Sulphonated alizarin fluorine blue (AFBS). Part IV. A critical comparison of the use of AFBS against alizarin fluorine blue (AFB) and the fluoride electrode for the determination of low fluoride concentrations: interferences with the AFBS method and their removal

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Abstract
The AFBS and AFB positive absorptiometric methods for the determination of fluoride in water were evaluated with respect to sensitivity, range, reproducibility, rate of complex formation and stability of the colours formed. The spectrophotometric methods were also compared statistically with the use of the fluoride electrode over the same working range. The AFBS method is shown to be slightly superior to the parent AFB method and more reproducible than the electrode except at very low concentrations. The interference of 24 common ions in the AFBS method was examined. Most interfering cations are removed by ion exchange; aluminium and iron require alternative treatment. The important anionic interferences of phosphate and sulphate are readily avoided. The interference study led to the identification of a 1 : 1 : 1 lanthanum-AFBS-mercury ternary mixed-cation complex.

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