Читать книгу Recent Advances in Polyphenol Research - Группа авторов - Страница 18

From eq. (36) to eq. (38) all equilibrium constants except those regarding the trans‐chalcones can be obtained. Moreover, the cis‐trans isomerization constants can be calculated from a reverse pH jump from the equilibrated solutions. Considering that formation of the flavylium cation from the trans‐chalcones is very slow, this kinetics should be followed by a standard spectrophotometer. The quinoidal bases, hemiketals, and cis‐chalcones are transformed to flavylium cation much faster than the trans‐chalcones and appear as an initial absorption.² From this point all the equilibrium constants have been calculated. The mole fraction of trans‐chalcone is thus obtained from the ratio of the absorbance of the trace amplitude/total absorbance. The mole fractions of the other species at equilibrium are obtained from those at pseudo‐equilibrium, calculating the respective proportion.³ For example, if at pseudo‐equilibrium A=0.3, B=0.2, and Cc=0.5 and the mole fraction of Ct at equilibrium is 0.5, the mole fractions of A, B, and Cc at equilibrium are the following: A=0.15, B=0.1, and Cc=0.25.