A PROCEDURE FOR THE QUALITATIVE 33 



Notes. — 1. The sodium cobaltinitrite reagent is made up 0.1 formal 

 in Na3Co(N02)6, 3n. in XaXOi and In. in CH3COOH. 



2. As little as 0.1 mg. of potassium can be detected in the above por- 

 tion after long standing even in the presence of a large excess of sodium. 



3. Incomplete precipitation of magnesium does not interfere with this 

 test. For example, as much as 100 mgs. of magnesium give no precipitate 

 and do not interfere with the precipitation of the potassium. 



4. Ammonium gives a precipitate of (NH4) iNaCoCNOj)^ which is 

 almost as insoluble as the potassium precipitate and very similar to it. There- 

 fore every precaution is taken to remove ammonium previous to this test. 

 Moreover, if the precipitate formed is scanty, the confirmatory test should 

 be made. 



Procedure 6. Confirmatory Test for Potassium. 



Boil the solution from P. 5 for a minute. Then cool and 

 add 3 c.c. of the NajCo (N02)6 reagent. Let stand for ten 

 minutes. A yellow precipitate is K2NaCo(N02)6 and confirms 

 the presence of potassium. 



Notes. — 1. The cobaltinitrite reagent contains an excess of sodium 

 nitrite and acetic acid. On boiling, anj' ammonium present is decomposed 

 by the nitrous acid. The cobaltinitrite is decomposed also, so that after 

 cooling more of the reagent is added. 



2. This confirmatorj' test is more satisfactory than the flame test 

 because the presence of sodium does not interfere. 



Procedure 7. Detection of Sodium. 



To the three-fourth portion from P.4 add 5 c.c. of the KjHj 

 SbjO; reagent. Let stand at least one hour or better overnight. 

 A white, crystalline precipitate adhering to the walls of the 

 test-tube is Na, HaSbaOy and shows the presence of sodium. 



Notes. — 1. The dipotassium dihydrogen pyroantimonate jeagent is 

 0.05 formal in KjHjSbzOy, O. In. in KOH and saturated with Na2H2Sbj07. 

 It must be freshly made because of its instability. 



2. When the portion analyzed is free from other substances, as little 

 as 0.5 mg. of sodium can be detected with this reagent. With large amounts 

 of potassium present, 8 mgs. of sodium maj'' escape detection. Therefore, 

 for more delicate work, potassium should be removed. Usual'v, however, 

 this accuracy is sufficient. 



