Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/558

Rh 546 CHEMISTRY [ORGANIC. combustion of (actual size). liquids presence of cupric oxide is oxidized to S0 2, and this gas if allowed to pass into the potash bulbs would be absorbed. These sources of error are avoided by the employment of lead chromate, because non-volatile lead chloride or bro mide are formed, and sulphur is oxidized to SO 3, which is fixed as PbS0 4. When substances rich in nitrogen are burnt with cupric oxide, nitrogen dioxide is likely to be formed, and this on meeting the air and potash in the bulbs is absorbed, and increases the weight of the C0 2 apparatus. To obviate this source of error it is necessary when dealing with nitrogenous bodies to plug the end of the combustion tube with a roll of freshly reduced copper gauze. This being kept at a bright red heat during the entire operation decomposes the oxides of nitrogen, and retaining the oxygen, allows only nitrogen to escape. 1 When the substance to be analyzed is a liquid, a known weight is sealed up in a small glass bulb (fig. 6). After sealing and weighing (the weight of the empty bulb having been previously determined) the neck of the bulb is broken off, and Fia 6 Glass Bulb for the the broken portion, together with the bulb itself, introduced into the combustion tube, which is afterwards filled up with cupric oxide, &amp;lt;fec., in the usual manner. In burning liquids the anterior portion of the combustion tube is, as with solids, first heated to redness ; the portion containing the bulb is then gradually warmed so as to expel the liquid, which is thus made to distil slowly over the red-hot cupric oxide which effects its combustion. The analysis of gaseous organic compounds is effected in eudiometers, or in special apparatus, of which several forms have been devised. (See Bunsen s Gasometry, Sutton s Volumetric Analysis, Thorpe s Quantitative Analysis, &c.) Determination of Nitrogen. Two methods are in use for determining this element. Will and Varrentrapp s method depends upon the fact that many nitrogenous bodies when heated with caustic alkalies yield their nitro gen in the form of ammonia. A known weight of the substance is intimately mixed with soda-lime, 2 and then heated in a combustion tube, the ammonia being absorbed by dilute hydrochloric acid contained in a glass apparatus attached to the end of the tube. (See fig. 7.) In conducting an operation of this kind some soda-lime is first introduced C into the tube ; the substance is then mixed intimately with more soda- lime in a mortar, and the mixture transferred to the tube ; the mortar Fia. 7 Determination of Nitrogen by Will and Varrentrapp s method. A B, combustion tube ; a b, soda-lime ; 6 c, mixture ; c d, rinsings ; d e, soda-lime ; /, asbestos plug; C, acid bulbs. is next rinsed out with more soda-lime, and the rinsings are added to the contents of the tube, which is finally filled up with pure soda-lime. When charged the tube is tapped so as to secure a free passage for the escaping gas 1 Although the principles upon which the determination of carbon and hydrogen in organic bodies depends remain unaltered, the process has received several modifications which it is unnecessary to describe in detail. Thus in some laboratories, instead of the potash bulbs, a tube filled with &quot; soda-lime&quot; is employed, and the substance, instead of being mixed with the CuO in the tube, is placed in a small boat of porcelain or platinum, and a slow stream of oxygen kept going during the whole process. Other oxidizers have been also proposed instead of cupric oxide, such as potassium perchlorate or a mixture of sulphuric acid and silver iodate, in which latter process the carbon is determined directly from the amount of C0 2 produced, and the hydrogen indirectly from the amount of oxygen consumed minus the quantity contained in the CO S. 2 Soda-lime is prepared by slaking quicklime with a strong solution of caustic soda, and then heating till thoroughly dry, (as in the combustion for the determination of C and H), and after it is laid in the trough of the gas furnace, the acid bulb is attached to it by means of a tightly fitting cork. The combustion is performed in precisely the same manner as in the process of burning with cupric oxide ; when it is completed the tail of the tube is broken off, and air drawn through the apparatus so as to draw the last traces of ammonia into the acid. If the substance contains much nitrogen it is advisable to mix it with some com pound which gives off a large quantity of gas when heated in the tube. Sugar or starch are convenient substances for this purpose. This dilution of the ammonia prevents its too rapid absorption by the acid, and thus diminishes the risk of acid being drawn back into the hot tube. Having by the above process obtained the nitrogen in the form of ammonium chloride, its quantity is deter mined by precipitation as ammonio-platinic chloride, (NH 4 ) 2 PtCl,5, the analysis being performed in the same manner as in the quantitative determination of ammonium by means of platinic chloride. In calculating the results the requisite data are furnished by the facts that 223 2 parts of the double salt contain 14 of nitrogen, or 197 5 parts of platinum correspond to 28 of nitrogen. Thus, if w = weight of substance taken, W the weight of double salt obtained, P the weight of platinum, and N the weight W x 14 P x 28 ICON of nitrogen, we have N = also for 223-2 197-5 the percentage of nitrogen. Instead of determining the ammonia gravimetrically it may be absorbed by a known quantity of dilute sulphuric acid of standard strength, and the amount of acid neutra lized determined by adding litmus, and then a standard solution of soda till complete neutralization is effected as in ordinary acidimetrical determinations. In cases where nitrogen-containing bodies do not yield the whole of that element as ammonia on ignition with soda-lime, Dumas s process is employed. This process depends upon the fact that all nitrogenous substances yield their nitrogen in the free state when burnt with cupric oxide, and in presence of ignited metallic copper. The method is thus carried out. A combustion tube of suitable length is sealed off at one end, and about 2 or 3 inches of hydrogen sodium carbonate (NaHC0 3 ) introduced, after which a small quantity of cupric oxide is added. A known quantity of the sub stance, intimately mixed with cupric oxide, is next intro duced, and then a further quantity of pure cupric oxide, the remainder of the tube being finally filled up with 3 or 4 inches of freshly reduced copper gauze plugging. No special precautions need be taken to keep the cupric oxide dry. A gas passage having been secured through the tube in the usual manner, a delivery tube is tightly adapted to the open end of the combustion tube. The tube is placed in a gas furnace, and the end of the delivery tube plunged beneath the surface of mercury con tained in a mercurial trough. The first proceeding is to expel the air from the apparatus. This is effected by heating a portion of the hydrogen sodium carbonate in the pos terior part of the combustion tube till a bubble of the gas received into caustic potash solution is wholly absorbed. When the apparatus is entirely filled with carbon dioxide, a graduated receiver containing about ^ of its volume of caustic potash solution, the remainder being filled with mercury, is inverted over the end of the delivery tube as in ordinary cases of gas-receiving over the mercurial trough. The combustion is then proceeded with in the usual manner, the anterior portion of the tube containing the metallic copper being first heated to redness, and the heat gradually carried back till the combustion is completed, when more of the NaHC0 3 in the posterior part is heated