Quantitative Analysis for a Color-Change of Humidity Indicator by Microscopic Absorption Spectrometry

A sensitive and easily distinguishable cobalt-free humidity indicator of porphyrin-silica gel-MgCl2 composite was prepared from pH-induced spectra changeable tetraarylporphyrin, silica gel (SiO2), and MgCl2. The pH change arose from proton release under dry conditions, and proton capture under humid conditions by a reversible reaction between MgCl2 and a silanol group of SiO2. A pink-orange porphyrin-Si(OH)2-MgCl2 composite was dried to give a green protonated porphyrin-SiO2Mg composite. The optimized concentrations of MgCl2 to make the concentrations of protonated porphyrin maximum under dry conditions were determined by absorption spectrometry of the green composite using a confocal laser scanning microscope as a microscopic spectrometer. Moreover, the green composite was prepared by heating dichloro(tetraarylporphyrinato)phosphorus chloride with MgCl2 and SiO2. The humidity-sensitivity of the green composite was evaluated by the absorption spectra under controlled humidity. A distinguishable color change of the green composite took place below 30% of relative humidity.