QUANTITATIVE ANALYSIS 479 cone in place, is carefully introduced under the combustion tube so that liquid is not driven out of the pressure gauge by the sudden thermal expansion. The heating is started about 2 cm. to the right of the asbestos square K; care should be taken that the burner is not left at this end of the tube long enough for the bung J to become appreciably warmed, otherwise errors will arise due to the evolution of volatile carbon-containing compounds. The Bunsen should be moved slowly along the combustion tube so that each part of the tube is heated to a bright red heat. Particular care should be taken as the burner approaches the boat. When burning the majority of organic compounds, an attempt should be made to drive the substance out of the boat, by heating, in an only partially combusted state, the greater part of the combustion taking place catalytically on the piece of silver gauze just protruding from the furnace mouth. Exceptions to this are non-volatile sub-stances such as glucose which may largely be oxidised in the boat. The chief danger and main source of error in a combustion is that of
This is the end of the preview. Sign up
access the rest of the document.