磷酸镧除氟的H+,Zn2+,La3+//SO4^2-,PO4^3--H2O体系中Zn2+的间接络合滴定

Indirect complexometric titration for the determination of Zn^2+in the system of H^+,Zn^2+,La^3+//SO4^2-,PO4^3--H2O for removal of fluoride using lanthanum phosphate

采用镧盐除氟剂LaPO4对锌电解液除F时,由于La^3+和PO4^3-的存在,H+,Zn2+,La3+//SO4^2-,PO4^3--H2O体系中Zn^2+不能直接用传统的滴定法测定。通过向待测液中加入适量NH3·H2O-NH4Cl缓冲溶液络合Zn^2+(Zn3(PO4)2溶于氨性溶液),使La^3+进入固相,抽滤、洗涤沉淀以分离Zn^2+和La^3+,收集滤液用EDTA(铬黑T作指示剂)滴定Zn^2+,建立了间接络合滴定该体系中Zn^2+的方法。实验结果表明:缓冲溶液采取较高的浓度配方、冲洗液采用稀释的缓冲溶液为宜。对于含0.1~1.0mmol Zn^2+的样液,确定缓冲溶液为54g NH4Cl溶于水,再加入350mL NH3·H2O,定容至1L,用量为20mL;冲洗液为缓冲溶液(1+2);为便于沉淀转移,加入沉淀La^3+理论量1.1倍的F-(La^3+物质的量的3.3倍),可使沉淀转为不易于粘附玻璃的LaF3。方法的定量限为0.01mol/kg。吸取含Zn^2+在0.1~1.0mmol的H^+,Zn^2+,La^3+//SO4^2-,PO4^3--H2O体系平衡液,按照实验方法测定Zn^2+,结果的相对标准偏差(RSD,n=6)小于0.1%,加标回收率为99%~100%。

During the removal of F in zinc electrolyte using LaPO4,the content of Zn^2+in the system of H^+,Zn^2+,La^3+∥SO4^2-,PO4^3--H2O cannot be directly determined by traditional titration method due to the presence of La^3+and PO^3-4.Proper amount of NH3·H2O-NH4Cl buffer solution was added into testing solution to complex Zn^2+(Zn3(PO4)2 was dissolved in ammonia solution),meanwhile,La^3+entered into solid phase.Thus Zn^2+and La^3+could be separated after suction filtration and rinsing of precipitate.The content of Zn^2+in filtrate was titrated with EDTA(eriochrome black T was used as indicator solution).The determination method of Zn^2+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 Zn^2+,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 La^3+,i.e.,3.3 times of molar quantity of La^3+)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 Zn^2+in equilibrium liquid of H^+,Zn^2+,La^3+∥SO4^2-,PO4^3--H2O(the content of Zn^2+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%.