INDUSTRIAL A N D ENGINEERING CHEMISTRY PUBLISHED BY THE A M E R I C A N C H E M I C A L SOCIETY W A L T E R J. M U R P H Y ,
EDITOR
Determination of Nitrogen by Combustion Improved Dumas Apparatus and Recycle Procedure HARRY GONICK', D. D. TUNNICLIFF, E. D. PETERS, LOUIS LYKKEN,
AND
VICTOR ZAHN
Shell Development Company, Emeryville, Calif.
Improved apparatus and procedure are described for the determination of nitrogen b y combustion according to. the method of Dumas. The improvements permit a rapid, highly accurate analysis of organic materials, particularly petroleum products, containing 0.01 to 50% nitrogen, regardless of whether the sample is a gas, liquid, or solid. The possible incomplete combustion of methane formed b y thermal cracking is overcome by recycling a mixture of the combustion products and pure oxygen through the combustion tube until a constant residual volume is obtained. A n y organic residue remaining from the pyrolysis of the sample is completely oxidized by passing oxygen over the hot residue. The apparatus contains no rubber connections, allows adequate control of the combustion, and provides for accurate determination of the small amounts of nitrogen introduced b y the carbon dioxide and oxygen. Means are described for preparetion of essentially nitrogen-free carbon dioxide. A skilled operator can make as many as eight determinations on liquid or solid samples or fifteen determinations on gas samples in an 8-hour day.
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11b: conventioriul Dumas combustion method for the determination of nitrogen in organic cornpounds generally gives satisi'uctory results on pure nitrogen-containing compounds but often gives high results when applied to the analysis of petroleum pruducts coritaiuirig little or no nitrogen. In the analysis of these products a coiisiderable quantity of methane may be formed by tliermal cracking of the sample as the vapors pass over the hut copper oxide. Sirice meth:tne is not readily oxidized by hot copper oxide at the temperatures usually employed, it may be wllected with the nitrogen, giving high results. With the coil\ eritiorial ~riethod,this error can be avoided only by decomposiiig the sarriple very slowly; iri the present instance, the sample is decomposed rapidly, making certain only that all the nitrogen compounds are decomposed. The combustion products after rlie removal of the carbon dioxide are mixed with nitrogen-free eii and the mixture is recycled through the hot combustion lulJt9. This process is repeated until a constant volume is obt:iiiied. The excess oxygen is removed by reduced copper in the hut combustion tube. Certain nitrogen compounds have been encountered that give low and erratic values by the conventional Dumas method because of the retention of nitrogen in the carbonaceous residue from the sample. This difficulty is overcome completely by introducing oxygen into the combustion tube to oxidize such residues and to reoxidize the combustion tube filling. This procedure avoids the necessity of any special treatment of the combustion tube bet.ween analyses, permitting consecutive analyses without interruption. 1
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The combustion tube filling is prepared from rolls of 20- and 40mesh copper gauze. The main filling consists of the usual section of copper oxide followed by a section of reduced copper. However, an additional section of copper oxide has been added after the reduced copper to reoxidize any hydrogen and carbon monoxide that may be formed by the interaction of water, carbon dioxide, and reduced copper. A short section of silver gauze is placed at the front end of the combustion tube to prevent darkening of the front portion of the tube which otherwise would result from the combustion of halogen-containing samples. Thi. darkening is undesirable because it prevents the operator from observing the sample. The carbon dioxide required for the analysis is prepared in a state of high purity from solid carbon dioxide. Pure oxygen is obtained by electrolysis in a high-capacity cell of special design. .4lthough neither of these gases should contain more than 0.02% nitrogen, the volume of oxygen and carbon dioxide used during the analysis is measured and an appropriate blank correction is applied to the final volume. The apparatus is also designed to permit the determination of nitrogen in gases. The gas is introduced into the azotometer and then measured in the buret. The gas is passed through the hot combustion tube, the residue mixed with oxygen, and recycled until a constant volume is obtained exactly as in the case of the cornbustion products of other samples. Since the gas is first, passed through the azotometer and then measured over alkaline brine, the analysis is conducted on a carbon dioxide-free basis. Volatile samples are handled in a special sample tube in w1iic.h the sample is confined between paraffin plugs. The sariiplc IS liberated at the proper time by the application of heat. APPARATUS
The gas manipulation and measurement appat dtus i q shown in Figure 1, the combustion tube and furnace assembly in Figure 2, and the constructional details of the componen~prtrts 111 Figure 3.
COMBUSTION TUBE. The combustion tube is constructed of Corning KO. 172 Pyrex 13 mm. in insido diameter, 17 nim. in outside diameter, and 105 cm. in length. External connecztioiis at the ends of the combustion tube arz made with 3/ls-inch copper tubing by means of special metal fittings which are easily attached to make a as-tight joint (refer to details in Figure 3). The metal fitting on &e front end of the combustion tube has a removable cap to permit introduction of the sample. The rear fitting has provision for removing any water condensed at the end of the tube. The combustion tube filling consists of a short roll of 20-mesh silver gauze, two rolls of 20-mesh and one roll of 40-mesh copper gauze; the length and position of these rolls of gauze are given in Figure 3. Before rolling, the copper gauze is annealed by heating to redness in a flamc froni a Bunsen burner. Loose rolling should be avoided t o prevent the possibility of insufficient contact with the gas stream. A hollow glass plug is placed in the primary heater section t o prevent diffusion of sample vapors into the cold end of the combustion tube. This plug is constructed from a 4-t o 5-cm. section of 11.5- to 12.5-mrn. outside diameter Pyrex tubing
Present address, Shell Oil Company, he., Mutines, Calif.
677
screen front, is placed over the exposed part of the combustion tube containing the sample t o protect the operator from harm in the event of an explosion. "
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Enough 'clean &er&y"k added t o the azotometer t o fill the capillary portion of alms tube between i t End stopcock E (Figure 3).
just f i r enough from the end so chat mercury williot riseabove
ported in a Dosition so that the h u i d level is 2.5 to 75 cm. (I to
are made of Pyrex and thcmaduatid portion is 500 to 520 mm.
5 5 - d buret is graduated in O.Ol-ml.-divisions from 0.0 tz 0.5 ml.
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t o the hoitom of the burets. Figure 1.
Gas Manipulation and Measurement Apparatus =~~~~~ ~
a t operating temperature. T& plug is iFovidedi
