Abstract :
Citrate-based buffers effectively decomplex AlFx complexes, a process which is not only pH-dependent, but also a function of both the fluoride and aluminium concentrations. At 1 mg/l fluoride, the lowest concentration examined, the tolerance for Al3+ is relatively high and constant in the pH range 5.5–8. At a pH of 8 and at 100 mg/l fluoride, the highest level tested, the tolerance for Al3+ is only about six times lower compared to the tolerance of a 1 mg/l fluoride solution. However, at a pH of 5.5, the tolerance for Al3+ decreases about 500 times compared to the tolerance for Al3+ at 1 mg/l F−. In other words, at a pH of 5.5, the tolerance for Al3+ decreases rapidly as F− levels increase, whereas the tolerance for Al3+ at a pH of 8 decreases very little towards higher F− levels. The intolerance for aluminium ions was also observed in calibration curves at high levels of Al3+ and low pH. The calibration curves are perfectly linear except in cases where the Al3+/F− ratio is too high (>10). Incomplete decomplexation caused the values of the slopes and intercepts to deviate from the regression values without Al3+. As a result, insufficient fluoride concentrations were measured when standard calibration curves were used without any Al3+ added.
