DETECTION OF ACIDS. 399 



ble gas is given off, the presence of a free metal is indicated. If the substance 

 simply dissolves and no acids are subsequently found, the presence of an oxide 

 or hydroxide is indicated, which can also be judged from a knowledge of the 

 known compounds of the metals present. 



The tests distinguishing between an arsenite and an arsenate (see Chapter on 

 Arsenic) cannot be applied when the substance is insoluble in water (except the 

 inolybdate test, which can be used in an acid solution), but the treatment with 

 hydrogen sulphide can be used to differentiate, because an arsenite gives a pre- 

 cipitate instantly even in cold solution, while an arsenate precipitates only after 

 a long time. 



If bismuth is present, remove it before testing for the acids by boiling with 

 sodium carbonate, filtering, etc., as described above. 



Substances insoluble in water and hydrochloric acid are next treated with 

 nitric acid. Ordinarily very few such substances are presented. If brown 

 vapors are evolved and sulphur separates, a sulphide is indicated, which the 

 appearance of the substance will also suggest. If brown vapors alone are 

 evolved, a free metal is indicated. 



If the preliminary tests for metals show the presence of mercury, and the 

 substance dissolves slowly on boiling with nitric acid, it is one of the halogen 

 salts. The mercury should be removed by boiling the substance with an excess 

 of caustic alkali, filtering, neutralizing the filtrate with nitric acid, and testing 

 it for chloride, bromide, or iodide. 



Before examining for metals, nitric acid solutions must be evaporated to dry- 

 ness to expel excess of the acid. The residue is dissolved in water. 



A few substances require nitro-hydrochloric acid for solution. The one most 

 likely to occur ordinarily is mercuric sulphide, which is indicated by the pres- 

 ence of mercury, its black or vermilion color, and volatility on heating. 



Of substances insoluble in all acids, the sulphates of barium, strontium, and 

 lead are the most likely to be presented ordinarily. The treatment of these by 

 fusion with sodium carbonate has already been mentioned. The presence of 

 silver (as shown by the preliminary tests for metals) would indicate a chloride, 

 bromide, iodide, or cyanide of this metal. The metal should be removed by 

 boiling with caustic alkali and the filtrate tested. Silver iodide does not yield 

 to this treatment, but its color and insolubility in strong ammonia is sufficient 

 evidence of iodide. Silver cyanide with hydrochloric acid forms silver chloride 

 and hydrocyanic acid, which is in solution and recognized by its odor. 



The insoluble halogen salts of silver, lead, and mercury, also mercuric chloride 

 and bromide, scarcely react when treated with concentrated sulphuric acid 

 (Table IX). 



Most of the points in the discussion above are shown in more convenient 

 form in the following Tables, XIII and XIV, which are more detailed than Table 

 IX, and will perhaps be of greater help to the student. Table XIV deals with 

 difficultly soluble or insoluble substances, which may be subnitrate ; subchlor- 

 ide; chloride (Pb, Hg(ous),Ag) ; bromide (Pb, Hg(ous),Ag) ; iodide; sulphate 

 (Ca, Sr, Ba, Pb, Hg(ous) ; sulphite (except of alkalies) ; sulphide (except of 

 alkalies and alkaline earths) ; carbonate, borate, phosphate, arsenate, arsenite 

 (except of alkalies in each case); chromate (high color) ; fluoride; cyanide; 

 oxide or hydroxide (except of alkali and alkaline earth metals) ; and a few 

 others. 



In the case of mixtures,, the tables may be used to determine as far as 



