236 § 119. FERTILIZERS. 



plioric acid. If the superphosphate was made from a 

 phosphatic guano, this precipitate of ferric phosphate will 

 generally be too large to allow an accurate volumetric 

 estimation of phosphoric acid, and a gravimetrical method 

 should be followed. (§ 93, II) 



2. To estimate the insoluble phosphate^ treat 20 grms. 

 of the substance with water to which 20 c.c. of nitric acid 

 (Sp. Gr. = 1.4) have been added, and digest the mixture 

 several hours on the water-bath. If a sufficient quantity 

 of acid was added, the insoluble residue left after diores- 

 tion can be plainly seen, when stirred up, to consist of 

 nothing but heavy sand and particles of coal ; if the so- 

 lution appears to be incomplete, add 10 c.c. more of the 

 acid and lieat the mixtui'e again several hours ; finally, 

 filter the solution into a litre flask, and when the filtrate 

 is cool, bring its volume up to 1000 c.c. ; determine phos- 

 phoric acid by the volumetric method (§ 115, a) in 50 c.c. 

 of this solution, using 20 c.c. of sodic acetate. 



This result gives the total amount of phosphoric acid 

 in the superphosphate, and, as the soluble phosphate has 

 already been determined, the amount of insoluble phos- 

 phate is readily estimated. 



The insoluble residue of sand in this examination may 

 le ignited and weighed. 



f. Complete analysis* — Examine the aqueous solution 

 prepared above, if it is particularly desired to learn its 

 composition, according to Scheme lY., § 94, taking 100 c.c. 

 for <7, and 100 c.c. for h with previous treatment with 

 sodic carbonate and potassic nitrate ; also determine chlo- 

 rine in 50 c.c. of the solution, by precipitation with ar- 

 gentic nitrate (§ 63). 



Or, for a more complete analysis, including the determi- 

 nation of what is soluble in acid with what is soluble in 

 water, examine the nitric-acid solution obtained in e ac- 

 cording to Scheme IV., except in case the phosphate 

 contains much organic matter, when it would be better to 



