COMPOSITION OF ORGANIC COMPOUNDS 33 



In practice, the combustion tube is filled, as described, with coarse copper 

 oxide, which has been heated to redness in a copper basin and allowed to 

 cool. The plugs and the roll are made of copper gauze which is rolled round 

 a piece of copper wire and heated in a blowpipe flame to oxidise the metallic 

 copper and burn away any organic matter. A space is left for the boat. 

 The tube is heated in the furnace at a low red heat and a current of dry clean 

 air or oxygen is passed through the tube from the gasometer. The carbon 

 dioxide and water present in the tube and on the copper oxide are thus 

 removed and the copper is completely oxidised to copper oxide. The heating 

 of that portion of the combustion tube into which the boat is to be placed is 

 discontinued so that this portion cools to room temperature whilst the rest of 

 the tube is kept at a red heat. The absorption tubes are filled and weighed. 

 About '2 gm. of substance is exactly weighed out into the boat which has been 

 heated and cooled in a desiccator. When the end of the combustion tube is 

 cool the absorption tubes are attached, that for water next to the tube. From 

 the other end the roll is removed with a hooked copper wire, the boat quickly 

 introduced and the roll replaced. The tube is closed and the pure air or 

 oxygen current passed through at a rate of about 3 bubbles every two 

 seconds. The roll is heated commencing at the end farthest from the boat 

 and the heating is gradually extended from this point towards the boat and 

 the coarse copper oxide until the substance has burnt away and the whole 

 tube is heated from end to end. The air or oxygen current is continued for 

 about half an hour after the combustion is finished so as to drive out the water 

 and carbon dioxide. Any water which condenses on the end of the com- 

 bustion tube is driven into the absorption tubes by means of a small flame, 

 or hot brick, held under the end of the combustion tube. When the oxida- 

 tion is completed, the absorption tubes are removed, allowed to cool for -J to 

 i hour and weighed. 



This method of analysis requires some modification if elements other than 

 hydrogen or oxygen are present in the substance. 



(a) Halogens. On combustion, the halogen in an organic compound is 

 evolved as hydrogen halide, or as halogen. To prevent its entry into the ab- 

 sorption tubes it is combined with silver as silver halide. This is effected by 

 putting at the end of the combustion tube a roll of silver gauze. 



(b) Nitrogen. Oxides of nitrogen may be evolved when organic nitrogen- 

 ous compounds are analysed. A roll of metallic copper gauze, prepared by 

 heating a roll in a blowpipe flame and dropping it into a few c.c. of methyl 

 alcohol contained in a test tube (held in a duster) and drying at 100, is intro- 

 duced at the end of the combustion tube. Any oxides of nitrogen are thus 

 reduced to nitrogen and prevented from being absorbed by the potash. 



(f) Sulphur and Phosphorus. To prevent hydrogen sulphide or hydrogen 

 phosphide being formed, the oxidation of the organic compound is effected 

 with lead chromate instead of copper oxide and the substance is mixed with 

 it instead of being placed in the boat. Lead chromate may replace the 

 irse copper oxide entirely or about half of it. 



Organic phosphorus compounds are very difficult to oxidise completely 

 id frequently give results for carbon which are too low by about -5 to i per 

 it. 



