EDTA滴定法测定白云石中氧化钙和氧化镁含量方法的改进

摘要

以盐酸、氢氟酸和硝酸分解试样,盐酸冒烟除去残余的硝酸和氢氟酸.经过氨水沉淀分离铁、铝等干扰元素后,在pH=10的NH3·H2O-NH4Cl缓冲体系中,以铬黑T为指示剂,EDTA滴定法测定氧化钙和氧化镁的合量;在pH≥12的氢氧化钾缓冲体系中,以钙试剂为指示剂,用EDTA滴定法测定氧化钙的含量.根据氧化钙和氧化镁的合量减去氧化钙的差值,得到氧化镁的含量.试验发现,采用盐酸、氢氟酸和硝酸溶解样品,代替国标方法用碳酸钠-硼酸混合熔剂熔融,稀盐酸浸取溶解样品,溶样省时,省力;两种方法溶样后,都需要加三乙醇胺掩蔽铁、铝等干扰元素,实验方法增加了氨水沉淀铁的步骤,进一步消除共存元素的干扰;EDTA滴定法测定氧化钙时,为了消除镁的干扰,实验方法加入甘露醇作辅助络合剂,代替国标方法中的设置预置滴定剂;在测定氧化钙和氧化镁的合量时,实验方法用铬黑T代替国标方法的酸性铬蓝K-萘酚绿B混合指示剂.精密度试验中,三个标准样品测定结果的相对标准偏差(RSD,n=6):氧化钙为 0.25％～0.35％;氧化镁为 0.35％～0.45％.采用国标方法和实验方法对不同的白云石样品进行测定,测定结果基本一致.%The sample was dissolved in HCl-HF-HNO3,the residual of HF and HNO3 were removed by smoking of HCl.The interference elements such as Fe,Al were separated by aqueous ammonia,the aggregate content of CaO and MgO was titrated by EDTA complexometric titration in the buffer system of NH3·H2O-NH4Cl at pH value of 10 with chrome black T as indicator;the content of CaO was titrated by EDTA titration in the buffer system of potassium hydroxide at pH≥12 with calcon as indicator.The content of MgO was obtained based on the difference between the content of CaO and the aggregate content of CaO and MgO.It was found that the sample was dissolved faster when HCl-HF-HNO3 was used instead of sodium carbonate-boric acid mixed flux melting,dilute hydrochloric acid leaching in national standard.Both dissolution methods needed to add triethanolamine to mask Fe,Al and other interference elements.The Fe was precipitated by aqueous ammonia in proposed method to futher eliminate the interferece of coexisting elements.In order to eliminate the interference of Mg while determining calcium oxide by EDTA titrimetry,the mannitol as auxiliary complexing agent was used in proposed method instead of setting present titrant in national standard.When the aggregate content of Ca and Mg was determined by EDTA,chrome black T was used instead of the national standard method of acid chromeblue K-naphthol green B mixed indicator.In precision test,the relative standard deviation of the determination results of three standard samples(RSD,n=6): CaO was 0.25％～0.35％;MgO was 0.35％～0.45％.Different dolomite samples were determined with national standard method and the proposed method.The results were basically consistent.