Silicon loss with hydrofluoric acid

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). 

Silicon may be present in various forms in some preparations, e.g. as tri-silicate in antacids, polydimethyl-siloxane or silicone oil (see also section VII, ref. 77). The sample may be evaporated to dryness and the residue fused with sodium bicarbonate or another fusion agent, or taken up in hydrofluoric acid provided strong heating and loss of silicon is avoided. A nitrous oxide/acetylene flame must be used. Chromium may be found in disinfectants and antiseptics. Unless dilution of the sample is possible the use of the injection-cup technique (see section II.A) may be preferable as otherwise large amounts of corrosive salts such as sodium hypochlorite will be aspirated. If iron is also present it may be necessary to use a nitrous oxide/ acetylene flame. Arsenic in arsenamide and lead arsenate preparations can be determined by boiling the sample in 5% nitric acid and aspiration of the sample [111]. Better sensitivity would be obtained using hydride generation (see section II.A). 

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