1977 年 97 巻 12 号 p. 1273-1278
A new colorimetric determination of tannin, measuring absorbances of the solutions after precipitating Methylene Blue with the sample, was investigated to develop an analytic method of higher sensitivity and reproducibility. Comparison of the calibration curves of tannic acid in Japanese Pharmacopoeia (J.P.), geraniin, chebulinic acid, and chebulagic acid at various pHs showed that the best result in sensitivity, linearity, and reproducibility common to these four tannins is obtained at pH 7.0. The determined value (RMB) of geraniin by this method at pH 7.0, based on the calibration curve of tannic acid J.P. (RMBpH7) was 1, but appreciable differences between these two tannins were observed at other pH range. The RMB values of samples at pH ranges other than 7.0 were based on the calibration curve of tannic acid J.P. at pH 7.0. Marked similarity between RMBpH7 and RApH6 was observed for variation of the activity induced by hydrolysis of geraniin, and extracts of Geranium thunbergii. The RMBpH7 values for seasonal variation of tannin content in G. thunbergii, for tannin content of Geranium species, and of several species of plants were obtained before and after treating the extracts with methanol. The RMB values of these specimens, based on the calibration curve of geraniin (RMBG), and RMB of geraniin and hydrolyzed geraniin based on the calibration curve of tannic acid J.P. at the same pH, were also obtained. The amount of the samples required for RMBpH7 determinations was ca. 1/50 of those for RApH6 determinations.