22 METHODS OF ANALYSIS [Chap. 



Strong Acid Digestion of the Soil 



1 1 preparation of soil solution.-official. 



Place aa amount of soil, as prepared under 2 and equivalent to 10 grams on a mois- 

 ture-free basis, in a 200-300 cc. non-soluble glass Erlenmeyer flask to which is fitted, by 

 a ground joint or 1-holed rubber stopper, a reflux tube 20 inches or more in length. 

 Add lOOcc.of hydrochloric acid of constant boiling point (approximate sp. gr. 1.115, 

 1350 cc. of acid (sp. gr. 1.19), and 1000 cc. of water), and digest continuously 

 for 10 hours on a steam or water bath, shaking the flask every hour. Allow to settle, 

 and avoiding more than very small quantities of the sediment, decant the solution 

 into a porcelain dish or non-soluble glass beaker. Transfer by means of hot water 

 the insoluble residue to a filter, wash until free from chlorin and add the washings 

 to the original solution. Concentrate, oxidize the organic matter present in the 

 solution with a few drops of nitric acid and evaporate to dryness on a water bath. 

 Treat with hot water, add a few cc. of hydrochloric acid and again evaporate to com- 

 plete dryness. When the final evaporation is complete and the dish cooled, moisten 

 the residue with a few drops of strong hydrochloric acid. Add 10-20 cc. of water, 

 warm on the bath to secure complete solution of the soluble salts, filter and wash 

 until free from chlorin. Again evaporate the solution to dryness to render insoluble 

 any silica that may remain in solution, and treat as above. The filtrate constitutes 

 the acid extract freed of soluble silica, and is made up to a definite volume (250 or 500 

 cc.) and designated as A. 



1 2 INSOLUBLE RESIDUE.— OFFICIAL. 



Combine the filters and the main residue obtained in 11 in a small dish, dry, ignite 

 over a Bunsen flame for an hour or more, carefully at first, then completely over 

 a blast lamp to constant weight. Weigh and calculate as the insoluble residue. 

 This residue may be analyzed by the usual methods applicable to silicates or it may 

 be employed in the determination of total alkalies as described under 26. If it is 

 desired to determine the silica soluble in alkalies, treat a separate portion of the soil 

 as directed in 11, except that all filtrations must be made through the same hard- 

 ened filter, then, without igniting, wash the insoluble residue into a platinum dish, 

 dry at 100°C. and complete the determination as directed under III, 4 (a). 



13 IRON, ALUMINnrM AND PHOSPHORIC ACID, COLLECTIVELY.— OFFICIAL. 



(l) To an aliquot (50 or 100 cc, according to the probable amount of iron 

 present) of A, under 11, add ammonium hydroxid, drop by drop, until the precipi- 

 tate formed requires several seconds to dissolve, thus leaving the solution but faint- 

 ly acid. Heat nearly to the boiling point, and add ammonium hydroxid to precipi- 

 tate all of the iron, aluminium, etc. Boil in a covered beaker for about 1 minute, 

 remove, and if no ammonia is given off (detected by smelling) continue the addi- 

 tion, drop by drop, until ammonia can be detected. Do not allow the precipitate 

 to settle, but stir and pour on 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, return it to the original beaker, dissolve with a few drops of hydrochloric 

 acid and warm. Reprecipitate the iron, aluminium and phosphoric acid with 

 ammonium hydroxid as above and wash until free from chlorin. Designate the 

 filtrate as B. 



Dry the filter and precipitate, remove the latter from the filter, ignite the filter 

 separately and add to its ash the precipitate. Then ignite to bright redness, cool 



