Indirect complexometric titration for the determination of Zn2+ in the system of H+, Zn2+, La3+//SO2-4, PO3-4-H2O for removal of fluoride using lanthanum phosphate
XU Chao1, ZHANG Ning2, ZENG De-wen*1
1. College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China; 2. College of Science, Central South University of Forestry and Technology, Changsha 410004, China
Abstract：During the removal of F in zinc electrolyte using LaPO4, the content of Zn2+ in the system of H+, Zn2+, La3+∥SO2-4, PO3-4-H2O cannot be directly determined by traditional titration method due to the presence of La3+ and PO3-4. Proper amount of NH3·H2O-NH4Cl buffer solution was added into testing solution to complex Zn2+ (Zn3(PO4)2 was dissolved in ammonia solution), meanwhile, La3+ entered into solid phase. Thus Zn2+ and La3+ could be separated after suction filtration and rinsing of precipitate. The content of Zn2+ in filtrate was titrated with EDTA (eriochrome black T was used as indicator solution). The determination method of Zn2+ in this system by indirect complexometric titration was established. The experimental results showed that it was better to use buffer solution with high concentration but use the diluted buffer solution as flushing fluid. For the sample solution with 0.1-1.0mmol of Zn2+, the following buffer solution was employed: 54g of NH4Cl was dissolved in water; then 350mL of NH3·H2O was added and the solution was diluted to 1L; the dosage was 20mL. The flushing fluid was buffer solution (1+2). For ease transfer of precipitates, excessive F- (1.1 times of theoretical requirements to precipitate La3+, i.e., 3.3 times of molar quantity of La3+) was added to convert the precipitates into LaF3, which was not easily adhered to the glass. The limit of quantification was 0.01mol/kg. The content of Zn2+ in equilibrium liquid of H+, Zn2+, La3+∥SO2-4, PO3-4-H2O (the content of Zn2+ was in range of 0.1-1.0mmol) was determined according to the experimental method. The relative standard deviation (RSD, n=6) of determination results was less than 0.1%. The spiked recoveries were between 99% and 100%.
徐超, 张宁, 曾德文. 磷酸镧除氟的H+,Zn2+,La3+∥SO2-4,PO3-4-H2O体系中Zn2+的间接络合滴定[J]. 冶金分析, 2020, 40(6): 1-7.
XU Chao, ZHANG Ning, ZENG De-wen. Indirect complexometric titration for the determination of Zn2+ in the system of H+, Zn2+, La3+//SO2-4, PO3-4-H2O for removal of fluoride using lanthanum phosphate. , 2020, 40(6): 1-7.
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