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Int. J. Electrochem. Sci., Vol. 14, 2019 5378 acid pH, and 5% and 1 % for neutral and basic pHs, respectively. In all cases, the curcumin is more unstable at acid pHs, which is quite the contrary to that ascertained by Tønnesen and Karlsen [15, 16] and Wang et al. [17] From the plots in Figure 2a and 2b the rate of degradation, Vdeg, can be defined as the time derivative of the curcumin degradation percent: Vdeg = d/dt (%CD) (2) Fort the first 20 minutes, the %CD variation as a function of time was practically linear, then the degradation rate can be estimated from the lines slope, see Table 1, from which it can be observed that the greatest degradation rate takes place at acid pH. From the Vdeg data, see Table 1, and that shown in Figure 1 and Figure 2 it may well be concluded that the curcumin in aqueous solution turns more stable as the pH becomes basic. Notwithstanding, control over the experimental conditions inevitably diminishes the degradation rate. It is quite important to mention that during all these experiments the presence of precipitates was not observed, therefore, the absorbance changes described of the curcumin solutions used are due solely to curcumin degradation and not to variations in the curcumin solubility. Table 1. Values of the curcumin degradation rate, Vdeg = d/dt(%CD), in an aqueous solution at different pH values and control of the experimental conditions. Vdeg/ %CD min-1 pH 3.576 ± 0.001 7.025 ± 0.001 10.526 ± 0.001 Without control of the experimental conditions 7.80 ± 0.75 4.78 ± 0.59 0.39 ± 0.02 Controlling the experimental conditions 2.86 ± 0.29 0.47 ± 0.03 0.34 ± 0.02 3.1.2 Electrochemical measurements In order to further verify the stability of curcumin, cyclic voltammetries were conducted, at different times, in each of the curcumin aqueous solutions having different pH values. Figure 3 depicts CVs where it is possible to note that for acid and neutral pH values, see Figure 3a and 3b, the CVs taken at the beginning of the experiment, named as 0 min, are quite different from those taken after 60 min, indicating that different curcumin chemical species were formed with time, however, for basic pH, see Figure 3c, the CVs are practically the same which indicates that curcumin was stable at this pH condition as the spectrophotometric studies previously revealed.PDF Image | Curcumin in Aqueous Media at Different pH
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