Silicon tetrafluoride determination

Another example is the determination of pure silica in an impure ignited silica residue. The latter is treated in a platinum crucible with a mixture of sulphuric and hydrofluoric acids the silica is converted into the volatile silicon tetrafluoride ... 

The commercial ores, beryl and bertrandite, are usually decomposed by fusion using sodium carbonate. The melt is dissolved in a mixture of sulfuric and hydrofluoric acids and the solution is evaporated to strong fumes to drive off silicon tetrafluoride, diluted, then analyzed by atomic absorption or plasma emission spectrometry. If sodium or silicon are also to be determined, the ore may be fused with a mixture of lithium metaborate and lithium tetraborate, and the melt dissolved in nitric and hydrofluoric acids (17). 

Silica (Si02) content in wood can be determined by treating the ash with hydrofluoric acid (HE) to form the volatile compound silicon tetrafluoride (SiF4). The weight loss is the amount of silica in the ash. Silica is rarely present in more than trace amounts in temperate climate woods, but can vary in tropical woods from a mere trace to as much as 9%. More than 0.5% silica in wood is harmful to cutting tools (55). 

Clearly some form of sample pretreatment is required for soils and sediments. Total levels may be obtained following sodium carbonate-boric acid fusion and the dissolution in hydrochloric acid employing lanthanum as a buffer and releasing agent. If the determination of silicon is not required, it may be volatilized as silicon tetrafluoride using hydrofluoric acid, although some calcium may also be lost as calcium fluoride. For many samples, however, it may be more appropriate to determine the exchangeable cation content of the sample. Here, the sample may be shaken with an extractant solution, for example, 1 mol 1 ammonium chloride, ammonium acetate, or disodium EDTA, prior to filtration and analysis. Where final solutions contain more than - 0.5% of dissolved material, the standards should also contain the major constituents, even where no chemical interference is expected, in order to match the viscosity and surface tension and avoid matrix effects. 

The gaseous products of the "Co y-radiolysis (in Pyrex ampoules) of liquid trifluoroacetic acid at 23 C and of the polycrystalline acid at - 196°C have been identified (CgF, CHFg, COg, CO, Ha, and CgF, CHF3, COg, respectively) and their C-values determined. Failure to detect the formation of carbon tetrafluoride, tetrafluoroethylene, or silicon tetrafluoride was taken as circumstantial evidence for the non-occurrence of reactions involving fluorine-atom abstraction by trifluoromethyl radical or the generation of... 

The primary use of hydrofluoric acid is for the decomposition of silicate rocks and minerals in the determination of species other than silica. In this treatment, silicon is evolved as the tetrafluoride. After decomposition is complete, the excess hydrofluoric acid is driven off by evaporation with sulfuric acid or perchloric acid. Complete removal is often essential to the success of an analysis because fluoride ion reacts with several cations to form extraordinarily stable complexes that interfere with the determination of the cations. For example, precipitation of aluminum (as AI2O3 XH2O) with ammonia is incomplete if fluoride is present even in small amounts. Frequently, it is so difficult and time-consuming to remove the last traces of fluoride ion from a sample that the attractive features of hydrofluoric acid as a solvent are negated. 

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