高浓度F~--Al~(3+)体系的热力学分析及氟的测定

设定了Visual minteq软件F~--Al~(3+)体系的物种及平衡常数。模拟滴定和实验滴定结果的相似性证明了软件设定的合理。对F-(0.01/0.1 mol/L)-Al~(3+)(0.02 mol/L)-OH-体系的模拟和分析表明,当F与Al摩尔比相差大时,体系平衡物种分布不同,OH-掩蔽铝的机理也不同,但是在p H值11~12铝都被成功掩蔽;不同的反应机理导致F与Al摩尔比大的体系模拟和滴定曲线差别较大。进一步模拟得到了此p H值区间的最大可掩蔽铝浓度。考虑OH-对氟离子电极的影响,最终确定高氟(0.01~0.1 mol/L)高铝体系氟的测定条件:控制p H值为(11.5±0.2),不大于0.02 mol/L的铝。误差分析表明,当电势测定误差较大时,标准曲线法比标准加入法误差小。

Thermodynamics Analysis of High Concentration F--Al3+ System for Fluorine Determination

The species and equilibrium constants were assigned for F^--Al³⁺ system in Visual MINTEQ software. The similarity between the simulated titration curves and experimental ones, and the accordance confirms this assignment. The masking effect of sodium hydroxide and masking mechanism was investigated according to the equilibrium distribution simulation of F^-(0.01/0.1 mol/L)-Al³⁺(0.02 mol/L)-OH^- system. Although the species and aluminum masking mechanism are different in these two systems, aluminum can be masked efficiently at pH 11~12. The larger discrepancy between the simulated titration curve and the experimental curve in the latter system is due to the slower reaction rate of the latter system. Furthermore, the permissible aluminum concentration at pH 11~12 was obtained by simulation. Considering the interference of OH^- to fluoride ion selective electrode, pH is limited to 11.5±0.2 for the determination of fluoride at 0.01~0.1 mol/L, and the maximum aluminum concentration masked at pH 11.5 is 0.02 mol/L. This method suits for the determination of high F concentration in the F-Al system. The error analysis indicates that the standard calibration curve method is more accurate than standard addition method if the potential error is larger than the error of electrode slope constant.