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Chemical Properties of Graphite





Combustion

When artificial graphite is heated in oxygen it begins to form carbon dioxide at 570° C., and the rate of formation increases up to 690° C., when inflammation occurs. With natural graphite the temperatures at which these changes take place are variable, but they are considerably below the corresponding temperatures for diamond (q.v.); and this difference constitutes one of the distinguishing tests between the two forms of carbon.


Action of Oxidising Agents

Graphite differs much from diamond in its reactivity towards liquid oxidising agents. Cold, concentrated nitric acid has no action on graphite; but some specimens, when strongly heated after being moistened with the acid, intumesce, i.e. swell up, because of the generation of gas within them. The gas consists of carbon dioxide and oxides of nitrogen, and is probably produced by the action of the acid on particles of amorphous carbon contained within the graphite. Luzi distinguishes between different kinds of graphite by this reaction of intumescence. Specimens that intumesce are known as "graphite," those which do not as "graphitite." The variety from Ceylon is "graphite," that from Borrowdale "graphitite." The artificial product is generally "graphitite." When molten cast-iron containing carbon is cooled by water the external layers contain graphitite, the interior graphite; intumescent graphite is, moreover, produced by dissolving carbon in molten platinum, and remains when the platinum is removed by aqua regia.

Graphite is slowly oxidised when heated with a mixture of nitric and sulphuric acids, or potassium chlorate with sulphuric or nitric acid. These reactions, which were discovered by Brodie, may be used for the purification of graphite or its conversion into "graphitic acid." If graphite is heated with a mixture of 1 part of nitric acid and 4 parts of sulphuric acid, or 14 parts of graphite are heated with 1 part of potassium chlorate and 28 parts of sulphuric acid, the graphite is partially oxidised and turns violet, but regains its black colour when washed with water. When strongly heated the mass swells up, loses gases, and yields a residue of pure graphite of density 2.25.

Mixtures of concentrated sulphuric acid and potassium permanganate or chromic acid may be substituted for Brodie's mixture.

By more prolonged treatment with similar reagents graphite is converted into graphitic acid.

For this purpose one part of graphite and 3 parts of potassium chlorate are mixed with nitric acid and heated for several days at 60° C., and after washing the process is repeated four or five times. Or the powdered graphite may be mixed with sulphuric and nitric acids at the ordinary temperature and a large excess of potassium chlorate be added. A green product results, which is changed into graphitic acid by treatment with potassium permanganate and sulphuric acid.

Graphitic acid is yellow, and appears to be crystalline because it preserves the form of the original graphite crystals. Its composition corresponds to the formula C11H405 or C11H406, but it is really an amorphous mixture of several compounds of high molecular weight.3 When the acid is heated it decomposes explosively, evolving carbon monoxide, carbon dioxide, and water, and leaves a black, charcoal-like residue called pyrographitic acid, which is converted by potassium chlorate and nitric acid into mellitic acid, C6(COOH)6.

Graphitic acid is reduced by stannous chloride to a black "pseudomorph" which resembles graphite in appearance, and may be oxidised again to graphitic acid.

The behaviour of graphite towards oxidising agents, by which graphitic acid is produced, distinguishes this substance from diamond and from amorphous carbon; for diamond is not acted upon by these reagents, and amorphous carbon is dissolved by them with the formation of humic and finally mellitic acid. According to Charpy, however, some forms of carbon hitherto regarded as amorphous, such as carbon from steel and the product obtained by heating graphitic acid, yield graphitic acid on oxidation. It therefore appears unsatisfactory to define graphite with reference to the action of oxidising agents.
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