SOILS. 15 



constitutes the acid extract freed of soluble silica, aiid is made up to a definite 

 volume (250 or 500 cc) and designated as solution A. 



Combine the two filters and the main residue and after drying ignite, prefer- 

 ably in a small dish over a Bunsen flame for an hour or more, then complete by 

 igniting over a blast until it ceases to lose weight. Weigh as the insoluble 

 residue. 



(b) FERRIC AND ALUMINIC OXIDS AND PHOSPHATES, COLLECTIVELY. OFFICIAL. 



To an aliquot of solution A (50 or 100 cc, according to the probable amount 

 of iron present) add ammonium hydroxid, drop by drop, until the precipitate 

 formed requires several seconds to dissolve, thus leaving the solution but faintly 

 acid. Iloat nearly to the boiling point, add sufficient ammonium hydroxid 

 to precipitate all of the iron, alumina, etc. Allow the covered beaker to boil 

 for about one minute, remove, and if no ammonia is given off (detect by smell- 

 ing), more is added drop by drop until it can be detected. Do not allow tho 

 precipitate to settle, but stir and pour onto the filter. Wash immediately with 

 hot water, using a fine jet which is played around the edge of the precipitate, 

 thus cutting it free from the paper in order to produce rapid filtration. Wash 

 the precipitate several times and return it to the original beaker, dissolve 

 with a few drops of hydrochloric acid and warm. Reprecipitate the iron, alum- 

 ina, and phosphoric acid with ammonium hydroxid as above and wash until 

 free from chlorids. The filtrate is designated as Solution B. 



Dry the precipitate, remove it from the filter, and ignite over a Bunsen flame, 

 the filter being incinerated separately and added to the precipitate* Then ignite 

 to bright redness, cool in a desiccator and weigh as ferric oxid (Fe 2 O 3 ), alumina 

 (A1,O 3 ), and phosphorus pentoxid (P 2 O 5 ). Transfer this residue to a flask and 

 digest with several cubic centimeters of sulphuric acid (1 to 4), heating to 

 accelerate solution. When solution is complete reduce with zinc and estimate 

 ferric oxid with a standard solution of permanganate. 



In lieu of the above, evaporate 50 or 100 cc of solution A with the addition 

 of 10 cc of sulphuric acid until all hydrochloric acid is expelled, dilute with 

 water, reduce with zinc, and estimate ferric oxid with a standard solution of 

 permanganate. 



The ferric oxid, together with the phosphorus pentoxid (to be determined 

 later), subtracted from the collective weights of ferric oxid, alumina, and phos- 

 phorus pentoxid, gives alumina. 



(c) MANGANESE. OFFICIAL. 



Concentrate Solution B to about 50 cc, cool,and make alkaline with ammonium 

 hydroxid, add bromin water until the solution is colored, and heat to boiling in 

 a covered beaker; again cool, and repeat the addition of ammonium hydroxid, 

 bromin water and boiling. If manganese be present, slightly acidify the solu- 

 tion with acetic acid, immediately filter, and wash with hot water. Dry the 

 precipitate and ignite over a Bunsen flame and weigh as mangano-manganic 

 oxid (Mn 3 O 4 ). The filtrate from this, or if there is no precipitate the original 

 solution, becomes Solution C. 



(d) CALCIUM. OFFICIAL. 



Evaporate Solution C to about 50 cc, make slightly alkaline with ammonium 

 hydroxid, and add, while still hot, ammonium oxalate solution, drop by drop, 

 so long as any precipitate is produced, adding a few cubic centimeters in excess 

 to convert the magnesium also into oxalate. Heat to "boiling, allow to stand 

 for 3 hours or longer, decant the clear solution on a filter, pour from 15 to 20 cc 



