6 METHODS OF ANALYSIS [Chap. 



by drop a 10 % solution of barium chlorid until no further jjrccii^itation occurs. 

 Continue the boiling for about 5 minutes, allow to stand for 5 hours or longer in a 

 warm place, pour the supernatant liquid on a tared Gooch or on an ashless filter, 

 treat the precipitate with 25-30 cc. of boiling water, transfer to the filter and wash 

 with boiling water until the filtrate is free from chlorin. Dry, ignite over a Bunsen 

 burner and weigh as barium sulphate. 



(C) Standard alkali solution. — Accurately determine the strength of this solution 

 by titration against the standard acid. N/10 solution is recommended. 



(d) Sulphuric acid. — Sp. gr. 1.84 and free from nitrates and ammonium sulphate. 



(e) Metallic mercury, or mercuric oxid. — Mercuric oxid should be prepared in the 

 wet way, but not from mercuric nitrate. 



(f) Copper sulphate. — Crystallized. 



(^) Potassium permanganate. — Finely pulverized. 



(h) Granulated zinc or pumice stone. — Added to the contents of the distillation 

 flask if necessary to prevent bumping. 



(i) Potassium sulphid solution. — Dissolve 40 grams of commercial potassium 

 sulphid in 1 liter of water. 



(j ) Sodium hydroxid solution. — A saturated solution, free from nitrates. 



(k) Cochineal solution. — Digest, with frequent agitation, 3 grams of pulverized 

 cochineal in a mixture of 50 cc. of strong alcohol and 200 cc. of water for 1 or 2 days 

 at ordinary temperature, and then filter. 



(1) Methyl red solution. — Dissolve 1 gram of methyl red (dimethyl-amino-azo- 

 benzene-ortho-carbonic acid) in 100 cc. of 95% alcohol. 



17 APPARATUS. 



(a) Kjeldahl flasks for both digestion and distillation. — Total capacity of about 

 550 cc, made of hard, moderately thick, and well-annealed glass. 



(b) Distillation flasks. — For distillation any suitable flask of about 550 cc. capac- 

 ity may be used. It is fitted with a rubber stopper through which passes the lower 

 end of a Kjeldahl connecting bulb to prevent sodium hydroxid being carried over 

 mechanically during distillation. The bulb should be about 3 cm. in diameter, and 

 the tubes should be of the same diameter as the condenser tube with which the upper 

 end of the bulb tube is connected by means of rubber tubing. 



1 8 DETERMINATION. 



Place 0.7-3.5 grams, according to the nitrogen content, of the substance to be 

 analyzed in a digestion flask with approximately 0.7 gram of mercuric oxid, or its 

 equivalent in metallic mercury, and add 20-30 cc. of sulphuric acid (0.1-0.3 gram of 

 crj'stallized copper sulphate may also be used in addition to the mercur}-, or in 

 place of it). Place the flask in an inclined position and heat below the boiling 

 point of the acid until frothing has ceased. (A small piece of paraffin may be added to 

 prevent extreme foaming.) Then raise the heat until the acid boils briskly and digest 

 for a time after the mixture is colorless or nearly so, or until oxidation is complete. 

 Remove the flask from the flame, hold it upright, and while still hot add care- 

 fully potassium permanganate in small quantities at a time until, after shaking, 

 the liquid remains green or purple. 



After cooling dilute with about 200 cc. of water, add a few pieces of granulated 

 zinc or pumice stone, if necessary to prevent bumping, and 25 cc. of potassium sul- 

 phid solution with shaking. Next add sufficient sodium hj^droxid solution to make 

 the reaction strongly alkaline, 50 cc. are usually enough, pouring it down the side of 

 the flask so that it does not mix at once with the acid solution. Connect the flask 



