Determination of impurities in silicon carbide by ICP-AES after coprecipitation with lanthanum hydroxide

Impurities (15 elements) in silicon carbide powder were determined by the following procedure : Fuse silicon carbide (0.5 g) with a mixture of sodium nitrate (2 g) and sodium carbonate (2 g) in a platinum crucible at 1000 deg C for 5 min after preheating at 450 deg C for longer than 6 h. Dissolve the melt with water and make the solution acidic with hydrochloric acid. Then, add ethanol and boil it to reduce chromium(VI) to (III). After adding lanthanum ion (20 mg) and phenolphthalein indicator, make the solution alkaline by the addition of 2.5 M sodium hydroxide solution with an excess of 4.5 ml until the solution changing to red to redissolve the precipitate of silicic acid and separate the precipitate of lanthanum hydroxide by filtration. After ashing the precipitate on the filter paper in the platinum crucible, remove the silicon (about 27 mg as SiO2) as fluorosilicate. Dissolve the residue in the crucible with hydrochloric acid, make the solution to 25 ml with water, and determine the elements in the solution by ICP-AES. Without the preheating procedure, the mixture of silicon carbide and fusion reagents would catch fire or bubble over the platinum crucible at about 500 deg C. In addition, iron, copper etc. in silicon carbide would alloy with the platinum crucible. This method is not simple or rapid, but nevertheless is sensitive and accurate for the determination of trace elements in silicon carbide. (author)