FERTILIZERS. 5 



gram, add 10 cc of concentrated nitric acid and ammonium hydroxid until a 

 slight permanent precipitate is formed, dilute to 60 cc, and proceed as under 

 the preceding method (2) (6,), page 4. 



(4) CITRATE-INSOLUBLE PHOSPHORIC ACID. 



Make the solution according to the directions given under "Gravimetric 

 method" (4), page 3, and determine the phosphoric acid In an aliquot corre- 

 sponding to 0.4 gram, as directed in (2) (&0, page 4. 



(5) CITRATE-SOLUBLE PHOSPHORIC ACID. 



The sum of the water-soluble and citrate-insoluble subtracted from the total 

 gives the citrate-soluble phosphoric acid. 



4. Nitrogen. 



(a) KJELDAHL METHOD. OFFICIAL. 

 (Not applicable in the presence of nitrates. ) 



(1) PREPARATION OF REAGENTS. 



(a) 'Standard acid solution. (aj Standard hydrochloric acid, the absolute 

 strength of which has been determined by precipitating with silver nitrate and 

 weighing the silver chlorid as follows : 



By means of a preliminary test with silver-nitrate solution, to be measured 

 from a burette, with excess of calcium carbonate to neutralize free acid and 

 potassium chromate as indicator, determine exactly the amount of nitrate 

 required to precipitate all the hydrochloric acid. To a measured and also to a 

 weighed portion of the standard acid add from a burette one drop more of 

 silver-nitrate solution than is required to precipitate the hydrochloric acid. 

 Heat to boiling, cover from the light, and allow to stand until the precipitate 

 is granular. Then wash with hot water through a Gooch crucible, testing the 

 filtrate to prove excess of silver nitrate. Dry the silver chlorid at 140 to 

 150 C. 



(a 2 ) Standard sulphuric acid. The absolute strength of the acid must be 

 determined by precipitation with barium chlorid. For ordinary work half-nor- 

 mal acid is recommended. For work in determining very small amounts of 

 nitrogen tenth-normal acid is recommended. In titrating mineral acids against 

 ammonium hydroxid solution use cochineal as indicator. 



(&) Standard alkali solution. The strength of this solution relative to the 

 acid must be accurately determined ; tenth-normal solution is recommended. 



(c) Sulphuric acid. The sulphuric acid used should have a specific gravity 

 of 1.84 and be free from nitrates and also from ammonium sulphate. 



(d) Metallic mercury, or mercuric oxid. If mercuric oxid is used, it should 

 be prepared in the wet way, but not from mercuric nitrate. 



(e) Potassium permanganate. This reagent is used in a finely pulverized 

 state. 



Mix 5 grains of the fertilizer with 25 cc of hot water and filter. To a portion of this 

 solution add two volumes of concentrated sulphuric acid, free from nitric acid and oxlds 

 of nitrogen, and allow the mixture to cool. Add cautiously a few drops of a concentrated 

 solution of ferrous sulphate, so that the fluids do not mix. If nitrates are present the 

 junction shows at first a purple, afterwards a brown color, or if only a very minute 

 quantity be present, a reddish color. To another portion of the solution add 1 cc of a 

 dilute solution of nitrate of soda (3 grams to 300 cc) and test as before to determine 

 whether sufficient sulphuric acid were added in the first test. (TJ. S. Dept. Agr., Division 

 of Chemistry, Bui. 49, p. 19.) 



