220 ANALYTICAL CHEMISTRY. 



35. DETECTION OF ACIDS. 



General remarks. There are no general methods (similar to 

 those for the separation of metals) by which all acids can be 

 separated, first into different groups, and afterwards into the in- 

 dividual acids. It is, moreover, impossible to render all acids 

 soluble (when in combination with certain metals) without de- 

 composition, as, for instance, in the case of carbonic acid when 

 in combination with calcium; calcium carbonate is insoluble in 

 water, and when the solution by means of acids is attempted, 

 decomposition takes place with liberation of carbon dioxide. 

 Many other acids suffer decomposition, in a similar manner, 

 when attempts are made to render soluble the substances in 

 which they occur. 



It is due to these facts that a complete separation of all acids 

 is not so easily accomplished as the separation of metals. 

 There is, however, for each acid a sufficient number of charac- 

 teristic tests by w r hich it may be recognized ; moreover, the pre- 

 liminary examination, as well as the respective solubility of the 

 substance, and the nature of the metal or metals present, will 

 aid in pointing out the acid or acids which are believed to be 

 present. 



If, for instance, a solid substance be completely soluble in 

 water, and if the only metal found were iron, it would become 

 unnecessary to test for carbonic, phosphoric, and hydrosulphuric 

 acids, because the combinations of these acids with iron are in- 

 soluble in water; there might, however, be present sulphuric, 

 hydrochloric, nitric, and many other acids, which form soluble 

 salts with iron. 



Detection of acids by means of the action of strong sulphuric 

 acid upon the dry substance. The action of sulphuric acid upon 

 a dry powdered substance often furnishes such characteristic in- 

 dications of the presence or absence of certain acids, that this 

 treatment should never be omitted when a search for acids is 

 made. 



When the substance under examination is liquid, a portion 

 should be evaporated to dryness, and, if a solid residue remains, 

 it should be treated in the same manner as solids. 



