Determination of free hydrochloric acid concentration in pickling solution in hydrochloric acid regeneration process of Ruthner method by potentiometric titration
CHEN Min, ZHU Zi-ping*
Manufacturing Department, Baoshan District Iron and Steel Co., Ltd., Shanghai 201900, China
Abstract：It was necessary to timely determine the concentration of free hydrochloric acid during the regeneration treatment of pickling solution with hydrochloric acid regeneration process of Ruthner method. When the concentration of free hydrochloric acid was obtained with the concentration of free H+ in the pickling solution which was determined by potentiometric titration, the presence of Fe2+ and Fe3+ in pickling solution would cause the interference. After the interference of a large amount of Fe2+ and a small amount of Fe3+ in pickling solution was eliminated with Ca-CaY solution (0.1mol/L EDTA-0.150mol/L CaCl2) as the masking agent, the determination of free hydrochloric acid concentration in pickling solution in hydrochloric acid regeneration process of Ruthner method was realized by potentiometric titration. The experiment results showed that the addition of Ca-CaY masking agent had no influence on the determination of free acid. Meanwhile, the jump range of pH narrowed. During the potentiometric titration, the influence of sudden jump near pH=6 on the end-point could be avoided, when the end-point judgment threshold value was set at 10, the end-point judgment range was pH=7-10, and the end-point recognition was maximum. Meanwhlie, the accurate end-point of titration could be obtained. The dosage of Ca-CaY solution was optimized and 10mL of Ca-CaY solution was used in experiments. One sample in each process of hydrochloric acid regeneration of Ruthner method was selected and determined by the proposed method. Moreover, the indirect method was also used for comparison. t test indicated that t (0.51-1.18) was lower than t(0.05,9)=2.26. In other words, there was no system error between the determination results by experimental method and indirect method. The relative standard deviations (RSD, n=10) of the determination results by experimental method were between 1.5% and 2.6%. Different pickling solution samples in the hydrochloric acid regeneration process of Ruthner method were selected and determined according to the proposed method, then certain amount of 0.1mol/L hydrochloric acid standard titration solution was added into samples for standard recovery test. The recoveries were between 96% and 103%.
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