钙沉淀-EDTA返滴定法测定稀土分离废液中草酸

采用过量加入钙标准溶液以沉淀稀土分离废液中草酸,过滤后,再以EDTA返滴定的方法测定了废液中草酸的含量。在样品溶液中加入400 g/L NaOH溶液调pH值至7.0～10.0,再加入10 mL NH₃-NH₄Cl缓冲溶液(pH 10.0),然后过量加入钙标准溶液沉淀废液中草酸,滤液再以NaOH溶液调pH值至12.0～12.5,铬蓝黑R为指示剂,用EDTA标准溶液返滴定过量钙。废液中大部分RE³⁺、Fe³⁺、Al³⁺等杂质离子在高pH值下可形成氢氧化物沉淀而得以分离,滤液中残余的Fe³⁺、Al³⁺用三乙醇胺等络合剂可消除其对铬蓝黑R指示剂的干扰。采用方法测定稀土分离废液合成样品中草酸的含量,结果与参考值一致。将方法应用于稀土分离废液实际样品中草酸的测定,测得结果相对标准偏差(RSD,n=6)为0.98%～1.9%,回收率为99%～101%。

Determination of oxalic acid in waste solution from rare earth separation process by calcium precipitation-EDTA back titration

Excessive standard solution of calcium was added to precipitate oxalic acid in waste solution. After filtration, the content of oxalic acid in waste solution from rare earth separation process was determined by EDTA back titration method. The pH of sample solution was adjusted to 7.0-10.0 with 400 g/L NaOH solution. After adding 10 mL of NH₃-NH₄Cl buffer solution at pH 10.0, the excessive standard solution of calcium was added to precipitate the oxalic acid in waste solution. The pH of filtrate was adjusted to 12.0-12.5 with NaOH solution. The content of excessive calcium was determined by EDTA back titration with chrome blue black R as indicator. Most impurity ions in waste solution (such as RE³⁺, Fe³⁺ and Al³⁺) could be precipitated to hydroxides and separated under high pH. The interference of residual Fe³⁺ and Al³⁺ in filtrate with chrome blue black R indicator was eliminated by adding triethanolamine as complexing agent. The proposed method was applied to determination of oxalic acid in synthetic sample of waste solution from rare earth separation process. The results were consistent with the reference values. The proposed method was applied to determination of oxalic acid in actual sample of waste solution from rare earth separation process. The relative standard deviations (RSD, n=6) were 0.98%-1.9%. The recoveries were between 99% and 101%.