190 METHODS OF ANALYSIS [Chap. 



the contents are thoroughly charred. Extract the charred mass with 25 cc. of 1% 

 nitric acid and filter. Return the residue to the dish, char and again extract with 

 25 cc. of 1% nitric acid, filter, wash with hot water, return to the dish and ignite 

 to a white ash. Dissolve the ash in 5% nitric acid and add the solution to the filtrates 

 previously obtained. Determine the chlorin in the combined filtrates either gravi- 

 metrically, as directed in I, 16 (a), or volumetrically, as directed in III, 15, using 

 N/50 solutions for greater accuracy. 



NITRITE NITROGEN.— TENTATIVE. 



19 REAGENTS. 



(a) Sulphanilic acid solution. — Dissolve 0.5 gram of sulphanilic acid in 150 cc. of 

 20% acetic acid. 



(b) Alpha-naphthylamin hydrochlorid solution. — Dissolve, by heating, 0.2 gram 

 of the salt in 150 cc. of 20% acetic acid. 



(C) Standard nitrite solution. — Dissolve 0.1097 gram of dry C. P. silver nitrite 

 in about 20 cc. of hot water, add 0.10 gram of C. P. sodivun chlorid, shake until 

 the silver chlorid flocculates and make up to 1 liter. Draw off 10 cc. of the 

 clear solution and dilute to 1 liter. Each cc. of the last solution is equivalent to 

 0.0001 mg. of nitrogen as nitrite. [Cf. IV, 12 (d)] 



The silver nitrite may be prepared as follows : To a cold solution of about 2 

 grams of sodium or potassium nitrite in 50 cc. of water, add a solution of silver 

 nitrate as long as a precipitate appears. Decant the liquid and thoroughly wash 

 the precipitate with cold water. Dissolve in boiling water. On cooling the silver 

 nitrite crystallizes out. Dry the crystals in the dark at ordinary temperature (pre- 

 ferably in a vacuima). 



20 DETERMINATION. 



(1) Select a series of 100 cc. volumetric flasks of uniform dimensions and color. 

 Place 2 grams of high-grade, nitrite-free flour in each; add approximately 70 cc. of 

 nitrite-free water and shake until the flour is thoroughly moistened. Add to these 

 flasks varying amounts of the standard sodium nitrite solution, so that a series of 

 comparison standards will be obtained having a range covering the probable nitrite 

 content of the unknown sample. Reserve 1 flask for a blank test. In order to 

 avoid making a large series of standards it is well to make a preliminary test to as- 

 certain the approximate nitrite content of the unknown. Where the quantity of 

 nitrite present is small, the nitrite solution in the flasks may be increased by 0.4 

 cc. each. Where bleaching is excessive, 1 gram of flour may be used throughout, 

 or the standards may be given a wider variation in nitrite content. 



To each of 2 similar flasks add 2 grams of the flour and 90 cc. of water; shake 

 thoroughly and digest all the flasks, including the blank, in a water bath at 40°C. 

 for at least 15 minutes; add 2 cc. each of the sulphanilic acid and alpha- 

 naphthylamin hydrochlorid solutions to each flask. Continue the digestion at 

 40°C. for an additional 20 minutes. The color must be developed in all the flasks 

 under conditions as nearly uniform as possible. Make up to the marks with nitrite- 

 free water and compare the unknown with the series of standards. This may be 

 done in a large, white, enameled pan; the effect of the turbidity, due to the flour, 

 being minimized by the white background. The solutions should be allowed to 

 subside and should not be shaken during comparison; or, 



(2) Weigh 20 grams of the flour into a 500 cc. Erlenmeyer flask, add 200 cc. of 

 nitrite-free water, previously warmed to 40°C., and close the flask with a rubber 

 stopper. Shake vigorouslj^ for 5 minutes and digest for an hour in a water bath, 



