SEPARATION OF SILICIC ACID 211 



general meeting of the delegates of the German agricultural ex- 

 periment stations, at Cassel, in 1903, both for slags and tricalcium 

 phosphates, and for the general separation of phosphoric acid. 58 



Bottcher, in a later communication, expresses the opinion that 

 if the direct precipitation of the phosphoric acid be carried on 

 with all the promptitude which he has recommended, the pre- 

 vious separation of the silicic acid, when an excess has been in- 

 dicated by the Keliner test, is rarely necessary. 59 



The great point to be observed is that all the manipulations be 

 conducted without delay. The preliminary test by the Keliner 

 method is chiefly valuable in showing with what samples special 

 precautions are necessary. 



185. Separation of Silicic Acid in the Estimation of Phosphoric 

 Acid in Basic Slag, Bone Meal, Etc. 60 Attention is called by 

 Bottcher to the fact that after the Association of Agricultural 

 Experiment Stations of the German Empire had determined to 

 estimate the citric-acid-soluble phosphoric acid in basic slag by 

 the direct precipitation method in all cases where the preliminary 

 test by boiling with 50 cubic centimeters of ammoniacal citrate 

 solution did not show a high content of silicic acid, the opinion 

 has again come into consideration that the separation of the silicic 

 acid by evaporation with hydrochloric acid is necessary with all 

 basic slags because the direct precipitation sometimes gives re- 

 sults which are too high, even if the preliminary test shows no 

 especially high content of silicic acid, and the solutions very often 

 filter too slowly. Bottcher, however, affirms anew that the 

 conduct of the direct precipitation citrate method never leads to 

 any difficulties of filtration nor to any differences in the results. 

 As he pointed out in a former place, and as he has shown by 

 subsequent analyses, which are given, he has obtained absolutely 

 correct results with all normal basic slags, which with two per cent, 

 citric acid solution gave bright green solutions, even when the pre- 

 liminary treatment has shown a high content of silica. 61 In 



58 Die landwirtschaftlichen Versuchs-Stationen, 1904, 60 : 221. 



59 Chemiker-Zeitung, 1903, 27 : 247. 



Zeitschrift fur angewandte Chemie, 1904, 1 7 : 988. 



60 Chemiker- Zeitung, 1905, 29 : 1293. 



61 Chemiker-Zeitung, 1903, 27 : 247. 



