THE CITRATE METHOD 105 



before filtering. In the practice of this method the presence of 

 aluminum oxid not exceeding 0.6 gram, or ferric oxid not ex- 

 ceeding o.u gram, may be present, but not in greater quantities 

 than these. Calcium oxid should also not be present in quantities 

 exceeding 0.03 gram. 



Detailed data are given by Jorgensen in connection with the 

 general determinations of all forms of phosphoric acid, but they 

 do not differ sufficiently from those already cited to warrant 

 their insertion in full in this manual. The student who desires to 

 study in complete detail this latest contribution to the methods 

 of determining phosphoric acid should consult the original article. 74 



97. The Citrate Method Applied to Samples with Small Content 

 of Phosphoric Acid. It is well established that the citrate method 

 does not give satisfactory results when applied to samples con- 

 taining small percentages of phosphoric acid, especially when 

 these are of an organic nature, as, for instance, cottonseed cake- 

 meal. An attempt has been made to remedy this defect in the 

 process so as to render the use of the method possible even in 

 such cases. 75 Satisfactory results have been obtained by adding 

 to the solution of the cake-meal a definite volume of a phosphate 

 solution of known strength. Solutions of ordinary mineral phos- 

 phates are preferred for this purpose. The following example will 

 show the application of the modified method : 



In a sample of cake-meal (cottonseed cake and castor pomace) 

 the content of phosphoric acid obtained by the molybdate method 

 was 2.52 per cent. 



Determined directly by the citrate method, the following data 

 were obtained : 



Allowing to stand 30 hours after adding magnesia mixture, 

 1.08 and 1.53 per cent, in duplicates. 



Allowing to stand 72 hours after adding magnesia mixture, 

 2.17 and 2.30 per cent, in duplicates. 



In each case 50 cubic centimeters of the solution were used, 

 representing half a gram of the sample. 



In another series of determinations 25 cubic centimeters 



74 Zeitschrift fur analytische Cheniie, 1906, 45 : 273. 



75 Journal of the American Chemical Society, 1895, 17 : 513. 



