COMPOSITION OF ORGANIC COMPOUNDS. 



A pure carbon compound which has been prepared is analysed, 

 i.e. the elements besides carbon contained in it and their amounts are 

 determined. 



Carbon compounds may contain the elements hydrogen, oxygen, 

 nitrogen, halogens, sulphur, phosphorus, etc., either singly or collectively. 

 Usually all the possible elements are not present, but the proteins 

 contain carbon, hydrogen, oxygen, nitrogen and sulphur ; some con- 

 tain also phosphorus and a few contain halogens. Proteins belong to 

 the most complex of the organic compounds. The method of analysis 

 of the compound is varied according to the elements which are present. 

 The elementary composition, or detection of the elements, precedes the 

 quantitative composition. Since all organic compounds contain carbon 

 and most of them contain hydrogen it is not absolutely essential that 

 the presence of these elements should be ascertained. Their quantita- 

 tive analysis is carried out simultaneously under the same conditions. 



A ELEMENTARY COMPOSITION. 



DETECTION OF THE ELEMENTS. 



i. Carbon and Hydrogen. 



(a) A small portion of the substance (e.g. cane sugar) is gently 

 heated in a test tube or on platinum foil. It melts and chars. The 

 charring denotes the presence of carbon. There is a condensation of 

 water on the sides of the tube where it is cool ; this denotes the 

 presence of hydrogen. 



(b) About 5 grms. of finely powdered cupric oxide are dried 

 thoroughly by heating in a small ^crucible. Whilst still warm 1 it is 

 mixed with a little of the substance (e.g. oxalic acid) and the mixture 

 is introduced into a hard glass tube. The end is closed with a cork 

 through which passes a glass tube, bent at right angles. This end 

 is dipped into a little baryta water contained in a small beaker or test 

 tube. On heating the mixture in the hard glass tube, water will 

 condense on the cooler parts of the tube presence of hydrogen ; and 

 the baryta water will become turbid owing to the formation of barium 

 carbonate presence of carbon. 



1 As cupric oxide takes up water on cooling it must be used warm, otherwise it must be 

 allowed to cool in a desiccator over sulphuric acid. 



28 



