A convenient calibrant for silicon determination

Using pure SiO2 or insoluble silicate certified reference materials for preparing calibration solutions is a time consuming process, which presents ample opportunity to incur error. In contrast, (NH4)2SiF6 is a water soluble salt from which calibration solutions can be quickly and simply prepared. It has been shown to be non-hygroscopic and at least 99.9% stoichiometric with regard to silicon content. No problems arise when (NH4)2SiF6 is used for calibrating analysis by molybdosilicate spectrophotometric methods. The silicon atomic absorption signal is suppressed by 6%–7% owing to the fluorine content of (NH4)2SiF6, but this error can be prevented by incorporating a buffer prepared from HBF4 and H3BO3, that is, 1 mol in both boron and fluorine. Aqueous solutions containing 2.9650 g l−1 (NH4)2SiF6 dissolve silicon from borosilicate glass surfaces at an initial rate of 0.11 μ m−2 s−1 SiO2, which is negligible over a period of hours and can be suppressed altogether by incorporating H3BO3 to give a B : Si atomic ratio, 1 : 2. Silicon is not lost from unacidified dilute aqueous solutions through permeation of SiF4 through plastic containers.