Ashless filter paper ignition

Unless otherwise directed, accurately weigh about 3 g of the sample in a tared crucible, ignite it at a low temperature (about 550°), not to exceed a very dull redness, until it is free from carbon, cool it in a desiccator, and weigh. If a carbon-free ash is not obtained, wet the charred mass with hot water, collect the insoluble residue on an ashless filter paper, and ignite the residue and filter paper until the ash is white or nearly so. Finally, add the filtrate, evaporate it to dryness, and heat the whole to a dull redness. If a carbon-free ash is still not obtained, cool the crucible, add 15 mL of ethanol, break up the ash with a glass rod, then burn off the ethanol, again heat the whole to a dull redness, cool it in a desiccator, and weigh. 

An alternative method for carrying out this test is to soak some ashless filter paper in aluminium salt solution, add a drop or two of cobalt nitrate solution and then to ignite the filter paper in a crucible the residue is coloured blue. 

An alternative method is to soak a piece of ashless filter paper in the zinc salt solution, add 1 drop cobalt nitrate solution and to ignite in a crucible or in a coil of platinum wire. The residue is coloured green. 

Ashless filter paper is generally used for quantitative work in which the paper is ignited away and leaves a precipitate suitable for weighing (see Chapter 10). There are various grades of filter papers for different types of precipitates. These are listed in Table 2.6 for Whatman (www.whatman.plc.uk > and for Schliecher and Schuell twww.s-und-s.de/english-index.htmD papers. 

Transfer the residue from the beaker to an ashless filter and examine for filth. If there is appreciable quantity of residue, place filter paper in a tared crucible, ignite, and determine sand and soil. Dry the spice in the Buchner funnel thoroughly, scrape the fine material from the paper and transfer to a 1-litre Wildman trap flask. Add about 150 ml of water, heat to boiling and simmer for fifteen minutes with stirring wash down the inside of the flask with water and cool to below 20°. Add 25 ml of petrol, mix thoroughly and allow to stand for five minutes, then fill the flask with water and allow to stand for thirty minutes. Stir every five minutes, trap off and filter. Add to the flask about 15 ml of petrol and mix well trap off and filter again after fifteen minutes. If the second extraction yields an appreciable quantity of filth decant most of the liquid from the flask, add 15 ml of petrol and make a third extraction. 

The last traces of precipitate that cling to the inside of the beaker are dislodged with a rubber policeman, which is a small section of rubber tubing that has been crimped on one end. The open end of the tubing is fitted onto the end of a stirring rod and is wetted with wash liquid before use. Any solid collected with it is combined with the main portion on the filter. Small pieces of ashless paper can be used to wipe the last traces of hydrous oxide precipitates from the wall of the beaker these papers are ignited along with the paper that holds the bulk of the precipitate. 

Total sulfur is determined by combustion in a bomb calorimeter (ASTM D-129, IP 61) and is often carried out with the determination of calorific value. The contents of the bomb are washed with distilled water into a beaker after which hydrochloric acid is added and the solution is raised to boiling point. Barium chloride is added drop by drop to the boiling solution to precipitate the sulfuric acid as granular barium sulfate. After cooling, and standing for 24 h, the precipitate is filtered off on an ashless paper, washed, ignited, and weighed as barium sulfate. 

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