32 ANALYSIS OF THE GROUP KING 



to dryness. Ignite the residue carefully with a free flame until 

 no more white fumes come off. (Hood.) Do not heat to red 

 heat. Be sure to heat the sides of the dish as well as the bottom. 

 Cool and add 10 c.c. of water. Heat just to boiling and filter 

 through a very small filter paper into a test-tube. Bubble 

 through this solution a fine stream of the oxides of nitrogen 

 made by warming 1 gram of starch with 4 c.c. of nitric acid 

 (Sp. Gr. 1.33) in a 100 c.c. flask. Then boil the solution in the 

 test tube. Divide the solution into one-fourth and three- 

 fourth portions. Treat the one-fourth portion by P. 5 and the 

 three-fourth portion by P. 7. 



Notes. — 1. The bulk of the ammonium salts is removed by volatiliza- 

 tion. Care must be taken, however, not to heat to incipient redness because 

 at this temperature NaCl and especially KCl are volatile. 



2. A residue so small as to be almost invisible maj^ contain sufficient 

 potassium or sodium to give tests. Therefore, the procedures 5 and 7 should 

 not be omitted. 



3. The reagents used throughout the analjsis ma\' have dissolved 

 enough silicic acid from the glass to give a white residue of silica at this point. 

 A brown residue may be due to organic matter derived from the filter paper 

 or to impurities in the ammonia used. The excess of phosphoric acid gives a 

 residue also. 



4. The oxides of nitrogen formed bj- the action of starch on nitric acid 

 of Sp. Gr. 1.33 are approximately a one to one mixture of NO and NOj. 

 They form nitrous acid in solution which reacts with the last traces of the 

 ammonium salts to give nitrogen. It is important to remove ammonium salts 

 completely because they interfere with the potassium test. 



5. The ammonia free solution is div'ided into two portions and the 

 potassium and sodium are tested for without separating them. For extreme 

 accuracy they may be separated by the perchlorate method as given in the 

 "Exact method of analysis of the alkali group," by .'\. A. Noyes, in his "Quali- 

 tative Chemical Analysis." 



Procedure 5. Detection of Potassinvi. 



Dilute the one-fourth portion from P. 4 to 5 c.c. and add 

 5 c.c. of Na3Co(N02) reagent. Let stand for at least 10 min- 

 utes if no immediate precipitate forms. A yellow precipitate 

 may be Kj NaCo(N02)6 and indicates the presence of potassium. 

 Treat by P. 6 without filtering. 



