38 



precipitate to settle and decant the clear fluid on a filter. Then wash 

 with 50 c. c. of boiling water, stir, allow to settle, and decant as before. 

 As some liine and magnesia may be carried down by the precipitation 

 of the hydrates, dissolve it in the beaker, with as little dilute hydro- 

 chloric acid as possible, reprecipitate by adding some ammonia and 

 boiling as before. Wash by decantation, three or four times, using 40 

 to 50 c. c. of boiling water each time. Finally transfer the precipitate 

 to the filter, with boiling water, and wash with the same, until a few 

 drops of the wash water, acidulated with nitric acid, do not show any 

 trace of chlorides when tested with argentic nitrate. 



Dry the filter and contents in an air-bath at 110 C., and when per- 

 fectly dry ignite it in a weighed platinum crucible, applying the heat 

 gently at first, until the filter paper is consumed, and then more in- 

 tensely, cool and weigh. This weight, after deducting that of the cruci- 

 ble and filter ash, will be that of the aluminium and ferric oxides with 

 the phosphoric acid contained in the 4 grams of air-dried soil. (Fres. 

 105, a, 113, I, a.) 



The weight of the phosphoric acid, determined by the method given 

 further on, is deducted from this weight, thus leaving the weight of the 

 two oxides. 



DETERMINATION OF THE FERRIC OXIDE BY TITRATION WITH POTAS- 

 SIUM PERMANGANATE. 



The ignited and weighed precipitate of the aluminium and ferric ox- 

 ides carrying the phosphoric acid is transferred to a small beaker, and 

 the crucible carefully washed with water to remove any adhering par- 

 ticles. Concentrated sulphuric acid is then cautiously added, about 

 10 c. c. is sufficient, and digested on the steam-bath until all the sub- 

 stance is in complete solution/ The solution is allowed to become 

 cold, and is then diluted with about 150 c. c. of water, and is passed 

 through a small filter into a 200 c. c. cylinder, and washed slightly. 

 The solution is made up to the mark and divided into two equal por- 

 tions of 100 c. c. each. 



Each of these two portions is transferred to a reducing bottle. Place 

 in each bottle a piece of amalgamated zinc, and a piece of platinum foil 

 an inch wide and 4 or 5 inches long, add 2 c. c. concentrated sulphuric 

 acid, fill with water to the shoulders, cover with watch-glasses and allow 

 to stand twenty-four hours. A strong current of gas should be induced 

 by contact between the zinc and the platinum. The zinc used must be 

 amalgamated, as it usually contains iron, which, in dissolving, it will 

 impart to the solution if this is not done. It has been found by experi- 

 ment that amalgamated zinc will not give up the iron it contains to the 

 solution until nearly if not quite , all the zinc is dissolved. Erlen- 

 meyer's flasks, holding about 150 c. c., will be found very convenient 

 bottles for reducing the ferric oxide. The platinum foil should be care- 



