408 EXPERIMENT STATION RECORD. 



potassium by the platinum chlorid method are discussed, and the author's views 

 as to the best means of avoiding them are stated. A summary of worli on this 

 subject is given." 



The determination of potash and soda in feldspar, G. Dillner {Jern- 

 Koiitorets Ann., 62 (1907), p. I'/J : ahs. in Chcm. Ztg., 31 (1907), No. 5',. Repert. 

 No. Ji9, p. 324). — A slightly modified Lawrence Smith method is described and 

 recommended. 



Contribution to the determination of phosphoric acid volumetrically, 

 W. D. Richardson {Jour. Amer. Cliem. Soc, 29 (1907), Nu. 9, pp. 131.'t, 1315).— 

 In view of the inaccuracy of results obtained by the Pemberton volumetric 

 method in the case of superphosphates, the author undertook some investiga- 

 tions which indicated that the error in this case was due to the presence of 

 sulphuric acid. He therefore proposes a method in which the sulpliuric acid is 

 precipitated as barium sulphate before the phosphoric acid is determined. 



The determination of citric-acid soluble phosphoric acid in Thomas slag, 

 P. Wagner, R. Kunze, and W. Si.mmermacher (Landic. I'ers. .S7af., 66 (1907), 

 No. Jt-5, pp. 257-28.',; abs. in Chem. Ztg., 31 (1907), No. 58, Repert. No. 53, p. 

 351; Jour. Chem. Soc. [London], 92 (1907), No. 537, II, p. 577).— This article 

 describes and discusses 5 methods which are believed to give reliable results 

 when carried out strictly according to the directions given in the article. These 

 are the German official method (separating silica by evaporation with hydro- 

 chloric acid), the Darmstadt method (separating silica by heating with mag- 

 nesium citrate mixture), the Naumann method, the molybdic method, and the 

 Lorenz method. 



The results obtained by direct precipitation agree with those obtained by the 

 methods named only when the slag extract gives no. or only slight, reaction by 

 the Kellner test. 



Of the 5 methods the molybdic method requires*"greatest care. Then follows 

 the official method (with se])aration of silicic acid by means of hydrochloric 

 acid), Xauniann's method, Lorenz's method, and the Dai'uistadt method. 



The I>orenz method is not suited to routine laboratory work. The results 

 are accurate, but tlie method requires much attention to details. The unavoid- 

 able nitric acid fumes are disagreeable and the weighing of the hygroscopic 

 precipitate must be done with care. 



Although the official method with direct precipitation in extracts giving no re- 

 action by the Kellner test yields reliable results, it is recommended that in all 

 cases silicic acid be precipitated by beating with citrate of magnesia mixture 

 and the phosphoric acid precipitated by addition of ammonia to the filtrate. 

 This method, which is designated the Darmstadt method, is as follows : 



To 1(K) cc. of the citric-acid extract of the slag in a 200 cc. flask add 50 cc. 

 of a citrate of magnesia mixture prepared by dissolving 2 kg. of citric acid and 

 400 gm. of auuuonium chlorid in 2 liters of water, adding .5 liters rf 20 per cent 

 annnonia, cooling, adding 7\oO gm. of magnesia chlorid, and making the volume 

 to 10 liters with water. Heat gently, for about 15 minutes, over a small Buusen 

 burner flame, until the silicic acid separates out. Swing the flask a few times 

 in order to collect the silicic acid in a mass, and heat over a strong flame to the 

 boiling point. Cool, add 25 cc. of hydrochloric acid of 1.124 sp. gr., and allow 

 to stand for one-half hour. Swing the flask twice during this time. Fill the 

 flask with water, close with a rubber stopper, and shake repeatedly and vigor- 

 ously until the silicic acid floccules ai-e flnely divided. Filter, and to 100 cc. of 

 the filtrate (corresponding to 0.5 gm. substance) add 50 cc. of 10 per cent am- 

 monia and .stir for one-half hour in a Stutzer apparatus. 



The method is recommended as accurate and well adapted to routine lab- 

 oratorj' work. 



