AMMONIUM. 269 



and more NH 3 unites with water to form NH 4 OH until the reaction is com- 

 plete. All ammonium salts are highly dissociated in dilute solutions. 



The reverse of the above action, namely, the liberation of ammonia from its 

 salts by an alkali, is discussed from the ionic point of view on page 194. 



Ammonium chloride, Ammonii chloridum, NH 4 C1~ 53.11 (Sal- 

 ammoniac). Obtained by saturating the "ammoniacal liquor" of the 

 gas-works with hydrochloric acid, evaporating to dryness, and puri- 

 fying the crude article by sublimation. 



Pure ammonium chloride either is a white, crystalline powder, or 

 occurs in the form of long, fibrous crystals, which are tough and 

 flexible ; it has a cooling, saline taste ; is soluble in 2 parts of cold, 

 and in 1 part of boiling water; and, like all ammonium compounds, 

 is completely volatilized by heat. 



Carbamic acid, CO.NH 2 .OH. This acid may be looked upon as carbonic 

 acid, CO.(OH) 2 , in which one of the hydroxyl groups is replaced by NH^ The 

 ammonium salt of this acid, CO.NH. 2 .ONH 4 , is formed when dry ammonia gas 

 and dry carbon dioxide are brought together, direct combination taking place, 



thus: 



NH 2 

 CO 2 + 2NH 3 = CO( 



\ONH 4 



Ammonium carbonate, Ammonii carbonas, NH 4 HCO 3 .NH 4 

 NH 2 CO 2 = 156. Ol (Ammonium sesquicarbonate, sal volatile, Preston 

 salt). Commercial ammonium carbonate is not the normal salt, but, 

 as shown by the above formula, a combination of acid ammonium 

 carbonate with ammonium carbamate. It is obtained by sublimation 

 of a mixture of ammonium chloride and calcium carbonate, when 

 calcium chloride is formed, ammonia gas and water escape, and am- 

 monium carbonate condenses in the cooler part of the apparatus : 



2CaC0 3 + 4NH 4 C1 = NH 4 HCO 3 NH 4 NH 2 CO 2 + 2CaCl 2 + H 2 O + NH 3 . 



Ammonium carbonate thus obtained forms white, translucent masses, losing 

 both ammonia and carbon dioxide on exposure to the air, becoming opaque, 

 and finally converted into a white powder of acid ammonium carbonate. 



NH 4 HCO 3 NH 4 NH 2 CO 2 = NH 4 HCO 3 + 2NH 3 -f CO 2 . 



When commercial ammonium carbonate is dissolved in water the carbamate 

 unites with one molecule of water, forming normal ammonium carbonate. 



NH 4 NH 2 CO 2 + H 2 O = (NH 4 ) 2 C0 8 . 



A solution of the common ammonium carbonate in water is, consequently, a 

 liquid containing both acid and normal carbonate of ammonium ; by the addi- 

 tion of some ammonia water the acid carbonate is converted into the normal 

 salt. The solution thus obtained is used frequently as a reagent. 



The Aromatic spirit of ammonia is a solution of normal ammonium carbonate 

 in diluted alcohol to which some essential oils have been added. 



