30 REPORT — 1877. 



by the Committee. This plan would cause the more perfect solution of the 

 precipitate, and probably yield somewhat higher results ; but the author's 

 experiments on pure potassium salts gave results sensibly below the truth. 

 As the value of the process is greatly limited by its uselessness in presence of 

 sodium compounds, the Committee did not think it necesssary to perform a 

 fresh series of experiments with more rigid adherence to Stolba's directions. 



Methods of determining Phosphoric Acid. 



With respect to the general method of procedure in the assay of com- 

 mercial phosphatic materials, the Committee has not thought it necessary to 

 make any original experiments, the published and collected evidence on the 

 subject being sufficient for the purpose. 



As the result of a very careful consideration of the subject, the Committee 

 make tho following recommendations and suggestions. In most eases theso 

 are quite free from novelty ; but as the evidence collected by the Committee, 

 and the results of many commercial analyses, show that the following con- 

 siderations and precautions are in many cases partly or wholly neglected, the 

 Committee is of opinion that the general adoption of the following sugges- 

 tions would tend greatly to diminish the number and extent of the dis- 

 crepancies common in determinations of phosphoric acid. 



Solution of the Manure. 



The Committee is of opinion that for dissolving the soluble phosphate 

 contained in a manufactured manure, cold water should invariably be em- 

 ployed. The water should be employed in successive small quantities, and 

 the treatment and digestion with tho solvent should not be extended over 

 more than two or three hours. Hot water should be wholly avoided, both 

 for the original extraction of the soluble matter and for washing the residue. 



The neglect of the above precautions may cause an error in either direc- 

 tion. The effect of employing hot water for dissolving the soluble phosphate 

 is shown by the fact that the cold aqueous extract of many superphosphates 

 yields a precipitate on boiling. On the other hand the di- and tricalcic 

 phosphates undergo change on boiling with water, with partial solution in 

 some cases. 



For the solution of the portion of the manure insoluble in water, or for 

 tho determination of the total phosphoric acid, hydrochloric acid is the most 

 suitable. In manures containing iron the addition of a few drops of nitric 

 acid is desirable, to ensure the complete peroxidation of any ferrous com- 

 pound which may be present. 



In manures containing silica the evaporation of the acid solution to 

 dryness should never be omitted. The neglect of this precaution causes the 

 precipitation of the silica at a subsequent stage, and is liable to cause a 

 serious error. Another advantage of the evaporation to dryness is the partial 

 elimination of any fluorine which may be present. 



In cases in which much organic matter is present, iron and aluminium cannot 

 be precipitated satisfactorily. In such cases the original sample or the 

 residue insoluble in water should be ignited with an alkaline oxidizing 

 mixture before treating it with acid. 



Fresenius, Neubauer, and Luck* havo recommended the employment of 

 dilute sulphuric acid for the extraction of the total phosphoric acid from a 

 manure. The advantage claimed for this modification is that the iron and 



* Zeitschrift, x. p. 103. 



