EDTA滴定法测定萤石中氟化钙

萤石中氟化钙的含量是评定其质量等级最重要的指标,因此测定萤石中氟化钙的方法受到关注。采用EDTA滴定法测定试样中全钙的含量,并提出了一种测定试样中碳酸钙的方法,再由全钙和碳酸钙的含量计算得到萤石中氟化钙的含量。称取两份不同质量试样,用定量氯化钙-盐酸溶液浸取,以酚酞为指示剂,调节溶液pH值约为7,将这两份溶液定容到相同体积,静置一段时间待氟化钙溶解达到平衡,此时两份溶液中氟化钙溶解量一致、试剂空白一致。对两份溶液进行干过滤,采用EDTA滴定法测定两份溶液中全钙的质量差(以碳酸钙计),此差值即称样量差中所含的碳酸钙质量。采用实验方法测定萤石标准样品和生产试样中全钙、碳酸钙和氟化钙含量,全钙测定结果与标准样品认定值或标准方法GB/T 5195.1—2006测定值相符,相对标准偏差(RSD,n=5)在0.11%~0.23%之间;碳酸钙测定结果与标准样品认定值或标准方法GB/T 5195.2—2006测定值相符,碳酸钙质量分数不小于0.30%的试样,相对标准偏差(RSD,n=5)在4.9%~7.6%之间,碳酸钙质量分数小于0.30%的试样,相对标准偏差(RSD,n=5)在12.6%~28.1%之间;氟化钙含量与认定值或上述两种标准方法测定后计算所得值基本相符,相对误差不大于0.40%。

Determination of calcium fluoride in fluorite by EDTA titration

The content of calcium fluoride in fluorite was the most important index to evaluate its quality level, so the determination method of calcium fluoride in fluorite had attracted much attention. The content of total calcium in sample was determined by EDTA titration, and a kind of determination method of calcium carbonate in sample was proposed. Then the content of calcium fluoride in fluorite could be calculated based on the content of total calcium and calcium carbonate. Two samples with different mass were leached with certain amount of calcium chloride-hydrochloric acid solution. The pH of solution was adjusted to about 7 with phenolphthalein as indicator. Then the two sample solutions was diluted to the same volume. After placing for a period of time, the dissolution of calcium fluoride reached the equilibrium. At this time, the content of dissolved calcium fluoride as well as reagent blank in the two solutions was the same. After dry filtration, the mass difference of total calcium (in calcium carbonate) in the two solutions was determined by EDTA titration. This difference was the mass of calcium carbonate contained in the difference of sampling mass. The proposed method was applied for the determination of total calcium, calcium carbonate and calcium fluoride in certified reference materials and production samples of fluorite. The determination results of total calcium were consistent with the certified values of certified reference materials or those obtained by standard method GB/T 5195.1-2006. The relative standard deviations (RSD, n=5) were between 0.11% and 0.23%. The determination results of calcium carbonate were consistent with the certified values of certified reference materials or those obtained by standard method GB/T 5195.2-2006. The RSDs (n=5) were between 4.9% and 7.6% for the sample with calcium carbonate mass fraction not lower than 0.30%, and the RSDs (n=5) were between 12.6% and 28.1% for the sample with calcium carbonate mass fraction lower than 0.30%. The determination results of calcium fluoride were basically consistent with the certified values of certified reference materials or those calculated by the found of the above two standard method. The relative error was not more than 0.40%.