132 Analysis and Valuation of Manures [209 



and add 12 c.c. of water. Evaporate on the water bath until 

 the liquid has diminished to about half its original bulk. Filter 

 into a 250-c.c. flask. Wash with water slightly acidulated with 

 HC1. Make up to 250 c.c. with distilled H 2 0. After mixing 

 take 25 c.c. and transfer to a 5-oz. conical flask fitted with a 

 caoutchouc stopper. Add 18 c.c. of the ammonium nitrate 

 solution. Heat to 85 ° C. Add 20 c.c. of ammonium molyb- 

 date solution, cork the flask, and shake vigorously for one 

 minute. Filter through a very smooth filter paper. Wash 

 three times with dilute HN0 3 (1-20). Then remove the 

 paper, and wash all the yellow precipitate from the paper into 

 a weighed platinum dish. Evaporate to dryness on the water 

 bath. Finally, dry in the steam oven, cool, and weigh. The 

 weight of the precipitate x 74*66 = percentage of P 2 5 . 



209. Molybdate and magnesia method. This is per- 

 haps the most reliable method for the estimation of phosphoric 

 acid, and certainly the one most generally applicable. 



Weigh out 1 gram of the sample. Dissolve in a mixture of 

 15 c.c. hydrochloric acid (S.G. 1*16) and 15 c.c. nitric acid 

 (S.G. 1*42), evaporating to dryness to separate the silica. Add 

 20 c.c. hydrochloric acid (S.G. 1*16), and evaporate until it 

 only occupies about half the original bulk. Add 20 c.c. boiling 

 water, and filter. Collect the filtrate in a 250-c.c. flask. Make 

 up to the 250-c.c. mark with water. Take 50 c.c. of this liquid 

 with a pipette, and introduce it into an 8-oz. conical flask 

 fitted with an india-rubber cork. Add ammonia until a per- 

 manent precipitate is formed. Dissolve this in the smallest 

 possible quantity of nitric acid. Heat to 85 C. Add 200 c.c. 

 ammonium molybdate solution (see paragraph 208). Cork 

 the flask, and shake vigorously for five minutes. Allow the 

 precipitate to settle, then filter. Wash once by decantation, 

 then two or three times on the filter paper, using 1 in 20 nitric 

 acid. This precipitate will contain all the phosphoric acid. 



