Abstract:

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.

Key words: thermodynamics analysis, sodium hydroxide, aluminum masking, fluoride ion selective electrode