以乙基紫为指示剂滴定法测定酸洗废液中盐酸浓度

采用传统酸碱滴定方法测定酸洗废液中的盐酸浓度,无掩蔽剂存在时,废液中铁离子和碱反应生成沉淀,干扰滴定反应;加入掩蔽剂后,如果指示剂不合适依然会判断不出滴定终点,导致测定结果不准确,进而会影响到后续酸洗废液的处理,造成不必要的资源浪费及环境伤害。因此找到一种合适的酸洗废液的盐酸浓度测定方法尤为重要。实验以乙基紫为指示剂,使用氢氧化钠标准溶液,采用滴定法对盐酸溶液、酸洗废液模拟溶液(w_HCI=15%、w_FeCI2=8%、w_FeCI3=1%)以及加入掩蔽剂后的酸洗废液模拟溶液中的盐酸浓度进行测定。通过分析、比较酸碱滴定曲线和滴定误差,探讨了Fe²⁺和Fe³⁺对该酸碱中和滴定的影响,探讨了乙基紫替代甲基橙作为指示剂采用滴定法测定酸洗废液模拟溶液中盐酸浓度的可行性。实验结果表明:加入掩蔽剂NH₄SCN和NaF后测定盐酸浓度,指示剂变色不再受铁离子影响,滴定终点时溶液由蓝色变为紫色,变化明显容易观察,此时溶液pH值在3.9左右,滴定误差为0.2%,小于酸碱滴定法中普遍使用的甲基橙指示剂的滴定误差。

Determination of hydrochloric acid concentration in waste pickling solution by titration using ethyl violet as an indicator

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 (w_HCl=15%, w_FeCl2=8%, w_FeCl3=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.