A Color-Changeable “Pearl” for Visually Displaying Chemical Complexation Reactions

An experimental design by combining hydrogel and ethylenediaminetetraacetic acid disodium salt (EDTA) for visually displaying chemical complexation reactions is developed. The Na+ ions on the α-l-guluronic acid (G) unit of the sodium alginate molecule undergo an ion exchange with a divalent cation, so that the G units are stacked to create a cross-linked network, forming a hydrogel. EDTA is inserted into the as-prepared hydrogel, producing transparent hydrogel microspheres containing EDTA, which are considered to be color-changeable pearls on the basis of the chelation of EDTA when reacting with several metal ions. By using different stability constants of the same typed coordination complexes, the color of pearls is changeable. The process and mechanism of the complex reaction are investigated by using elemental mapping, Fourier transform infrared reflection (FTIR) and Raman spectra. Moreover, all the instruments and chemicals for this experiment are included in one single portable experimental kit. The kit is small, easy to carry, cheap, and safe, and it enables easy display of related chemical reactions. By using the developed kit, the students' hands-on experience can be improved. The experimental operation is simple, green, safe, and interesting, which enables the engineering of color-changeable pearls to be broadly used including the chemical experiment courses in universities, experimental teaching in schools, and the chemical popularization activities for the public.