Mohamed M. Shahata* and Seddique M. Ahmed Pages 130 - 136 ( 7 )
Enhancement the oxidation constant (Kox) of Vitacid-C (Vit-C) in potassium chromate catalyzed by phosphate was studied. Molecular mechanics calculations suggest that the potential energy of the optimum molecular geometric structure of Vit-C is at least ten times of magnitude more stable than the PE of the molecular geometric structure of the same matrix. The oxidation constant Kox of Vit-C by K2CrO4 catalyzed by different mineral acids and different phosphate buffer are computed and discussed.
Background: Ascorbic acid can be easily degraded depending on several variables. It has been reported that the degradation kinetics are significant affected by many environment factors such as pH, temperature, light, and the presence of enzymes, oxygen, and metallic catalyzers. The multifaceted nature of the debasement instruments prevents the improvement of mechanistic models, and pseudo- kinetic model, for example, zero order, first- order or second- order kinetics are frequently connected so as to get a solid match to the test information.
Methods: Stock solutions of Vit-C and potassium chromate (K2CrO4) (Aldrich), Milwaukee, WI) were prepared. Thermo Electron-Vision pro Software V 4.10 UV/Vis spectrophotometer (190-1100 nm) with 1.0 cm quartz cell (scan speed, 5.0 nms-1) was used for spectrophotometric measurements.
Results: The absorption spectra of K2CrO4 (0.5 mmol L-1) in different mineral acids were recorded as shown in Fig. (2). The decrease of the absorbance at about λmax =350 nm with increasing the concentration of Vit-C was observed. A good linearity (r ≥ 0.98) of the plotting absorbance vs. Vit-C concentration at constant an oxidant and acid, the slope of this straight line corresponding to the oxidation constant (Kox). The Kox increases according to the following series. HClO4 > H2SO4 >HNO3 >HCl >H3PO4.
Conclusion: The Vit-C -Na3PO4 complex is a more favorable for the oxidative degradation than the Vit-C-H3PO4 matrix. The molar absorpitivity (ε) of the third-derivative (D3) is more sense about threeorder (2.36x103) of magnitude higher than D0 order (normal spectra). The limit of detection can be as low as 4.93 ppm (mgL-1) of Vit-C without treatment. This is about eight times (8.21) lower than that reported previously The molar absorpitivity (ε) of the third-derivative (D3) is more sense about threeorder (2.36 x 103) of magnitude higher than the normal spectra (zero-order, D0). The oxidation constant Kox of Vit-C by K2CrO4 catalyzed by different mineral acids according to the following series: HClO4 > H2SO4 > HNO3 > HCl > H3PO4. The oxidation constant Kox of Vit-C by K2CrO4 catalyzed by different phosphate buffer were computed and discussed.
Mechanism of oxidation, molecular mechanics calculations (MM+), oxidative degradation, oxidation constant, phosphate buffer, vitacid C tablets.
Environmental Affairs Department, Assiut University Hospitals, Assiut, Chemistry Department, Faculty of Science, Assiut University, 71517, Assiut