ポルフィリンをpH指示薬に用いる顕微吸光光度分析によるマグネシウム塩-シリカゲル複合固体の酸性度測定

概要

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The acidity of magnesium salts-silica gel composites was measured by absorption spectrophotometry in the micro-region using a confocal laser scanning microscope (CLSM). It has been reported that MgCl2-SiO2 releases protons under dry conditions, and goes back to neutral under wet conditions. At first, using five kinds of tetraarylporphyrins, the absorbance (AP) at 515 nm due to a free base type and that (AG) at 650 nm due to the protonated porphyrins were measured in a CHCl3-MeOH-water solution, whose apparent pH was adjusted by HClO4. Fraction (F-value) of AP [F = AP/(AG + AP)] was calculated at every apparent pH to produce apparent pH-profiles of the F-values for all porphyrins. The profiles were fitted by sigmoid curves. Second, the porphyrin was adsorbed on MgCl2-SiO2. The porphyrin-adsorbed composite was kept under the given relative humidity conditions for 3 days and subjected to CLSM analysis. The F-values of the porphyrins adsorbed on the composites were obtained from the absorption spectra to determine the apparent pH using fitting curves. As a result, the MgCl2-SiO2 had an apparent pH of 1.73 under dry conditions, and 4.39 under wet conditions. Moreover, the pH values of the Mg(NO3)2-SiO2 composite and the MgSO4-SiO2 composite were measured.