Determination of hydrochloric acid concentration in waste pickling solution by titration using ethyl violet as an indicator
CHEN Jie1,2,3,4, TIAN Feng5, ZHAO Mingjie1,3, LIU Xiaojing1,3 LI Yongtian*1,2,3, ZHANG Zhenbin6
1. Qinhuangdao Tianda Environmental Protection Research Institute Co., Ltd., Hebei Key Laboratory of River Water Quality Purification and Ecological Restoration,Qinhuangdao 066004, China; 2. School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China; 3. Qinhuangdao Water Pollution Monitoring and Treatment Engineering Technology Research Center, Qinhuangdao 066000, China; 4. Tianjin Dawei Debao Marine Technology Co., Ltd.,Tianjin 301700, China; 5. School of Water Conservancy and Environment, Jinan University, Jinan 250022, China; 6. School of Chemical and Environmental Engineering, Liaoning University of Technology, Jinzhou 121000, China
Abstract：During the determination of hydrochloric acid concentration in waste pickling solution by traditional acid-base titration, if there is no masking agent, the iron ions in waste solution can react with base to form precipitate, which interferes the titration reaction. After adding masking agent, the endpoint of titration is hardly judged if the indictor is improper, leading to the inaccurate determination results. Moreover, it will also influence on the treatment of subsequent waste pickling solution, causing unnecessary waste of resources and environmental damage. Therefore, it is very important to find a suitable method for the determination of hydrochloric acid concentration in waste pickling solution. In experiments, the concentrations of hydrochloric acid in hydrochloric acid solution, simulated waste pickling solution (wHCl=15%, wFeCl2=8%, wFeCl3=1%) and simulated waste pickling solution with masking agent were determined by titration method with sodium hydroxide standard solution using ethyl violet as the indicator. The effect of Fe2+ and Fe3+ on the acid-base neutralization titration was discussed by analyzing and comparing the acid-base titration curve and titration error. The feasibility of using ethyl violet instead of methyl orange as the indicator to accurately determine the concentration of hydrochloric acid in the simulated waste pickling solution was discussed. The results showed that the coloring of indictor was not affected any more after adding masking agents (NH4SCN and NaF) for the determination of hydrochloric acid concentration. The color of solution changed from blue to purple at the end of titration, which was easily observed. At this time, the pH of solution was about 3.9, and the titration error was 0.2%, which was less than the titration error of methyl orange indicator commonly used in acid-base titration.
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