POTASSIUM. 261 



2. To a neutral or slightly acid solution of a potassium salt add solu- 

 tion of sodium cobaltic nitrite: a yellow precipitate of potassium cobaltic 

 nitrite, (KNO 2 ) 6 .Co 2 (NO 2 ) 6 + H 2 O, is produced. (The reaction is 

 not influenced by the presence of alkaline earths, earths, or metals of 

 the iron group, but is not suitable in case of potassium iodide, since 

 iodine is liberated by the nitrous acid of the cobalt solution, and 

 interferes with the test.) 



3. Add to a concentrated solution of a neutral potassium salt a 

 freshly prepared strong solution of tartaric acid : a white precipitate 

 of potassium acid tartrate, KHC 4 H 4 O 6 , is slowly formed. Addition 

 of alcohol facilitates precipitation. 



Tartaric acid, H 2 .C 4 H 4 O 6 , is dibasic and dissociates chiefly into H* 

 and HC 4 H 4 O/ ions. Potassium ions, K', and HC 4 H 4 O 6 ' ions unite 

 to form the difficultly soluble acid tartrate ; 



K- + NO./ -f H- + HC 4 H 4 O 6 ' = KHC 4 H 4 6 + H' + NO,'. 



One part of the salt is soluble in about 200 parts of water, but prac- 

 tically insoluble in alcohol, even when diluted. 



4. Potassium compounds color violet the flame of a Bunsen burner 

 or of alcohol. The presence of sodium, which colors the flame in- 

 tensely yellow, interferes with this test, as it masks the violet caused 

 by potassium. The difficulty may be overcome by observing the 

 flame through a blue glass or through a thin vessel filled with a solu- 

 tion of indigo. The yellow light is absorbed by the blue medium, 

 while the violet light passes through and can be recognized. 1 



With few exceptions, potassium compounds are white, soluble in 

 water, and not volatile at a low red heat. Of the above tests, the 



1 The flame reaction for metals is one of the steps taken in qualitative analysis. For this 

 purpose the platinum wire should be kept immersed in hydrochloric acid in a test-tube. 

 When needed, it is cleaned by alternately holding it in the flame and dipping it in the acid, 

 until no color is given to the flame. The salt best adapted for flame tests is a chloride ; hence 

 the substance to be tested should be moistened in a dish with hydrochloric acid before intro- 

 ducing it into the flame on the loop of the wire. Chlorides are readily volatilized. Unless the 

 substance is volatile, there will be 110 flame reaction. 



QUESTIONS. How is potassium found in nature, and from what sources is 

 the chief supply of potassium salts obtained ? What color have the salts of the 

 alkali metals, and which are insoluble ? Mention two processes for making potas- 

 sium hydroxide, and what are its properties? Show by symbols the conversion 

 of carbonate into bicarbonate of potassium. Explain the principle of the man- 

 ufacture of potassium nitrate, and what is the office of the latter in gunpowder? 

 How is potassium chlorate made, and what are its properties ? Give the proc- 

 esses for manufacturing iodide and bromide of potassium, both in words and 

 symbols. State the composition of potassium sulphate and sulphite. How can 

 they be obtained? Mention tests for potassium compounds. How much iodine 

 is contained in 33 grammes of potassium iodide? 



