钼酸铅重量法测定钼铁中钼的方法改进

GB/T 5059.1—2014测定钼铁中钼含量的钼酸铅重量法使用硝酸和氟化铵溶解钼铁,用氢氧化钠分离铁等干扰元素后,加Na₂EDTA进一步屏蔽干扰元素,以甲基橙指示剂和单宁指示剂确定反应终点。氟化铵遇热分解形成的氟化氢会腐蚀玻璃烧杯,且存在过滤和洗涤氢氧化物沉淀耗时长、指示剂变色不明显、所用化学试剂种类多的问题。为缩短检测时间,简化实验流程,对上述方法进行了改进:用乙酸-硝酸在50～70 ℃低温溶解钼铁试样,煮沸并蒸发至近干除去氮氧化物后,加水稀释至150 mL,加入30 mL Na₂EDTA溶液络合铁、铜等杂质离子。在乙酸-乙酸铵缓冲溶液环境下,加入120 g/L乙酸铅溶液,使钼全部形成钼酸铅沉淀。120 ℃烘干沉淀,在马弗炉中180 ℃灰化并在550 ℃灼烧后进行称重,计算钼含量。实验方法用于测定3个钼铁中钼,结果的相对标准偏差(RSD,n=5)小于0.2%;按照实验方法测定钼铁标准样品中钼,结果与标准值相一致。改进后的方法不仅解决了溶解液腐蚀玻璃烧杯的问题,而且不用氢氧化钠沉淀剂,无需使用变色指示剂,减少了实验误差,缩短了测试时间,适合生产型企业快速测定钼铁中钼含量。

Improvement of determination method of molybdenum in ferromolybdenum by lead molybdate gravimetric method

Nitric acid and ammonium fluoride are used to dissolve ferromolybdenum in the lead molybdate gravimetric method for determining the content of molybdenum in ferromolybdenum in GB/T 5059.1-2014.After separating the interference elements such as iron with sodium hydroxide, Na₂EDTA is added to further mask the interfering elements.Methyl orange indicator and tannin external indicator are used to determine the reaction endpoint.The hydrogen fluoride generated in thermal decomposition of ammonium fluoride will corrode the glass beaker.Moreover,this method also has some other shortcomings,for example,the operation of filtration and washing of hydroxide precipitation is time-consuming,the coloring of indicator is unclear,and many kinds of chemical reagents are used.To shorten the detection time and simplify the experimental process,the method above was improved:the ferromolybdenum sample was dissolved at low temperature of 50-70 ℃ with a mixed solution of acetic acid-nitric acid, the solution was boiled and evaporated to nearly dryness to remove the nitrogen oxides;then it was diluted with water to 150 mL,and 30 mL of Na₂EDTA solution was added for complexation with iron,copper,and other impurity ions.In the medium of acetic acid-ammonium acetate buffer solution, 120 g/L lead acetate solution was added to fully convert molybdenum into lead molybdate precipitate. The precipitate was dried at 120 ℃, ashed in muffle furnace at 180 ℃, and then burned at 550 ℃.The sample was weighed to calculate the content of molybdenum.The experimental method was applied for the determination of molybdenum in three ferromolybdenum samples,and the relative standard deviation (RSD, n=5) of the results was less than 0.2%. The content of molybdenum in certified reference materials of ferromolybdenum was determined according to the experimental method,and the found results were consistent with the certified values.The improved method not only solved the corrosion problem of glass beaker by the solution, but also did not require sodium hydroxide precipitant and coloring indicator. It reduced the experimental error and shortened the test time,so this method was suitable for the rapid determination of molybdenum content in ferromolybdenum in production-type enterprises.