Evaporation and sublimation of boric acid: Application for boron purification from organic rich solutions

We report in this paper a series of experiments whose purpose is to test th e long held idea of boron volatility when neutral to acidic solutions conta ining traces of boron are evaporated. Boron recoveries have been measured p recisely by isotopic dilution and boron isotopic ratios have also been dete rmined. Most of the evaporations have been conducted at 60-65 degreesC. It was found that no loss of boric acid occurs when c. 1 mug of B is evaporate d in water, HCl, HF and acetone solutions. At the same time, we did not obs erve any associated B isotopic fractionation. In contrast to previous studi es on B volatilisation, it was not found necessary to use mannitol to preve nt B losses during the evaporation of acidic solutions. Important losses of boron were observed from solutions of methanol, ethanol and organic-rich n atural solutions. In the presence of methanol and ethanol, a volatile methy l (or ethyl) borate is likely to be formed. In contrast to these studies, w e show that boron is highly volatile when the dried residues of an evaporat ion are heated further. The sublimation of boron that then occurs can be mo derated by the use of mannitol, but a slight increase of the temperature al lows B volatilisation. Despite the substantial losses of B by sublimation, no B isotopic fractionation was observed at 60-65 degreesC. The property of boron to sublime was used to separate B from on organic matrix. A miniatur ized sublimation apparatus, comprising a 5 ml Teflon beaker, and the associ ated "microsublimation" technique are described. The 100% boron recovery an d the absence of B isotopic fractionation make this method suitable for ext racting B from organic-rich samples.