of AA in human beings, the quantitative analysis of AA has gained increased significance in several areas of analytical chemistry such as pharmaceutical and food applications. It is also clinically important to determine its concentration in blood, urine and some tissues. Many methods depend on the reaction of AA by means of suitable reagents, then the product of reaction or excess reagent monitored with optically (spectrophotometric, fluorimetric or chemiluminimetric), electrochemically (amperometric, voltammetric, potentiometric or conductometric) and chromatographically (High Pressure Liquid Chromatography). Spectrofluorimetric analysis is well known for its sensitivity and speed. Recently, fluorimetric determinations of AA have been developed based on the condensation reactions of AA with o-phenylenediamine and on the oxidation with mercury(II) of vitamin C to form quinoxaline derivate. The reaction products of these methods exhibit fluorescence. A simple, sensitive and selected fluorimetry for determination of AA is based on the reaction of AA in alkaline medium with 2,3-diaminonaphthalene without oxidant. The addition of beta-cyclodextrin (beta-CD) was greatly enhancing the fluorescence of the system. In another study, Thionine blue is reacted with AA in order to develop a simple, sensitive, selective and rapid fluorimetric method for determining AA. The product of this photochemical reaction, Leucothionine blue, is a highly florescent specimen, which is used for sensitive determination of AA in flow injection system. In our study, Methylene Blue (MB), a member of thiazine dye groups like thionine blue, was used for the fluorimetric determination of AA. MB is widely used in a number of different areas e.g. a photo sensitizer to produce singlet oxygen in photodynamic therapy for the treatment of cancer. The highly colored oxidized form of MB can be reduced to the colorless leuco form Leuco-Methylene Blue (LMB), as shown in Fig. 1. The redox indicator properties of this couple have been used extensively in chemical analyses. The reaction between AA and MB was used for AA determination in pharmaceuticals or fruit juices by using spectrophotometry, amperometry or voltammetry. In this study, the determination of AA is based on the measurement of decreasing of the fluorescence intensity of MB due to the reaction between AA and MB, where MB is reduced to colorless LMB and AA is oxidized to dehydroascorbic acid (DHAA). This method allows us to determine the amount of AA in the purified materials, specifically Vitamin C tablets.