Condensed Milk and Milk Powder 203 



PREPARATION OF REAGENTS 



"(a) Potassium sulphate. — 'This reagent should be pulverized 

 before using. 



(b) Sulphuric acid. — The sulphuric acid should have a specific 

 gravity of 1.84. It should be C. P. containing no nitrates nor am- 

 monium sulphate. 



(c) Sulphuric acid. — N-10 solution. 



(d) Standard alkali solution. — The strength of this solution 

 relative to the acid must be accurately determined, N-10 solution. 



(e) Metallic mercury or mercuric oxid. — If mercuric oxid is 

 used, it should be prepared in the wet way, but not from mercuric 

 nitrate. 



(/) Granulated zinc or pumice stone. — One of these reagents 

 is added to the contents of the distillation flasks, when found nec- 

 essary, in order to prevent bumping. 



(g) Potassium sulphid solution. — A solution of forty grams 

 of commercial potassium sulphid in one liter of water. 



(h) Sodium hydroxid solution. — A saturated solution of so- 

 dium hydroxid free from nitrates. 



(i) Indicator. — A solution of cochineal is prepared by digest- 

 ing and frequently agitating three grams of pulverized cochineal in 

 a mixture of 50 c.c. of strong alcohol and 200 c.c. of distilled water 

 for a day or two at ordinary temperatures. The filtered solution is 

 employed as indicator." 



DETERMINATION 



Place the substance to be analyzed in a digestion flask, employ- 

 ing from 0.7 to 3.5 grams, according to its proportion of nitrogen. 

 Add ten grams of powdered potassium sulphate and from 15 to 25 

 c.c. (ordinarily about 20 c.c.) of sulphuric acid. Conduct the di- 

 gestion by starting with a temperature below boiling point and in- 

 creasing the heat gradually until frothing ceases. Digest for a time 

 after the mixture is colorless or nearly so, or until oxidation is com- 

 plete. Do not add either potassium permanganate or potassium 

 sulphid. Dilute, neutralize, distil, and titrate with standard alkali. 

 in neutralizing, it is convenient to add a few drops of phenol- 

 phthalein indicator, by which one can tell, when the acid is complete- 

 ly neutralized, remembering that the pink color, which indicates 



