390 ANALYTICAL CHEMISTRY. . 



The precipitation of sulphur, in the absence of metals of the arsenic group, 

 frequently leads beginners to the assumption that metals of this group are 

 present. The precipitate consisting only of sulphur is white and milky, but 

 flocculent, and more or less colored in the presence of the metals of the 

 arsenic group. 



TABLE VII. Treatment of the precipitate formed by ammonium 



carbonate. 



The precipitate may contain the carbonates of barium, caloium, and 

 Strontium. 1 Dissolve the precipitate in acetic acid, and add potassium dichromate. 



TABLE VIII. Detection of the alkalies and of magnesium. 



The fluid which has been treated with hydrochloric acid, hydrogen sulphide, am- 

 monium hydroxide, sulphide, and carbonate, may contain magnesium and the 

 alkalies. 



Divide solution into two portions. 



To the first portion add sodium phosphate. A white crystalline precipitate indi- 

 cates magnesium. 2 



The second portion is evaporated to dryness, further heated (or ignited) until all 

 ammonium compounds are expelled, and white fumes are no longer given off. The 

 residue is dissolved in water, and sodium cobaltic nitrite added. A yellow precipitate 

 indicates potassium. The residue is also examined by flame test : a yellow color 

 indicating sodium, a red color lithium. 



Ammonium compounds have to be tested for in the original fluid by treating 

 it with calcium hydroxide, when ammonia gas is liberated 



1 If an insufficient quantity of ammonium chloride should have been present, some magnesia 

 may also be contained in this precipitate, and may be redissolved by treating it with ammonium 

 chloride solution. 



2 If an insufficient quantity of ammonium chloride has been produced in the original solution 

 by the addition of hydrochloric acid and ammonium hydroxide, a portion of the magnesia may 

 have been precipitated by the ammonium hydroxide or carbonate. 



QUESTIONS. By what tests can mercurous chloride be distinguished from 

 the chloride of silver or lead ? How can it be proved that a precipitate pro- 

 duced by hydrogen sulphide in an acid solution contains a metal or metals of 



