Role of barium chemical modifier in the determination of fluoride by laser-excited molecular fluorescence of magnesium fluoride in a graphite tube furnace

Abstract

Some improvement in the determination of fluorine in urine and tap water by use of laser-exicted molecular fluorescence spectrometry of magnesium monofluoride in a graphite tube furnace has previously been reported. The use of barium as a chemical modifier increased the size of the signal by a factor of 100. The work reported in the present paper was carried out in an attempt to elucidate the mechanism of the enhancement of the magnesium fluoride fluorescence by barium, and to explain some other experimental characteristics of the method, such as the vaporization temperature, which was lower at 1800 °C than the 2400–2700 °C reported by other workers. The mechanism of formation of gaseous magnesium fluoride molecules from sodium fluoride and magnesium nitrate solutions in a graphite tube furnace during atomic absorption measurements was investigated with and without the presence of barium. It was shown that, without chemical modification, the formation of magnesium fluoride in the gaseous phase proceeded mainly via interaction between magnesium difluoride molecules and excess of free magnesium atoms [Mg(g)+ MgF₂(g)→ 2MgF(g)]. The efficiency of this process was fairly low, primarily because of the difference between the vaporization temperatures of the reacting species (1400 °C for magnesium difluoride and 1800 °C for magnesium vaporized as magnesium oxide). The presence of barium changed the mechanism of formation of magnesium fluoride. It was calculated that the formation of barium difluoride, rather than magnesium difluoride, was thermodynamically preferable in the first step of the mechanism. Experimental data indicated that the formation of magnesium fluoride then proceeded with higher efficiency than without barium because the reaction Mg(g)+ F(g)→ MgF(g) followed the appearance of magnesium from magnesium oxide, and fluorine from barium difluoride at coincidental temperatures in the range 1700–1900 °C.