23 



sand or pulverized glass added, to prevent the formation of a dense clot, 

 the liquid and precipitate are to be heated to about 90 C , filtered, and 

 the precipitate washed thoroughly on the filter with water at about the 

 same temperature. This third filtration may be carried out on the 

 same filter already used for the second, but as a general rule it will be 

 found better to use a new filter, thus avoiding possible delay due to 

 partial drying of the previously used one and subsequent clogging of 

 its pores. 



Assuming now that nitrogen is present in the sample under exami- 

 nation only in the two forms of proteids and simpler amidic compounds,, 

 the three (or two) filters used and their contents are to be submitted 

 to the Gunning-Kjeldahl process for the determination of proteid nitro- 

 gen. By subtraction of this from the total nitrogen previously deter- 

 mined the amount of this element present in the simpler amidic 

 compounds will be obtained. 



In cases involving the presence of ammonia or its salts, nitrates, or 

 alkaloids, the nitrogen occurring in these forms must, of course, also be 

 deducted from the total nitrogen before recording the residue as nitro- 

 gen of the simpler amids and amido-acids. In like manner a separa- 

 tion of lecithin, when present, may be effected by the use of ether as a 

 solvent, 1 a determination of phosphorus made the basis of a calcula- 

 tion of lecithin nitrogen, and this in turn subtracted from the total 

 nitrogen found. 



When peptones are present, these are to be precipitated by tannic 

 acid from the solution which has been acidified with acetic acid and 

 heated. After this has completely cooled down, and before adding 

 phospho tungstic acid, the filter on which the tannic- acid precipitate 

 is collected and washed with cold water is, with its contents, to be 

 submitted to the modified Kjeldahl process, and the nitrogen obtained 

 counted as part of the proteid nitrogen. 



The several filters and precipitates from which the proteid nitrogen 

 is obtained may either be treated separately by the Kjeldahl process 

 or, preferably, may all be brought together and submitted to this pro- 

 cess in a single operation. If the latter course be pursued, it will be 

 well to introduce each filter with its contents as soon as washed into 

 the strong sulphuric acid, so as to avoid any possible decomposition 

 and loss of nitrogen as ammonia until all the filters have been brought 

 together and the moist combustion process can be proceeded with. 



When proteoses are present it may be well to make a check determi- 

 nation of their amount by saturation of the aqueous solution, after 

 acidification with acetic, acid, heating and subsequent cooling, with zinc 

 sulphate, 2 and determining nitrogen in the precipitate so formed by 

 means of the Kjeldahl process. 



1 Extraction with the ether alone will remove only a portion of the lecithins. A 

 mixture of ether and alcohol should follow the ether in order to secure a complete 

 extraction. 



-As suggested by Boemer, Zeitschrift fur annlyt. Chemio, 1895, 34,562. 



