2017 Volume 43 Issue 2 Pages 123-128
A cobalt-free humidity indicator (HDI) for silica gel was developed using tetraphenylporphyrin (Tpp), which showed a distinguishable color change depending on pH. The HDI was prepared by mixing dichloro(tetraphenylporphyrinato)phosphorus chloride (PTpp) and MgCl2 with SiO2 and drying at 130°C for 24 h. During the preparation, the PTpp was decomposed into the protonated Tpp (H2Tpp2+). The pH change arose from proton release by the reaction of MgCl2 with silanol of the SiO2 under dry conditions and neutralization under humid conditions. The HDI showed green color due to H2Tpp2+ under dry conditions and orange color due to Tpp under humid conditions. However, the HDI underwent partial decoloration on extended exposure to sunlight. Here, to prevent this decoloration, UV-absorbents coumarin and dibenzosuberenone were loaded onto the HDI. The HDIs with coumarin and without coumarin were irradiated at 352 nm and their microscopic absorption spectra were measured on a confocal laser scanning microscope. The absorbance at 650 nm and 515 nm of the HDI were monitored under dry and wet conditions, respectively. The light-protecting ability of UV-absorbent was analyzed by assuming that the decomposition process obeyed first-order kinetics. It was concluded that the additions of 0.125 wt% of coumarin and 0.050 wt% of dibenzosuberenone were effective to prevent decoloration of green color of H2Tpp2+ as well as orange color of Tpp.
