Abstract The blue-green phenomenon in the coordination titration using eriochrome black T (EBT) as the indicator was investigated. The pH measurement results indicated that the ammonia buffer solution maintained the stability of the solution acidity during the titration process. The slight change of pH did not cause the solution to appear blue-green. The absorption spectrum analysis showed that excessive ethylene diamine tetraacetic acid disodium salt (EDTA) affected the absorption spectrum of EBT, reduced the absorption of EBT at yellow light 575 nm and enhanced the absorption at red light 622 nm and 664 nm. Thus, blue-green with complementary red light appeared. Reversed phase high performance liquid chromatography analysis demonstrated that excessive EDTA prolonged the retention time of EBT, which shown that EDTA reduced the polarity of EBT. Methanol did not alter the different colors of EBT. Hexadecyltrimethylammonium chloride causes different colors of EBT to appear blue-green. It can be inferred that in ammonia buffer solution, static electricity occurred between the positively charged group ≡NH+in EDTA and the negatively charged group —SO-3 or —O- in EBT, and EBT phenolic hydroxyl group may form hydrogen bonds withNor —COO- of EDTA, resulting to the changes of absorption spectrum and HPLC retention of EBT. As a result, the electrostatic and hydrogen bonds between EDTA and EBT may be the nature of blue-green phenomenon.
HU Ya-Lan, LIU Su-Mei, CHEN Huai-Xia. Exploration of the Blue Green Phenomenon of Eriochrome Black T in Coordination Titration[J]. Chinese Journal of Chemical Education, 2024, 45(12): 112-116.
