II] SOILS 25 



and heat to boiling; let settle for a few minutes, and determine if the precipitation 

 is complete by the addition of barium hydroxid solution to a little of the clear liquid. 

 When no further precipitate is produced, filter and wash thoroughly with hot water. 

 Heat the filtrate to boiling, add ammonium hydroxid and ammonium carbonate to 

 complete the precipitation of the barium, calcium, etc., let stand a short time on the 

 water bath, filter, and wash the precipitate thoroughly with hot water; evaporate 

 the filtrate and washings to dryness, expel ammonium salts by heating below red- 

 ness, treat with a little hot water, add a few drops of ammonium hydroxid, 1 or 2 

 drops of ammonium carbonate, and a few drops of ammonium oxalate; let stand 

 a few minutes on the water bath, set aside for a few hours, filter, evaporate to complete 

 dryness on the water bath, and heat to dull redness until all ammonium salts are 

 expelled and the residue is nearly or quite white. Dissolve in a minimum amount 

 of water, filter into a tared platinum dish, add a few drops of hydrochloric acid, evap- 

 orate to dryness on the water bath, heat to dull redness, cool in a desiccator, and 

 weigh as potassium and sodium chlorids. Repeat the heating until constant weight 

 is obtained. Dissolve in a small amount of water; if any residue remains, the sepa- 

 ration must be repeated until the residue of potassium and sodium chlorids is entirely 

 soluble. Dissolve the residue with water, add an excess of platinic chlorid solution 

 [1,40 (b)], proceed as directed under I, 45 and calculate to potassium oxid (KjO); or, 

 (2) Instead of the foregoing, evaporate to dryness a fresh aliquot of A, under 11, 

 redissolve in water, treat directly with barium hydroxid solution, and from this point 

 proceed as directed above in (1). 



22 Method II.— Tentative. 



Proceed as in 21 through "let stand a short time on the water bath" (the point 

 at which the barium, calcium, etc., have been precipitated with ammonium hydroxid 

 and ammonium carbonate) and then proceed as follows: 



Filter into a beaker, add 1 or 2 drops of hydrochloric acid and 1 cc. of ammonium 

 sulphate (75 grams to 1 liter), digest several hours on a water bath, and filter into a 

 tared platinum dish. Evaporate to dryness, heat to full redness, add 1 gram of 

 powdered ammonium carbonate; heat to expel excess of ammonium carbonate, 

 cool, and weigh the sulphates of sodium and potassium. Determine potassium as 

 directed under I, 42 (a) and calculate to potassium oxid (K2O). 



TOTAL PHOSPHORUS. 



23 Magnesium Nitrate Method. — Official. 



Place 5 grams of soil, as prepared under 2, in a porcelain dish. Moisten with 5-7 

 cc. of magnesium nitrate solution [I, 4 (e) ]. Dry on the water bath and burn off 

 the organic matter at low redness. Cool, moisten slightly with water, add 10 cc.of 

 concentrated hydrochloric acid, and digest 2 hours on the water bath, keeping the 

 dish covered with a watch glass and stirring 2 or 3 times during the digestion. Trans- 

 fer to a 250 cc. graduated flask, cool, fill to the mark, mix well, and pass through 

 a dry folded filter, pouring back on the filter until the filtrate becomes clear. Pipette 

 an aliquot corresponding to 2 or 4 grams of the soil, depending upon the amount 

 of phosphorus present, into a porcelain dish, evaporate to dryness, treat with hydro- 

 chloric acid and water, filter, and wash; the combined volume should not exceed 

 40 cc. Make alkaline with ammonium hydroxid, and dissolve the precipitate by 

 the slow addition of concentrated nitric acid, using a slight excess. Add gradu- 

 ally, while shaking, 5-15 cc. of molybdate solution [I, 7 (a) ]. Keep the solution 

 at 40°-50°C. for an hour^ let stand overnight at room temperature, filter, and wash 

 well with cold water. Return the filter and precipitate to the same flask and deter- 

 mine phosphorus volumetrically, as directed under 19. 



